Patent Publication Number: US-2023147072-A1

Title: Gap Measurement Tool Assembly, System, and Method for Measuring a Gap Between Mating Parts of a Structure

Description:
FIELD 
     The disclosure relates generally to assemblies, systems, and methods for measuring gaps between mating parts, and more specifically, to assemblies, systems, and methods for measuring part interface gaps between mating aircraft parts using an air gage system. 
     BACKGROUND 
     In the manufacture of large structures, such as aircraft, rotorcraft, spacecraft, watercraft, and other large structures, the proper joining of parts is important to minimize any residual stresses incorporated into such large structures during the assembly process. Such residual stresses may be created when parts that do not fit together properly at all interface points are tightly fastened together, which may result in bending of the parts and introduction of such residual stresses into the assembly. To prevent such bending of the parts and introduction of residual stresses, manufacture and assembly requirements typically specify a maximum part interface gap between mating parts that is allowed with a minimum force to hold the parts together. For example, for mating or assembled aircraft parts, such as for an aircraft fuselage section, a maximum gap, such as a part interface gap, may be 0.005 inch with an assembly force of 5 pounds per foot applied. 
     Known apparatuses, systems, and methods exist for measuring gaps, such as part interface gaps. For example, one such known apparatus, system, and method includes use of a flat feeler gage manually inserted by an operator at gaps, such as part interface gaps, at edges of the mating or assembled parts to ensure that the feeler gage having a specified thickness, such as 0.005 inch, does not fit into the gap. This may require multiple gap checks by repeatedly trying different sizes of feeler gages. For large parts, such as having a length of greater than 10 feet, this may be a tedious and time consuming process. 
     Moreover, the assembly geometry of the mating or assembled parts may be such that the edges of the parts are covered or not accessible to be edge-checked. In this case, temporary fasteners may be used to fasten the parts together at predetermined clamp loads and intermediate unfilled holes may be used to measure for the existence of gaps, such as part interface gaps. The flat feeler gage used for such measuring is typically modified to have a 90 degree bend to probe inside the intermediate unfilled holes, so that the gap check process may be made through such intermediate unfilled holes, which may be very small in size. 
     To obtain a gap measurement using a modified feeler gage with a 90 degree bend, for example, a modified feeler gage having a thickness of 0.005 inch, the modified feeler gage is inserted into the gap, and a gap width is determined. This may require multiple gap checks by repeatedly trying different sizes of feeler gages, such as successively larger and larger feeler gages, until either a gap measurement is obtained, that is, until the largest size that fits within the gap is determined and designated as a “gap width”, or it is determined that the feeler gage, such as the 0.005 inch feeler gage, does not fit and the gap is less than the maximum allowance. Such gap measuring may have to be manually performed on thousands of individual intermediate unfilled holes on a single aircraft assembly. 
     Obtaining such gap measurements through intermediate unfilled holes is a two-step process and may be difficult to perform and may require manual dexterity and operator “feel” to determine the existence of a gap with any reasonable certainty and may produce a wide variation in results. Moreover, obtaining such gap measurements through intermediate unfilled holes may be time consuming, labor intensive, and ergonomically challenging for operators repeatedly taking such gap measurements. 
     Accordingly, there is a need in the art for an improved assembly, system, and method for measuring gaps, such as part interface gaps, between mating parts, that are fast, that may be automated, that are simple and stable, that provide accurate, robust, and repeatable measurements, and that provide advantages over known assemblies, systems, and methods. 
     SUMMARY 
     Example implementations of the present disclosure provide an assembly, system, and method for measuring part interface gaps between mating parts. As discussed in the below detailed description, versions of the assembly, system, and method may provide significant advantages over known assemblies, systems, and methods. 
     In a version of the disclosure, there is provided a gap measurement tool assembly for measuring a gap at a through hole between mating parts of a structure. The gap measurement tool assembly comprises a gap measurement tool. 
     The gap measurement tool comprises a first end configured to couple to an air gage system and an air supply source and a second end configured to be inserted into the through hole. The gap measurement tool further comprises a body formed between the first end and the second end. The body comprises a hollow inner channel, an exterior annular crevice, a first exterior annular groove having a cross-hole intersected by the hollow inner channel, and a second exterior annular groove positioned distal to the first exterior annular groove. 
     The gap measurement tool assembly further comprises a first seal element fitted around the exterior annular crevice. The gap measurement tool assembly further comprises a second seal element fitted around the second exterior annular groove. The gap measurement tool assembly further comprises a third seal element inserted into a portion of the hollow inner channel at the second end of the gap measurement tool. 
     The gap measurement tool assembly is configured for insertion and sealing into the through hole and through the mating parts, and the cross-hole is configured to align with the gap, such that when compressed air is passed into the gap, via the hollow inner channel, the air gage system takes a measurement of one of, a back pressure, an air flow, or a differential pressure, and correlates the measurement to a predetermined gap measurement, to determine a gap size of the gap at the through hole between the mating parts of the structure. 
     In another version of the disclosure, there is provided a gap measurement system for measuring a gap at a through hole between mating parts of a structure. The gap measurement system comprises a gap measurement tool assembly configured for insertion into the through hole and for alignment with the gap between the mating parts. 
     The gap measurement tool assembly comprises a gap measurement tool. The gap measurement tool comprises a first end, a second end, and a body formed between the first end and the second end. The body comprises a hollow inner channel, an exterior annular crevice, a first exterior annular groove having a cross-hole intersected by the hollow inner channel, and a second exterior annular groove positioned distal to the first exterior annular groove. 
     The gap measurement tool assembly further comprises a first seal element fitted around the exterior annular crevice. The gap measurement tool assembly further comprises a second seal element fitted around the second exterior annular groove. The gap measurement tool assembly further comprises a third seal element inserted into a portion of the hollow inner channel at the second end of the gap measurement tool. 
     The gap measurement system further comprises an air gage system coupled to the first end of the gap measurement tool. The gap measurement system further comprises an air supply source having compressed air. The air supply source is coupled to the air gage system. 
     When the gap measurement tool assembly is inserted into the through hole and through the mating parts, so that the cross-hole is aligned with the gap, and an entry side and an exit side of the through hole are sealed, the compressed air is passed into the gap, via the hollow inner channel, and the air gage system takes a measurement of one of, a back pressure, an air flow, or a differential pressure, and correlates the measurement to a predetermined gap measurement, to determine a gap size of the gap at the through hole between the mating parts of the structure. 
     In another version of the disclosure, there is provided a method for measuring a gap at a through hole between mating parts of a structure. The method comprises the step of providing a gap measurement tool assembly. The gap measurement tool assembly comprises a gap measurement tool comprising a first end, a second end, and a body formed between the first end and the second end. The body comprises a hollow inner channel, an exterior annular crevice, a first exterior annular groove having a cross-hole intersected by the hollow inner channel, and a second exterior annular groove positioned distal to the first exterior annular groove. 
     The gap measurement tool assembly further comprises a first seal element fitted around the exterior annular crevice. The gap measurement tool assembly further comprises a second seal element fitted around the second exterior annular groove. The gap measurement tool assembly further comprises a third seal element inserted into a portion of the hollow inner channel at the second end of the gap measurement tool. 
     The method further comprises the step of coupling the gap measurement tool assembly to an air gage system and an air supply source having compressed air, to obtain a gap measurement system. The method further comprises the step of inserting the gap measurement tool assembly into the through hole and through the mating parts, so that the cross-hole of the gap measurement tool is aligned with the gap at the through hole between the mating parts, and an entry side and an exit side of the through hole are sealed. 
     The method further comprises the step of passing the compressed air through the hollow inner channel of the gap measurement tool and into the gap. The method further comprises the step of using the air gage system to take a measurement of one of, a back pressure, an air flow, or a differential pressure. The method further comprises the step of correlating the measurement to a predetermined gap measurement, to determine a gap size of the gap at the through hole between the mating parts of the structure. 
     The features, functions, and advantages that have been discussed can be achieved independently in various versions of the disclosure or may be combined in yet other versions, further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure can be better understood with reference to the following detailed description taken in conjunction with the accompanying drawings, which illustrate preferred and exemplary versions, but which are not necessarily drawn to scale. The drawings are examples and not meant as limitations on the description or claims. 
         FIG.  1 A  is an illustration of a perspective side view of an exemplary version of a gap measurement tool of the disclosure; 
         FIG.  1 B  is an illustration of a side view of the gap measurement tool of  FIG.  1 A ; 
         FIG.  1 C  is an illustration of a top view of the gap measurement tool of  FIG.  1 A ; 
         FIG.  1 D  is an illustration of a cross-section of the gap measurement tool of  FIG.  1 B , taken along lines  1 D- 1 D; 
         FIG.  1 E  is an illustration of a partial sectional perspective side view of the gap measurement tool of  FIG.  1 A ; 
         FIG.  1 F  is an illustration of a perspective side view of another exemplary version of a gap measurement tool of the disclosure; 
         FIG.  1 G  is an illustration of a top view of the gap measurement tool of  FIG.  1 F ; 
         FIG.  2 A  is an illustration of a perspective side view of an exemplary version of a gap measurement tool assembly of the disclosure; 
         FIG.  2 B  is an illustration of a cross-section of the gap measurement tool assembly of  FIG.  2 A ; 
         FIG.  2 C  is an illustration of a partial sectional perspective side view of the gap measurement tool assembly of  FIG.  2 A ; 
         FIG.  3 A  is an illustration of a perspective side view of an exemplary version of a gap measurement tool assembly of the disclosure, inserted in a through hole through mating master blocks having a known gap size; 
         FIG.  3 B  is an illustration of a top view of the gap measurement tool assembly and a mating master block of  FIG.  3 A ; 
         FIG.  3 C  is an illustration of a cross-section of the gap measurement tool assembly and the mating master block of  FIG.  3 B , taken along lines  3 C- 3 C; 
         FIG.  3 D  is an illustration of a partial sectional perspective side view of the gap measurement tool assembly of  FIG.  3 A , showing an air flow path through the gap measurement tool and a gap having the known gap size between the mating master blocks; 
         FIG.  3 E  is an illustration of another partial sectional perspective side view of the gap measurement tool assembly of  FIG.  3 A , showing an air flow path through the gap measurement tool and no gap between the mating master blocks; 
         FIG.  4    is an illustration of a block diagram of an exemplary gap measurement system with a gap measurement tool assembly for measuring a gap at a through hole between mating parts, in accordance with an illustrative version of the disclosure; 
         FIG.  5 A  is an illustration of an exemplary version of a portable gap measurement system of the disclosure; 
         FIG.  5 B  is an illustration of an exemplary version of an automated gap measurement system of the disclosure, coupled to a robot; 
         FIG.  6    is an illustration of a schematic diagram of an exemplary version of a differential pressure gauge air gage system and a gap measurement tool assembly of the disclosure; 
         FIG.  7    is an illustration of a graph showing a correlation between gap size and back pressure, air flow, and differential pressure; 
         FIG.  8    is an illustration of a flow diagram of an exemplary version of a method of the disclosure; 
         FIG.  9    is an illustration of a perspective view of an aircraft incorporating assembled aircraft parts that may be measured for gaps using exemplary versions of a gap measurement tool assembly, a gap measurement system, and a method of the disclosure; 
         FIG.  10    is an illustration of a flow diagram of an exemplary aircraft manufacturing and service method; and 
         FIG.  11    is an illustration of an exemplary block diagram of an aircraft. 
     
    
    
     The figures shown in this disclosure represent various aspects of the versions presented, and only differences will be discussed in detail. 
     DETAILED DESCRIPTION 
     Disclosed versions will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all of the disclosed versions are shown. Indeed, several different versions may be provided and should not be construed as limited to the versions set forth herein. Rather, these versions are provided so that this disclosure will be thorough and fully convey the scope of the disclosure to those skilled in the art. 
     This specification includes references to “one version” or “a version”. The instances of the phrases “one version” or “a version” do not necessarily refer to the same version. Particular features, structures, or characteristics may be combined in any suitable manner consistent with this disclosure. 
     As used herein, “comprising” is an open-ended term, and as used in the claims, this term does not foreclose additional structures or steps. 
     As used herein, “configured to” means various parts or components may be described or claimed as “configured to” perform a task or tasks. In such contexts, “configured to” is used to connote structure by indicating that the parts or components include structure that performs those task or tasks during operation. As such, the parts or components can be said to be configured to perform the task even when the specified part or component is not currently operational (e.g., is not on). 
     As used herein, the terms “first”, “second”, etc., are used as labels for nouns that they precede, and do not imply any type of ordering (e.g., spatial, temporal, logical, etc.). 
     As used herein, an element or step recited in the singular and preceded by the word “a” or “an” should be understood as not necessarily excluding the plural of the elements or steps. 
     As used herein, the phrase “at least one of,” when used with a list of items, means different combinations of one or more of the listed items may be used, and only one of each item in the list may be needed. In other words, “at least one of” means any combination of items and number of items may be used from the list, but not all of the items in the list are required. The item may be a particular object, a thing, or a category. 
     There is disclosed herein a gap measurement tool  10  (see  FIGS.  1 A- 1 E ), which is part of a gap measurement tool assembly  12  (see  FIGS.  2 A- 2 B ), a gap measurement system  14  (see  FIG.  4   ), which includes the gap measurement tool assembly  12 , and a method  180  (see  FIG.  8   ), discussed in further detail below. The gap measurement tool assembly  12 , the gap measurement system  14 , and the method  180  are designed to measure a gap  16  (see  FIGS.  4 ,  5 A- 5 B ), such as a part interface gap  16   a  (see  FIGS.  4 ,  5 A- 5 B ), at a through hole  18  (see  FIGS.  4 ,  5 A- 5 B ), such as a fastener through hole  18   a  (see  FIGS.  4 ,  5 A- 5 B ) between mating parts  20  (see  FIGS.  4 ,  5 A- 5 B ), such as aircraft mating parts  22  (see  FIGS.  4 ,  5 A,  9   ), of a structure  24  (see  FIGS.  4 ,  5 A- 5 B ), such as an aircraft structure  26  (see  FIGS.  4 ,  5 A,  9   ), for example, cylindrical or curved objects such as fuselage sections  26   a  (see  FIGS.  4 ,  5 A ), or an aircraft structure  26  such as a tail section  26   b  (see  FIG.  4   ), or another suitable aircraft structure. The mating parts  20  comprise a stack-up of mating parts  20  that are adjoined. The structure  24  may comprise a fully assembled structure  24   a  (see  FIG.  5 A ), or a partially assembled structure  24   b  (see  FIG.  5 B ). 
     The gap measurement tool assembly  12 , the gap measurement system  14 , and the method  180 , are used to conduct gap analysis and to determine a gap size  28  (see  FIGS.  4 ,  5 A- 5 B ), such as a gap width  30  (see  FIGS.  4 ,  5 A- 5 B ), of any gaps  16  that occur when joining together mating parts  20 , such as aircraft mating parts  22 , of the structure  24 , such as the aircraft structure  26 , for example, the fuselage section  26   a , the tail section  26   b , or another suitable aircraft structure, using through holes  18  configured to receive fasteners  19  (see  FIGS.  5 A- 5 B ), such as permanent fasteners  19   a  (see  FIG.  5 A ), or temporary fasteners  19   b  (see  FIG.  5 B ), for example, rivets, bolts, screws, or other suitable fasteners. 
     The gap measurement system  14  and the method  180  use an air gage system  32  (see  FIGS.  4 ,  5 A- 5 B ) and an air supply source  34  (see  FIGS.  4 ,  5 A- 5 B ), with air  36  (see  FIG.  4   ), such as compressed air  36   a  (see  FIGS.  4 ,  5 A- 5 B ). The air gage system  32  is coupled, or attached, to the gap measurement tool assembly  12 , and the gap measurement tool assembly  12  is inserted, partially or completely, through the through hole  18  and through the mating parts  20 , and properly sealed, as discussed below. The air  36 , such as compressed air  36   a , from the air supply source  34 , is passed through the gap measurement tool  10  and into the gap  16 . 
     The air gage system  32  is used to take a measurement  38  (see  FIG.  4   ) of one of, a back pressure  40  (see  FIG.  4   ), an air flow  42  (see  FIG.  4   ), or a differential pressure  44  (see  FIG.  4   ), and correlates the measurement  38  to a predetermined gap measurement  46  (see  FIG.  4   ), to determine the gap size  28  of the gap  16 , or gap measurement  48  (see  FIG.  4   ), at the through hole  18  between the mating parts  20  of the structure  24 . The air flow  42 , such as the flow of the air  36 , for example, the compressed air  36   a , is proportional to the gap size  28  of the gap  16 . Further, leakage of air  36 , such as compressed air  36   a , through the gap measurement tool  10  and through the gap  16  between the mating parts  20  causes a corresponding change in the back pressure  40  or the differential pressure  44  and is proportional to, or correlated to, the gap size  28  of the gap  16 . 
     The gap measurement system  14  and the method  180  use only a single measurement  38   a  (see  FIG.  4   ) in a one-step process  49  (see  FIG.  4   ), to measure and determine the gap size  28  of the gap  16 , or gap measurement  48 . The use of the gap measurement tool assembly  12 , the gap measurement system  14 , and the method  180  avoid use of known feeler gauges and a two-step process using such known feeler gauges, and avoid multiple gap checks by repeatedly trying different sizes of feeler gages. The use of the gap measurement tool assembly  12 , the gap measurement system  14 , and the method  180  are simple to use and provide accurate, robust, and repeatable measurements. Further, the gap measurement tool assembly  12 , the gap measurement system  14 , and the method  180  provide the ability to take gap measurements  48  (see  FIG.  4   ) of gaps  16  between mating parts  20 , or adjoining parts, where it may not be possible to take a gap measurement  48  because of geometry. 
     Now referring to  FIGS.  1 A- 1 E ,  FIGS.  1 A- 1 E  show an exemplary version of a gap measurement tool  10  of the disclosure.  FIG.  1 A  is an illustration of a perspective side view of the exemplary version of the gap measurement tool  10 .  FIG.  1 B  is an illustration of a side view of the gap measurement tool  10  of  FIG.  1 A .  FIG.  1 C  is an illustration of a top view of the gap measurement tool  10  of  FIG.  1 A .  FIG.  1 D  is an illustration of a cross-section of the gap measurement tool  10  of  FIG.  1 B , taken along lines  1 D- 1 D.  FIG.  1 E  is an illustration of a partial sectional perspective side view of the gap measurement tool  10  of  FIG.  1 A . 
     As shown in  FIGS.  1 A- 1 E , in one version, the gap measurement tool  10  is in the form of a machined circular mandrel  50  that is configured to fit, and fits, for example, to partially fit, and partially fits, closely or snugly in the through hole  18  and through the mating parts  20  at the gap  16  to be measured. The gap measurement tool  10  is preferably made of a metal material  52  (see  FIG.  4   ) comprising one or more of, stainless steel  52   a  (see  FIG.  4   ), steel  52   b  (see  FIG.  4   ), aluminum  52   c  (see  FIG.  4   ), or another suitable metal material. The gap measurement tool  10  may also be made of another suitably sturdy and durable material. The gap measurement tool  10  is preferably precision machined using a precision machining process. The gap measurement tool  10  may also be 3D (three-dimensional) printed using a 3D (three-dimensional) printing process. In one exemplary version, the gap measurement tool  10  has a length of one (1) inch to two (2) inches. However, the gap measurement tool  10  may have a length of greater than two (2) inches, or may have a length of 0.20 inch to 0.99 inch, depending on the size of the through hole  18 , the geometry of the mating parts  20  being measured, and where the gap  16  is located. 
     As shown in  FIGS.  1 A- 1 E , the gap measurement tool  10  comprises a first end  54 , such as a proximal end  54   a , configured to couple to, or be coupled to, the air gage system  32 , and in turn, the air supply source  34 . As further shown in  FIGS.  1 A- 1 B,  1 D- 1 E , the gap measurement tool  10  comprises a second end  56 , such as a distal end  56   a , configured to be inserted into the through hole  18  and through the mating parts  20 . 
     As further shown in  FIGS.  1 A- 1 B,  1 D- 1 E , the gap measurement tool  10  comprises a body  58  formed between the first end  54  and the second end  56 . In one version, as shown in  FIGS.  1 A- 1 B,  1 D- 1 E , the body  58  comprises a head portion  60 , and a shaft portion  62  coupled to, and extending from, the head portion  60 . As shown in  FIG.  1 A , the head portion  60  has a first end  64   a  and a second end  64   b , and the shaft portion  62  has a first end  66   a  and a second end  66   b . The first end  64   a  of the head portion  60  comprises an interior connector portion  68  (see  FIG.  1 D ) configured to couple, or attach, to the air gage system  32 . As shown in  FIG.  1 D , the head portion  60  has an outer diameter  70   a , and the shaft portion  62  has an outer diameter  70   b , and the outer diameter  70   a  of the head portion  60  is greater than the outer diameter  70   b  of the shaft portion  62 . In one version, the outer diameter  70   a  of the head portion  60  of the body  58  is ½ (one-half) inch to 1 (one) inch in size, and the outer diameter  70   b  of the shaft portion  62  of the body  58  is ¼ (one-quarter) inch to ½ (one-half) inch in size, and in proportion to the size of the head portion  60 . In other versions, the outer diameter  70   a  of the head portion  60  is greater than 1 (one) inch and the outer diameter  70   b  of the shaft portion  62  is greater than ½ (one-half) inch, and in proportion to the size of the head portion  60 . Although the body  58  of the gap measurement tool  10 , shown in  FIGS.  1 A- 1 E , has a head portion  60  and a shaft portion  62 , the gap measurement tool  10  may have another suitable design or configuration. 
     In one version, the head portion  60  (see  FIG.  1 C ) and the shaft portion  62  (see  FIG.  1 A ) comprise an outer cross-section shape  72  (see  FIGS.  1 A,  1 C ) in the form of a circle  72   a  (see  FIGS.  1 A,  1 C ). In another version, the head portion  60  (see  FIG.  1 G ) and the shaft portion  62  (see  FIG.  1 F ) comprise an outer cross-section shape  72  (see  FIGS.  1 F,  1 G ) in the form of a square  72   b  (see  FIGS.  1 F,  1 G ). In other versions, the head portion  60  and the shaft portion  62  may comprise an outer cross-section shape  72  in the form of a hexagon  72   c  (see  FIG.  4   ), a triangle  72   d  (see  FIG.  4   ), or another suitable outer cross-section shape. 
     As further shown in  FIGS.  1 C,  1 D- 1 E , the body  58  of the gap measurement tool  10  comprises an exterior  74   a  and an interior  74   b , and a hollow inner channel  76  having a length  78  (see  FIG.  1 D ) extending along a longitudinal axis  79  (see  FIG.  1 D ) through the interior  74   b  from the first end  54  to the second end  56 . As shown in  FIG.  1 E , in this version, the hollow inner channel  76  has a generally funnel shape  80 , where a first diameter  82   a  of the hollow inner channel  76  in the head portion  60  is greater than a second diameter  82   b  in the hollow inner channel  74  in the shaft portion  62 . In other versions, the hollow inner channel  76  has another suitable shape. As further shown in  FIGS.  1 C,  1 E , the hollow inner channel  76  of the body  58  has a top opening  84   a  and a bottom opening  84   b.    
     As further shown in  FIGS.  1 A- 1 B,  1 D- 1 E , the body  58  of the gap measurement tool  10  comprises an exterior annular crevice  86  formed, or notched, on a portion of the exterior  74   a  of the body  58 . As shown in  FIG.  1 A , the exterior annular crevice  86  is positioned between the second end  64   b  of the head portion  60  and the first end  66   a  of the shaft portion  62 . The exterior annular crevice  86  has an annular surface  87  (see  FIG.  1 A ). 
     The body  58  of the gap measurement tool  10  further comprises a first exterior annular groove  88  (see  FIGS.  1 A- 1 B,  1 D- 1 E ) formed on another portion of the exterior  74   a  of the body  58 , and positioned distal to the exterior annular crevice  86 . The first exterior annular groove  88  has an outer diameter  90  (see  FIG.  1 D ) extending through the interior  74   b  of the body  58 , parallel to axis  92  (see  FIG.  1 D ), and intersecting the hollow inner channel  76  through the interior  74   b  of the body  58 . 
     The first exterior annular groove  88  further has a cross-hole  94  (see  FIGS.  1 A- 1 B,  1 E ) intersected by the hollow inner channel  76 . The cross-hole  94  has a cross-hole diameter  95  (see  FIG.  1 B ). The cross-hole  94  comprises cross-hole openings  96  (see  FIGS.  1 A- 1 B,  1 D- 1 E ) positioned opposite each other and in alignment with each other. The cross-hole openings  96  comprise a first cross-hole opening  96   a  (see  FIGS.  1 A- 1 B,  1 D- 1 E ) positioned opposite a second cross-hole opening  96   b  (see  FIGS.  1 D- 1 E ) to form the cross-hole  94 . The cross-hole openings  96  are preferably cross-drilled, and are configured to align with, and do align with, the gap  16  between the mating parts  20  when the gap measurement tool assembly  12  is inserted in the through hole  18  through the mating parts  20 . 
     The first exterior annular groove  88  further has an annular slot  98  (see  FIGS.  1 B,  1 D- 1 E ) extending beyond the cross-hole  94  comprising the cross-hole openings  96 . The annular slot  98  has a width  100  (see  FIG.  1 B ) that is greater than the cross-hole diameter  95  (see  FIG.  1 B ) of the cross-hole  94 , and that is greater than the gap width  30  (see  FIGS.  4 ,  5 A- 5 B ) of the gap  16  to be measured between the mating parts  20 , to ensure there is a sufficient overlap of the annular slot  98  and to the gap  16  to take the measurement  38 . 
     As shown in  FIGS.  1 A- 1 B,  1 D- 1 E , the body  58  of the gap measurement tool  10  further comprises a second exterior annular groove  102  formed on another portion of the exterior  74   a  of the body  58 , and positioned distal to the first exterior annular groove  88  and near the second end  56 , such as the distal end  56   a , of the gap measurement tool  10 . The second exterior annular groove  102  has an annular surface  104  (see  FIG.  1 A ). 
     Now referring to  FIGS.  1 F- 1 G ,  FIGS.  1 F- 1 G  show another exemplary version of a gap measurement tool  10   a  of the disclosure, such as a machined square mandrel  51 , where the head portion  60  (see  FIG.  1 G ) and the shaft portion  62  (see  FIG.  1 F ) of the body  58  (see  FIG.  1 F ) comprise an outer cross-section shape  72  (see  FIGS.  1 F,  1 G ) in the form of a square  72   b  (see  FIGS.  1 F,  1 G ).  FIG.  1 F  is an illustration of a perspective side view of the gap measurement tool  10   a  of the disclosure.  FIG.  1 G  is an illustration of a top view of the gap measurement tool  10   a  of  FIG.  1 F . 
       FIG.  1 F  shows the gap measurement tool  10   a , such as in the form of the machined square mandrel  51 , with the first end  54 , such as the proximal end  54   a , the second end  56 , such as the distal end  56   a , the body  58  with the exterior  74   a  and the interior  74   b  (see  FIG.  1 G ) and the head portion  60  and the shaft portion  62 , the exterior annular crevice  86  with the annular surface  87 , the first exterior annular groove  88  with the cross-hole  94  and annular slot  98 , and the second exterior annular groove  102  with the annular surface  104 . 
       FIG.  1 G  shows the gap measurement tool  10   a  with the hollow inner channel  76 .  FIG.  1 G  shows the gap measurement tool  10   a  with the outer cross-section shape  72  in the form of a square  72   b , and with the first end  54 , such as the proximal end  54   a , and the body  58  with the exterior  74   a , the interior  74   b , the head portion  60 , and the hollow inner channel  76 . 
     Now referring to  FIGS.  2 A- 2 C ,  FIGS.  2 A- 2 C  show an illustration of an exemplary version of a gap measurement tool assembly  12  of the disclosure.  FIG.  2 A  is a perspective side view of the exemplary version of the gap measurement tool assembly  12  of the disclosure. FIG.  2 B is an illustration of a cross-section of the gap measurement tool assembly  12  of  FIG.  2 A .  FIG.  2 C  is an illustration of a partial sectional perspective side view of the gap measurement tool assembly  12  of  FIG.  2 A . 
     As shown in  FIGS.  2 A- 2 C , the gap measurement tool assembly  12  comprises the gap measurement tool  10 , as discussed above with respect to  FIGS.  1 A- 1 E . As shown in  FIG.  2 B , the gap measurement tool  10  comprises the first end  54 , such as the proximal end  54   a , configured to couple to the air gage system  32  (see  FIGS.  4 ,  5 A- 5 B ) and the air supply source  34  (see  FIGS.  4 ,  5 A- 5 B ), comprises the second end  56 , such as the distal end  56   a , configured to be inserted into the through hole  18  and through the mating parts  20 , and comprises the body  58  formed between the first end  54  and the second end  56 . 
     As further shown in  FIG.  2 B , the body  58  comprises the exterior  74   a  and the interior  74   b , the head portion  60  having the first end  64   a  and the second end  64   b , the shaft portion  62  having the first end  66   a  and the second end  66   b , and the hollow inner channel  76  extending through the interior  74   b  from the first end  54  of the gap measurement tool  10  to the second end  56  of the gap measurement tool  10 . As further shown in  FIG.  2 B , the hollow inner channel  76  of the body  58  has the top opening  84   a  and the bottom opening  84   b.    
       FIGS.  2 A- 2 C  further show the exterior annular crevice  86 . As shown in  FIG.  2 B , the exterior annular crevice  86  is positioned between the second end  64   b  of the head portion  60  and the first end  66   a  of the shaft portion  62 . 
     As shown in  FIG.  2 A , the gap measurement tool assembly  12  further comprises a plurality of seal elements  105 . As shown in  FIGS.  2 A- 2 C , the plurality of seal elements  105  of the gap measurement tool assembly  12  comprise a first seal element  106  (see  FIGS.  2 A- 2 C ), a second seal element  116  (see  FIGS.  2 A- 2 C ), and a third seal element  120  (see  FIGS.  2 A- 2 C ). 
     The first seal element  106  is fitted around, and seated in, the exterior annular crevice  86  (see  FIGS.  2 A- 2 C ). The first seal element  106  is seated against the annular surface  87  (see  FIG.  1 A ) of the exterior annular crevice  86  (see also  FIGS.  2 A- 2 C ). The first seal element  106  is a mechanical seal comprising one of, a flat elastomeric seal  106   a , a flat gasket seal  106   b , or another suitable flat seal element. The first seal element  106  is made of a flexible, durable, and resilient material, such as elastomer or rubber, a polymer material, synthetic rubber copolymer of acrylonitrile (ACN) and butadiene, nitrile butadiene rubber (NBR), silicone, silicone rubbers, neoprene, fluoroelastomers, or another suitable material. 
     When the gap measurement tool assembly  12  is inserted into the through hole  18  through the mating parts  20  (see  FIGS.  5 A- 5 B ), or through mating master blocks  21  (see  FIG.  3 A ), the first seal element  106 , such as the flat elastomeric seal  106   a , is configured to seal, and seals a circumference  108   a  (see  FIGS.  3 C,  5 A- 5 B ) of the through hole  18  at an entry side  110  (see  FIGS.  3 C,  5 A- 5 B ), or entry point, of the through hole  18 . The first seal element  106  is squeezed to prevent air  36 , such as compressed air  36   a , from leaking out of an interface  112  (see  FIGS.  3 C,  5 A- 5 B ) between the second end  64   b  of the head portion  60  and a top end  114   a  (see  FIG.  3 C ) of a first mating master block  21   a  (see  FIG.  3 C ) or a top end  128   a  (see  FIGS.  5 A- 5 B ) of first mating part  20   a  (see  FIGS.  5 A- 5 B ). The first seal element  106 , such as the flat elastomeric seal  106   a , is configured to seal the interface  112  and around the exterior annular crevice  86  to prevent air  36 , such as compressed air  36   a , from escaping, or leaking. 
       FIGS.  2 A- 2 C  further show the first exterior annular groove  88  positioned distal to the exterior annular crevice  86  and having the cross-hole  94  intersected by the hollow inner channel  76  (see  FIGS.  2 B- 2 C ).  FIGS.  2 B- 2 C  show the cross-hole  94  comprising cross-hole openings  96 , for example, the first cross-hole opening  96   a  positioned opposite the second cross-hole opening  96   b .  FIGS.  2 A- 2 C  further show the annular slot  98  of the first exterior annular groove  88 . 
       FIGS.  2 A- 2 C  further show the second exterior annular groove  102  positioned distal to the first exterior annular groove  88  and near, or at, the second end  56  (see  FIG.  2 B ), such as the distal end  56   a  (see  FIG.  2 B ), of the gap measurement tool  10 . As shown in  FIGS.  2 A- 2 C , the gap measurement tool assembly  12  further comprises the second seal element  116  fitted around, and seated in, the second exterior annular groove  102 . The second seal element  116  is seated against the annular surface  104  (see  FIG.  1 B ) of the second exterior annular groove  102 . The second seal element  116  is a mechanical seal comprising one of, an O-ring seal  116   a  (see  FIGS.  2 A- 2 C,  3 C,  4 ,  5 A- 5 B ), a gasket seal  116   b  (see  FIG.  4   ), or another suitable seal element. As used herein, “O-ring seal” means a mechanical seal in the shape of a ring or torus having a circular cross-section, and designed to be seated in a groove and compressed during insertion into a through hole, to create a seal. 
     When the gap measurement tool assembly  12  is inserted into the through hole  18  through the mating parts  20 , the second seal element  116  seals a circumference  108   b  (see  FIGS.  3 C,  5 A- 5 B ) of the through hole  18  at, or near, an exit side  118  (see  FIGS.  3 C,  5 A- 5 B ), or exit point, of the through hole  18 . The second seal element  116  is made of a flexible, durable, and resilient material, such as elastomer or rubber, a polymer material, synthetic rubber copolymer of acrylonitrile (ACN) and butadiene, nitrile butadiene rubber (NBR), silicone, silicone rubbers, neoprene, fluoroelastomers, or another suitable material. 
     As shown in  FIGS.  2 A- 2 C , the gap measurement tool assembly  12  further comprises the third seal element  120 . The third seal element  120  is completely, or partially, inserted into a portion  76   a  (see  FIG.  2 C ) of the hollow inner channel  76  (see  FIG.  2 C ) at the second end  56  (see  FIG.  2 C ) of the gap measurement tool  10  (see  FIG.  2 C ), to seal the second end  56  of the gap measurement tool  10 . As shown in  FIGS.  2 A- 2 C , the third seal element  120  is a mechanical seal comprising one of, a plug seal  120   a , a set screw seal  120   b , for example, a flat point set screw seal or a cup point set screw seal, a threaded screw seal or threaded screw plug, a pressed-in elastomeric seal, or another suitable seal element. When the gap measurement tool assembly  12  is inserted into the through hole  18  through the mating parts  20 , the third seal element  120  seals the second end  56  of the gap measurement tool  10 , to prevent air  36 , such as compressed air  36   a , from leaking out the second end  56  of the gap measurement tool  10 , when the air  36 , such as compressed air  36   a , is passed through the hollow inner channel  76  and into the gap  16 . In one version, the third seal element  120  is made of a flexible, durable, and resilient material, such as an elastomer or rubber material, a polymer material, synthetic rubber copolymer of acrylonitrile (ACN) and butadiene, nitrile butadiene rubber (NBR), silicone, silicone rubbers, neoprene, fluoroelastomers, or another suitable material. In another version, the third seal element  120  comprises a metal material, such as one or more of, stainless steel, steel, aluminum, or another suitable metal material. 
     Now referring to  FIGS.  3 A- 3 E ,  FIGS.  3 A- 3 E  show an exemplary version of the gap measurement tool assembly  12  of the disclosure, inserted in a through hole  18  (see  FIGS.  3 A,  3 C- 3 E ), such as a master block through hole  18   b  (see  FIGS.  3 A,  3 C- 3 E ), of mating master blocks  21 , such as precisely machined master blocks, having a gap  16 , such as a precisely machined gap, with a known gap size  28   a  (see  FIGS.  3 C- 3 D ), such as a known gap width  30   a  (see  FIGS.  3 C- 3 D ), to calibrate the gap measurement system  14  (see  FIGS.  3 D- 3 E ). The mating master blocks  21  may be made of stainless steel, steel, chrome, aluminum, tungsten carbide, composite material, or another suitable material, and are manufactured to specific tolerances. 
       FIG.  3 A  is an illustration of a perspective side view of an exemplary version of the gap measurement tool assembly  12  of the disclosure, inserted in the through hole  18 , such as the master block through hole  18   b , through the mating master blocks  21 , comprising a first mating master block  21   a  and a second mating master block  21   b , with the gap  16  having the known gap size  28   a , such as a known gap width  30   a  (see  FIGS.  3 C- 3 D ) between the mating master blocks  21 .  FIG.  3 B  is an illustration of a top view of the gap measurement tool assembly  12  and the mating master block  21  comprising the first mating master block  21   a , of  FIG.  3 A . 
       FIG.  3 C  is an illustration of a cross-section of the gap measurement tool assembly  12  and the mating master block  21  of  FIG.  3 B , taken along lines  3 C- 3 C. As shown in  FIG.  3 C , the gap measurement tool assembly  12  is inserted into the through hole  18 , such as the master block through hole  18   b , through the mating master blocks  21 , comprising the first mating master block  21   a  and a second mating master block  21   b , with the gap  16  having the known gap size  28   a , such as the known gap width  30   a , between the mating master blocks  21 . As shown in  FIG.  3 C , each of the mating master blocks  21 , comprising the first mating master block  21   a  and the second mating master block  21   b , has a top end  114   a  and a bottom end  114   b .  FIG.  3 C  shows the gap measurement tool  10  of the gap measurement tool assembly  12 , and shows the head portion  60  with the first end  64   a  and the second end  64   b , and shows the shaft portion  62  with the first end  66   a  and the second end  66   b.    
     As shown in  FIG.  3 C , with the gap measurement tool assembly  12  inserted into the through hole  18  through the mating master blocks  21 , the first seal element  106 , such as the flat elastomeric seal  106   a , seals the circumference  108   a  of the through hole  18  at the entry side  110 , or entry point, of the through hole  18 . The first seal element  106 , such as the flat elastomeric seal  106   a , seals the interface  112  (see  FIG.  3 C ) between the second end  64   b  of the head portion  60  and the top end  114   a  (see  FIG.  3 C ) of the first mating part  20   a  (see  FIG.  3 C ), and seals around the exterior annular crevice  86  (see  FIG.  3 C ), to prevent air  36  (see  FIG.  4   ), such as compressed air  36   a  (see  FIG.  4   ), from escaping, or leaking, between the head portion  60  and the top end  114   a  of the first mating master block  21   a , where the head portion  60  rests against the top end  114   a.    
       FIG.  3 C  further shows the first exterior annular groove  88  having the cross-hole  94  aligned with the gap  16  and intersected by the hollow inner channel  76 .  FIG.  3 C  further shows the second exterior annular groove  102  with the second seal element  116 , such as the O-ring seal  116   a , fitted around, and seated in, the second exterior annular groove  102 . With the gap measurement tool assembly  12  inserted into the through hole  18  through the mating master blocks  21 , the second seal element  116  seals the circumference  108   b  (see  FIG.  3 C ) of the through hole  18 , at, or near, the exit side  118  (see  FIG.  3 C ), or exit point, of the through hole  18 .  FIG.  3 C  further shows the third seal element  120 , such as the plug seal  120   a , inserted into the hollow inner channel  76  at, or near, the second end  56  of the gap measurement tool  10 , to seal the second end  56  of the gap measurement tool  10 . 
       FIG.  3 D  is an illustration of a partial sectional perspective side view of the gap measurement tool assembly  12  of  FIG.  3 A , showing an air flow path  122   a  from the air gage system  32  and the air supply source  34 , through the hollow inner channel  76  of the gap measurement tool  10  and into the gap  16  having the known gap size  28   a , such as the known gap width  30   a , between the mating master blocks  21 , comprising the first mating master block  21   a  and the second mating master block  21   b .  FIG.  3 D  shows the gap  16 , such as a maximum gap  16   b , having a large gap area, for the air  36 , such as the compressed air  36   a , to escape. With the maximum gap  16   b , the back pressure  40  (see  FIG.  4   ) measured by the air gage system  32  is low, and the air flow  42  measured by the air gage system  32  is high.  FIG.  3 D  further shows the cross-hole  94  through the first exterior annular groove  88  aligned with the gap  16 .  FIG.  3 D  further shows the gap measurement tool  10  sealed with the first seal element  106  at the exterior annular crevice  86 , with the second seal element  116  at the second exterior annular groove  102 , and with the third seal element  120  at the second end  56  of the gap measurement tool  10 . 
       FIG.  3 E  is an illustration of another partial sectional perspective side view of the gap measurement tool assembly  12  of  FIG.  3 A , showing an air flow path  122   b  from the air gage system  32  and the air supply source  34 , through the hollow inner channel  76  of the gap measurement tool  10  and at a master block interface  124  with a minimum gap  16   c , such as no gap or zero gap, or a very small gap area, between the mating master blocks  21 , comprising the first mating master block  21   a  and the second mating master block  21   b . With the minimum gap  16   c , the back pressure  40  (see  FIG.  4   ) measured by the air gage system  32  is high, and the air flow  42  measured by the air gage system  32  is low. 
       FIG.  3 E  further shows the gap measurement tool  10  sealed with the first seal element  106  at the exterior annular crevice  86 , with the second seal element  116  at the second exterior annular groove  102 , and with the third seal element  120  at the second end  56  of the gap measurement tool  10 .  FIG.  3 E  further shows the cross-hole  94  aligned with the minimum gap  16   c  and intersected by the hollow inner channel  76 . 
     In one version, as shown in  FIGS.  3 A- 3 E , the gap measurement system  14  is calibrated with a first set  21   c  (see  FIG.  3 D ) of mating master blocks  21  (see  FIG.  3 D ) with the known gap size  28   a  (see  FIGS.  3 C- 3 D ), such as the known gap width  30   a  (see  FIGS.  3 C- 3 D ), between the mating master blocks  21 . As shown in  FIG.  3 D , the known gap size  28   a  of the first set  21   c  is for the maximum gap  16   b  that actual mating parts  20  (see  FIG.  4   ) of a structure  24  (see  FIG.  4   ) would have, for example, 0.005 inch. As shown in  FIG.  3 E , the gap measurement system  14  is further calibrated with a second set  21   d  of mating master blocks  21  with the minimum gap  16   c , which is zero or no gap, or a very small gap area, between the mating master blocks  21 . 
     By calibrating the gap measurement tool assembly  12  with the first set  21   c  of mating master blocks  21  of the maximum gap  16   b  of known gap size  28   a , and the second set  21   d  of mating master blocks  21  with the minimum gap  16   c  of no gap, or a very small gap area, the range  47  (see  FIG.  4   ) of predetermined gap measurements  46  (see  FIG.  4   ) may be set with the air gage system  32 , so that when an unknown measurement is measured, an operator or user can use the maximum gap  16   b  and the minimum gap  16   c  measurements to extrapolate measurements in between the maximum gap  16   b  and the minimum gap  16   c  measurements. In another version, the gap measurement tool assembly  12  may be calibrated with more than two sets of mating master blocks  21 , one set with the maximum gap  16   b , one set with the minimum gap  16   c , and additional sets with known gap sizes  28   a  in between the maximum gap  16   b  and the minimum gap  16   c.    
     Once the gap measurement system  14  is properly mastered or calibrated, an operator or user has the ability to quickly measure gaps  16  between mating parts  20  in a structure  24 , such as an aircraft structure  26 . The predetermined gap measurement  46  (see  FIG.  4   ) is selected from the range  47  (see  FIG.  4   ) of predetermined gap measurements  46  obtained using the mating master blocks  21  with the known gap sizes  28   a , such as the maximum gap  16   b  and the minimum gap  16   c  of known gap sizes  28   a.    
     Now referring to  FIG.  4   ,  FIG.  4    is an illustration of a block diagram of an exemplary gap measurement system (GMS)  14  having the gap measurement tool assembly  12 , as discussed in detail above, for measuring the gap  16  at the through hole  18  between mating parts  20 , in accordance with an illustrative version of the disclosure. The blocks in  FIG.  4    represent elements, and lines connecting the various blocks do not imply any particular dependency of the elements. Furthermore, the connecting lines shown in the various Figures contained herein are intended to represent example functional relationships and/or physical couplings between the various elements, but it is noted that other alternative or additional functional relationships or physical connections may be present in versions disclosed herein. One or more of these blocks may be combined, divided, or combined and divided into different blocks when implemented in an illustrative example. Further, the illustration of the gap measurement system  14  and gap measurement tool assembly  12  in  FIG.  4    is not meant to imply physical or architectural limitations to the manner in which an illustrative example may be implemented. Other components in addition to or in place of the ones illustrated may be used. Some components may be unnecessary. 
     As shown in  FIG.  4   , the gap measurement system  14  may be in the form of a portable gap measurement system (GMS)  14   a  (see also  FIG.  5 A ), an automated gap measurement system (GMS)  14   b  (see  FIG.  5 B ), or another suitable form. The portable gap measurement system  14   a  is configured to be independently and easily carried, or transported, and is independently and easily carried, or transported, by an operator or user, to the mating parts  20  having the gap  16  to be measured, or to the mating master blocks  21  to calibrate the portable gap measurement system  14   a.    
     The automated gap measurement system  14   b  comprises the gap measurement system  14  mounted to a robot  125  (see  FIGS.  4 ,  5 B ). The robot  125  is configured to couple to, or attach to, and couples to, or attaches to, the gap measurement tool assembly  12  (see  FIGS.  4 ,  5 B ) and the air gage system  32  (see  FIGS.  4 ,  5 B ) and air supply source  34  (see  FIGS.  4 ,  5 B ). The automated gap measurement system  14   b  automatically measures the gap  16  at the through hole  18  between the mating parts  20 . 
     As shown in  FIG.  4   , the gap measurement system  14  comprises the air gage system  32  and the air supply source  34  coupled to the air gage system  32 . The air gage system  32  is configured for coupling to the first end  54  (see  FIGS.  4 ,  5 A- 5 B ) of the gap measurement tool  10 . The air supply source  34  has air  36  (see  FIGS.  4 ,  5 A- 5 B ), such as compressed air  36   a  (see  FIGS.  4 ,  5 A- 5 B ), that is passed into the hollow inner channel  76  (see  FIGS.  4 ,  5 - 5 B ) of the gap measurement tool  10 , and into the gap  16  (see  FIGS.  4 ,  5 A- 5 B ), such as the part interface gap  16   a  (see  FIGS.  4 ,  5 A- 5 B ). 
     The air gage system  32  is used to take a measurement (MEAS.)  38  (see  FIG.  4   ) of one of, a back pressure  40  (see  FIG.  4   ), an air flow  42  (see  FIG.  4   ), or a differential pressure  44  (see  FIG.  4   ), and is used to correlate the measurement  38  to a predetermined gap measurement (PREDET. GAP MEAS.)  46  (see  FIG.  4   ), to determine the gap size  28  (see  FIG.  4   ) of the gap  16 , or gap measurement (MEAS.)  48  (see  FIG.  4   ), at the through hole  18  between the mating parts  20  of the structure  24 . The gap measurement system  14  uses only a single measurement (MEAS.)  38   a  (see  FIG.  4   ) in a one-step process  49  (see  FIG.  4   ), to measure and determine a gap measurement  48  (see  FIG.  4   ), such as the gap size  28  (see  FIG.  4   ), for example, the gap width  30  (see  FIG.  4   ). 
     As shown in  FIG.  4   , the gap measurement tool assembly  12  comprises the gap measurement tool  10 , and a plurality of seal elements  105 . As shown in  FIG.  4   , the gap measurement tool  10  comprises a metal material  52  such as stainless steel  52   a , steel  52   b , aluminum  52   c , or another suitable metal material. The gap measurement tool  10  may also be made of another suitably sturdy and durable material. 
     The gap measurement tool  10  comprises the first end  54  (see  FIG.  4   ), configured to couple to, or be coupled to, the air gage system  32 , and in turn, to the air supply source  34 . The gap measurement tool  10  comprises the second end  56  (see  FIG.  4   ) configured to be inserted into the through hole  18  and through the mating parts  20  or mating master blocks  21 . The gap measurement tool  10  further comprises the body  58  (see  FIG.  4   ) formed between the first end  54  and the second end  56 , and in one version, comprising the head portion  60  (see  FIG.  4   ) and the shaft portion  62  (see  FIG.  4   ) coupled to, and extending from, the head portion  60 . As shown in  FIG.  4   , the head portion  60  and the shaft portion  62  of the gap measurement tool  10  comprise an outer cross-section shape  72  in the form of one of, a circle  72   a , a square  72   b , a hexagon  72   c , a triangle  72   d , or another suitable outer cross-section shape. 
     As further shown in  FIG.  4   , and as discussed above in detail, the gap measurement tool  10  comprises the hollow inner channel  76 , comprises the exterior (EXT.) annular crevice  86 , comprises the first exterior annular groove (AG)  88  positioned distal to the exterior annular crevice  86 , and having the cross-hole  94  intersected by the hollow inner channel  76  and having the annular slot  98 , and comprises the second exterior (EXT.) annular groove (AG)  102  positioned distal to the first exterior annular groove  88 . 
     As further shown in  FIG.  4   , and as discussed above in detail, the gap measurement tool assembly  12  comprises the plurality of seal elements  105  comprising, in one version, the first seal element (ELEM.)  106  fitted around the exterior annular crevice  86 , the second seal element  116  fitted around the second exterior annular groove  102 , and the third seal element  120  inserted into a portion  76   a  (see  FIG.  2 C ) of the hollow inner channel  76  (see  FIG.  2 C ) at the second end  56  of the gap measurement tool  10 . As shown in  FIG.  4   , the first seal element  106  may comprise a flat elastomeric seal  106   a  or a flat gasket seal  106   b . The first seal element  106  may also comprise another suitable seal element. As shown in  FIG.  4   , the second seal element  116  may comprise an O-ring seal  116   a  or a gasket seal  116   b . The second seal element  116  may also comprise another suitable seal element. As shown in  FIG.  4   , the third seal element  120  may comprise a plug seal  120   a  or a set screw seal  120   b . The third seal element  120  may comprise another suitable seal element. 
     As further shown in  FIG.  4   , the gap measurement system  14  is coupled, or attached, to the structure  24 , such as the aircraft (AC) structure  26 , for example, a fuselage section  26   a , a tail section  26   b , or another suitable aircraft structure. The structure  24  comprises the mating parts (MP)  20  (see  FIG.  4   ), such as aircraft (AC) mating parts  22  (see  FIG.  4   ). As shown in  FIG.  4   , the mating parts  20  comprise the first mating part (MP)  20   a  and the second mating part (MP)  20   b . The mating parts  20  have the through hole (TH)  18 , such as the fastener through hole (TH)  18   a , formed or drilled through the mating parts  20 . As shown in  FIG.  4   , the through hole  18  has the entry side  110 , or entry point, and the exit side  118 , or exit point. The through hole  18  comprises a first through opening  126   a  (see  FIGS.  4 ,  5 A- 5 B ) formed or drilled through the first mating part  20   a , and further comprises a second through opening  126   b  (see  FIGS.  4 ,  5 A- 5 B ) formed, or drilled, through the second mating part  20   b . The first through opening  126   a  is aligned with the second through opening  126   b  to form the through hole  18 . 
     As further shown in  FIG.  4   , there is a gap  16 , such as a part interface gap  16   a , between the mating parts  20 , such as the first mating part  20   a  and the second mating part  20   b . As shown in  FIG.  4   , the gap  16 , such as the part interface gap  16   a , comprises a gap size  28 , such as a gap width  30 , or another suitable gap size or gap dimension. When mating master blocks  21  (see  FIGS.  3 A- 3 E ) are used to calibrate the gap measurement system  14 , the gap  16 , such as the part interface gap  16   a , has a known gap size  28   a  (see  FIGS.  3 C- 3 D ), such as a known gap width  30   a  (see  FIGS.  3 C- 3 D ), or another suitable known gap size or known gap dimension. 
     When the gap measurement tool assembly  12  of the gap measurement system  14  is inserted into the through hole  18 , such as the fastener through hole  18   a , and through the mating parts  20 , such as the first mating part  20   a  and the second mating part  20   b , so that the cross-hole  94  in the first exterior annular groove  88  is aligned with the gap  16 , such as the part interface gap  16   a , and the entry side  110  and the exit side  118  of the through hole  18  are properly sealed, the air  36 , such as the compressed air  36   a , is passed into the gap  16 , via the hollow inner channel  76 , and the air gage system  32  takes a measurement  38  of one of, the back pressure  40 , the air flow  42 , or the differential pressure  44 , and correlates the measurement  38  to a predetermined gap measurement  46 , to determine a gap size  28  of the gap  16  at the through hole  18  between the mating parts  20  of the structure  24 . 
     Now referring to  FIG.  5 A ,  FIG.  5 A  is an illustration of an exemplary version of a gap measurement system  14 , such as in the form of a portable gap measurement system  14   a , of the disclosure, for measuring the gap  16 , such as the part interface gap  16   a , at the through hole  18 , such as the fastener through hole  18   a , between mating parts  20 , such as aircraft mating parts  22 , of a structure  24 , such as an aircraft structure  26 , for example, a fuselage section  26   a . As shown in  FIG.  5 A , the structure  24 , such as the aircraft structure  26 , comprises a fully assembled structure  24   a , having multiple through holes  18 , such as fastener through holes  18   a , configured to each receive a fastener  19 , such as a permanent fastener  19   a , for example, a rivet, to fasten, or join, the mating parts  20  together. In this version, where one or more gap measurements  48  (see  FIG.  4   ) are desired to be taken on the fully assembled structure  24   a , certain of the permanent fasteners  19   a  are removed from the fastener through holes  18   a  in order to take the one or more gap measurements  48  with the gap measurement system  14 , and certain of the permanent fasteners  19  remain in the through holes  18 . As shown in  FIG.  5 A , each through hole  18  comprises the first through opening  126   a  formed or drilled through the first mating part  20   a , and comprises the second through opening  126   b  formed, or drilled, through the second mating part  20   b , where the first through opening  126   a  is aligned with the second through opening  126   b  to form the through hole  18 . As shown in  FIG.  5 A , in one version, the mating parts  20 , such as the first mating part  20   a  and the second mating part  20   b , are held together with one or more clamping devices  127 , near the through hole  18  in which the gap measurement tool  18  is inserted to take the gap measurement  48 . In another version, the mating parts  20 , such as the first mating part  20   a  and the second mating part  20   b , are held together with temporary fasteners  19   b  (see  FIG.  5 B ) inserted through one or both through holes  18  adjacent the through hole  18  in which the gap measurement tool  10  is inserted to take the gap measurement  48 . Upon completion of taking the one or more gap measurements  48  for the fully assembled structure  24   a , the one or more clamping devices  127  are removed, or the one or more temporary fasteners  19   b  are removed and replaced with the permanent fasteners  19   a  installed in the through holes  18 . This clamping and/or temporary fastening process allows the fully assembled structure  24   a  to maintain its structural integrity in the proper assembled state, while the one or more gap measurements  48  are taken. 
     As shown in  FIG.  5 A , the gap measurement system  14 , such as the portable gap measurement system  14   a , comprises the gap measurement tool assembly  12  coupled, or attached, to the air gage system  32 , which, in turn, is coupled, or attached, to the air supply source  34 . The gap measurement tool assembly  12  is inserted into the through hole  18  and aligned with the gap  16  between the mating parts  20 . In particular, as shown in  FIG.  5 A , the shaft portion  62  of the body  58  of the gap measurement tool  10  of the gap measurement tool assembly  12  is inserted in a through hole  18  through the mating parts  20 , in the form of aircraft mating parts  22 . As further shown in  FIG.  5 A , the gap measurement tool  10  has the first end  54  and the second end  56 . 
     As further shown in  FIG.  5 A , the mating parts  20 , in the form of aircraft mating parts  22 , comprise the first mating part  20   a  and the second mating part  20   b , in the form of a first aircraft mating part  22   a  and a second aircraft mating part  22   b , having the gap  16 , such as the part interface gap  16   a . Each of the mating parts  20 , comprising the first mating part  20   a  and the second mating part  20   b , has a top end  128   a  (see  FIG.  5 A ) and a bottom end  128   b  (see  FIG.  5 A ). The head portion  60  of the gap measurement tool  10  is seated against, and interfaces with, the top end  128   a  of the first mating part  20   a , such as the first aircraft mating part  22   a . The mating parts  20  may be made of composite material, metal material, a combination of composite and metal material, or another suitable material. 
     As shown in  FIG.  5 A , with the gap measurement tool assembly  12  inserted into the through hole  18 , such as the fastener through hole  18   a , through the mating parts  20 , the first seal element  106 , seals the circumference  108   a  of the through hole  18 , at the entry side  110 , or entry point, of the through hole  18 . As further shown in  FIG.  5 A , the first seal element  106  seals the interface  112  between the second end  64   b  of the head portion  60  and the top end  128   a  of the first mating part  20   a , and seals around the exterior annular crevice  86  of the gap measurement tool  10 , to prevent air  36 , such as compressed air  36   a , from escaping, or leaking, between the head portion  60  and the top end  128   a  of the first mating part  20   a , where the head portion  60  rests against the top end  128   a.    
       FIG.  5 A  further shows the first exterior annular groove  88  having the cross-hole  94  aligned with the gap  16  and intersected by the hollow inner channel  76 .  FIG.  5 A  further shows the second exterior annular groove  102  positioned distal to the first exterior annular groove  88 , and with the second seal element  116  fitted around, and seated in, the second exterior annular groove  102 . As shown in  FIG.  5 A , with the gap measurement tool assembly  12  inserted into the through hole  18 , such as the fastener through hole  18   a , through the mating parts  20 , the second seal element  116  seals the circumference  108   b  of the through hole  18 , at, or near, the exit side  118 , or exit point, of the through hole  18 .  FIG.  5 A  further shows the third seal element  120  inserted into the hollow inner channel  76  at, or near, the second end  56  of the gap measurement tool  10 , to seal the second end  56  of the gap measurement tool  10 . 
       FIG.  5 A  shows the air  36 , such as the compressed air  36   a , flowing from the air gage  130 , or air gage probe, of the air gage system  32  inserted in the hollow inner channel  76  in the head portion  60  of the gap measurement tool  10 . The air  36 , such as the compressed air  36   a , flows from the air supply source  34  with the air supply  34   a  (see  FIG.  5 A ), to the air gage  130 , or air gage probe, and through the hollow inner channel  76  of the gap measurement tool  10 , and into the gap  16  between the mating parts  20 , such as the aircraft mating parts  22 . 
     As shown in  FIG.  5 A , with the gap measurement tool assembly  12  inserted into the through hole  18  and through the mating parts  20 , so that the cross-hole  94  is aligned with the gap  16 , and the entry side  110  and the exit side  118  of the through hole  18  are sealed, the air  36 , such as the compressed air  36   a  is passed through the gap  16 , via the hollow inner channel  76 , and the air gage system  32  takes a measurement  38  (see  FIG.  4   ) of one of, the back pressure  40  (see  FIG.  4   ), then air flow  42  (see  FIG.  4   ), or the differential pressure  44  (see  FIG.  4   ), and correlates the measurement  38  to a predetermined gap measurement  46  (see  FIG.  4   ), to determine a gap size  28 , such as gap width  30 , of the gap  16  at the through hole  18  between the mating parts  20  of the structure  24 . 
     As shown in  FIG.  5 A , in one version, the air gage system  32  comprises a non-contact air gage system  32   a . As further shown in in  FIG.  5 A , the air gage system  32  comprises the air gage  130 , or air gage probe, having a nozzle tip  132  configured for connection with the interior connector portion  68  within the head portion  60  of the gap measurement tool  10 . The nozzle tip  132  may comprise an exterior threaded connection  134 , or another suitable connection, to securely attach the air gage  130  to the interior connector portion  68  and to the first end  54  of the gap measurement tool  10  of the gap measurement tool assembly  12 . As further shown in  FIG.  5 A , the air gage  130 , or air gage probe, has a handle  135  that is connected to an amplifier  136 , via a connection element  138 , such as in the form of a first air tube  138   a , or air hose. 
     The amplifier  136  is a device containing the necessary hardware to measure one or more of the back pressure  40 , the air flow  42 , or the differential pressure  44 , depending on the type of amplifier  136  used. As shown in  FIG.  5 A , in one version, the amplifier  136  has a housing  140  with a flat panel display  142  that displays measurements  38  on a scale  144  as dimensional values  145 , such as digitally displayed values, and the housing  140  of the amplifier  136  has connector ports  146 , controls  148 , red and green status indicators  149 , and a universal serial bus (USB) port  150  for insertion of a universal serial bus (USB) memory stick  152  that may be used to transfer measurement data to a computer (not shown). The amplifier  136  provides visual display of the measurements  38  taken, enabling an operator or user to quickly take measurements  38  of back pressure  40 , air flow  42 , or differential pressure  44 . 
     The air gage system  32 , such as the non-contact air gage system  32   a , may further comprise one or more of, a filter  153  (see  FIG.  6   ), a pressure regulator  154  (see  FIG.  6   ), a pressure gauge  155  (see  FIGS.  5 A,  6   ), an equalizing jet  156  (see  FIG.  6   ) such as a non-adjustable equalizing jet, a master jet  157  (see  FIG.  6   ) such as a non-adjustable master jet, a differential pressure gauge  158  (see  FIG.  6   ), and a zone setting valve  160  (see  FIG.  6   ). The air gage system  32 , such as the non-contact air gage system  32   a , may further comprise other suitable components and parts. 
     As further shown in  FIG.  5 A , the air supply source  34  is connected to the amplifier  136 , via a connection element  138 , such as in the form of a second air tube  138   b , or air hose. The air supply source  34  comprises a sealed container, tank, or other suitable holding element, that holds and stores an air supply  34   a  (see  FIG.  5 A ) within the air supply source  34 . As shown in  FIG.  5 A , the air supply source  34  is coupled to the pressure gauge  155 . 
     Now referring to  FIG.  5 B ,  FIG.  5 B  is an illustration of an exemplary version of a gap measurement system  14 , such as in the form of an automated gap measurement system  14   b , of the disclosure, coupled to a robot  125 , to automatically measure the gap  16 , such as the part interface gap  16   a , at the through hole  18 , such as the fastener through hole  18   a , between the mating parts  20 , such as the first mating part  20   a  and the second mating part  20   b . As shown in  FIG.  5 B , the structure  24  comprises a partially assembled structure  24   b , having multiple through holes  18 , such as fastener through holes  18   a , configured to each receive a fastener  19 , such as a temporary fastener  19   b , for example, a temporary rivet, bolt, screw, or other suitable fastener, to temporarily fasten, or join, the mating parts  20  together. In this version, where one or more gap measurements  48  (see  FIG.  4   ) are desired to be taken on the partially assembled structure  24   b , certain of the temporary fasteners  19   b  are removed from the fastener through holes  18   a  in order to take the one or more gap measurements  48  with the gap measurement system  14 , and certain of the temporary fasteners  19   b  remain in the through holes  18 . As shown in  FIG.  5 A , each through hole  18  comprises the first through opening  126   a  formed or drilled through the first mating part  20   a , and comprises the second through opening  126   b  formed, or drilled, through the second mating part  20   b , where the first through opening  126   a  is aligned with the second through opening  126   b  to form the through hole  18 . As shown in  FIG.  5 B , in one version, the mating parts  20 , such as the first mating part  20   a  and the second mating part  20   b , are held together with one or more clamping devices  127 , and one or more temporary fasteners  19   b , near the through hole  18  in which the gap measurement tool  18  is inserted to take the gap measurement  48 . In another version, the mating parts  20 , such as the first mating part  20   a  and the second mating part  20   b , may be held together only with temporary fasteners  19   b  inserted through one or both through holes  18  adjacent the through hole  18  in which the gap measurement tool  10  is inserted to take the gap measurement  48 . After one gap measurement  48  is taken, the gap  16  at the through hole  18  that was measured may be clamped and a different temporary fastener  19   b  may be removed in order to take an additional gap measurement  48  with the gap measurement system  14 . Upon completion of taking the one or more gap measurements  48  for the partially assembled structure  24   b , the one or more clamping devices  127  are removed, and/or the one or more temporary fasteners  19   b  are removed and replaced with permanent fasteners  19   a  (see  FIG.  5 A ) that are installed. This clamping and/or temporary fastening process allows the partially assembled structure  24   b  to maintain its structural integrity in the proper assembled state, while the one or more gap measurements  48  are taken. 
     As shown in  FIG.  5 B , the robot  125  comprises a robotic arm  162  having a first end  163   a  attached to an end effector  164  and a second end  163   b  attached to a gantry  165 . The end effector  164  is coupled, or attached, to the handle  135  of the air gage  130  of the air gage system  32 , such as the non-contact air gage system  32   a . As shown in  FIG.  5 B , the gantry  165  has a platform  166  for holding the amplifier  136  of the air gage system  32  and for holding the air supply source  34 . The robot  125  may be controlled with a control unit, such as in the form of a computer numerical control (CNC) machine, having one or more computers, for controlling the robot  125 . The robot  125  is powered with a power unit for providing power to the robot  125 . The control unit and the power unit may be connected to the robot  125  by one or more connections, such as wired or wireless connections. 
     As shown in  FIG.  5 B , the gap measurement system  14 , such as the automated gap measurement system  14   b , comprises the gap measurement tool assembly  12  coupled, or attached, to the air gage system  32 , which, in turn, is coupled, or attached, to the air supply source  34 . The gap measurement tool assembly  12  is inserted into the through hole  18  and aligned with the gap  16  between the mating parts  20 . In particular, as shown in  FIG.  5 B , the shaft portion  62  of the body  58  of the gap measurement tool  10  of the gap measurement tool assembly  12  is inserted in a through hole  18  through the mating parts  20 . As further shown in  FIG.  5 B , the gap measurement tool  10  has the first end  54  and the second end  56 . 
     As further shown in  FIG.  5 B , the mating parts  20  comprise the first mating part  20   a  and the second mating part  20   b , having the gap  16 , such as the part interface gap  16   a , between the mating parts  20 . Each of the mating parts  20 , comprising the first mating part  20   a  and the second mating part  20   b , has the top end  128   a  (see  FIG.  5 B ) and the bottom end  128   b  (see  FIG.  5 B ). The head portion  60  of the gap measurement tool  10  is seated against, and interfaces with, the top end  128   a  of the first mating part  20   a . The mating parts  20  may be made of composite material, metal material, a combination of composite and metal material, or another suitable material. 
     As shown in  FIG.  5 B , with the gap measurement tool assembly  12  inserted into the through hole  18 , such as the fastener through hole  18   a , through the mating parts  20 , the first seal element  106 , seals the circumference  108   a  of the through hole  18 , at the entry side  110 , or entry point, of the through hole  18 . As further shown in  FIG.  5 B , the first seal element  106  seals the interface  112  between the second end  64   b  of the head portion  60  and the top end  128   a  of the first mating part  20   a , and seals around the exterior annular crevice  86  of the gap measurement tool  10 , to prevent air  36 , such as compressed air  36   a , from escaping, or leaking, between the head portion  60  and the top end  128   a  of the first mating part  20   a , where the head portion  60  rests against the top end  128   a.    
       FIG.  5 B  further shows the first exterior annular groove  88  having the cross-hole  94  aligned with the gap  16  and intersected by the hollow inner channel  76 .  FIG.  5 B  further shows the second exterior annular groove  102  positioned distal to the first exterior annular groove  88 , and with the second seal element  116  fitted around, and seated in, the second exterior annular groove  102 . As shown in  FIG.  5 B , with the gap measurement tool assembly  12  inserted into the through hole  18 , such as the fastener through hole  18   a , through the mating parts  20 , the second seal element  116  seals the circumference  108   b  of the through hole  18 , at, or near, the exit side  118 , or exit point, of the through hole  18 .  FIG.  5 B  further shows the third seal element  120  inserted into the hollow inner channel  76  at, or near, the second end  56  of the gap measurement tool  10 , to seal the second end  56  of the gap measurement tool  10 . 
       FIG.  5 B  shows the air  36 , such as the compressed air  36   a , flowing from the air gage  130 , or air gage probe, of the air gage system  32  inserted in the hollow inner channel  76  in the head portion  60  of the gap measurement tool  10 . The air  36 , such as the compressed air  36   a , flows from the air supply source  34  with the air supply  34   a  (see  FIG.  5 B ), to the air gage  130 , or air gage probe, and through the hollow inner channel  76  of the gap measurement tool  10 , and into the gap  16  between the mating parts  20 , such as the aircraft mating parts  22 . 
     As shown in  FIG.  5 B , with the gap measurement tool assembly  12  inserted into the through hole  18  and through the mating parts  20 , so that the cross-hole  94  is aligned with the gap  16 , and the entry side  110  and the exit side  118  of the through hole  18  are sealed, the air  36 , such as the compressed air  36   a  is passed through the gap  16 , via the hollow inner channel  76 , and the air gage system  32  takes the measurement  38  (see  FIG.  4   ) of one of, the back pressure  40  (see  FIG.  4   ), the air flow  42  (see  FIG.  4   ), or the differential pressure  44  (see  FIG.  4   ), and correlates the measurement  38  to a predetermined gap measurement  46  (see  FIG.  4   ), to determine the gap size  28 , such as the gap width  30 , of the gap  16  at the through hole  18  between the mating parts  20  of the structure  24 . 
     As shown in  FIG.  5 B , in one version, the air gage system  32  comprises a non-contact air gage system  32   a . As further shown in in  FIG.  5 B , the air gage system  32  comprises the air gage  130 , or air gage probe, having the nozzle tip  132  with the exterior threaded connection  134  configured for connection with the interior connector portion  68  within the head portion  60  of the gap measurement tool  10  and to the first end  54  of the gap measurement tool  10  of the gap measurement tool assembly  12 . As further shown in  FIG.  5 B , the air gage  130 , or air gage probe, has the handle  135  that is connected to the amplifier  136 , via the connection element  138 , such as in the form of the first air tube  138   a , or air hose. As shown in  FIG.  5 B , in one version, the amplifier  136  has the housing  140  with the flat panel display  142  that displays measurements  38  on the scale  144  as dimensional values  145 , such as digitally displayed values, and the housing  140  of the amplifier  136  has connector ports  146 , controls  148 , red and green status indicators  149 , and the universal serial bus (USB) port  150  for insertion of a universal serial bus (USB) memory stick  152 . 
     The air gage system  32 , such as the non-contact air gage system  32   a , may further comprise one or more of, a filter  153  (see  FIG.  6   ), a pressure regulator  154  (see  FIG.  6   ), a pressure gauge  155  (see  FIGS.  5 B,  6   ), an equalizing jet  156  (see  FIG.  6   ) such as a non-adjustable equalizing jet, a master jet  157  (see  FIG.  6   ) such as a non-adjustable master jet, a differential pressure gauge  158  (see  FIG.  6   ), and a zone setting valve  160  (see  FIG.  6   ). The air gage system  32 , such as the non-contact air gage system  32   a , may further comprise other suitable components and parts. 
     As further shown in  FIG.  5 B , the air supply source  34  is connected to the amplifier  136 , via the connection element  138 , such as in the form of the second air tube  138   b , or air hose. The air supply source  34  comprises a sealed container, tank, or other suitable holding element, that holds and stores the air supply  34   a  (see  FIG.  5 B ) within the air supply source  34 . As shown in  FIG.  5 B , the air supply source  34  is coupled to the pressure gauge  155 . 
     Now referring to  FIG.  6   ,  FIG.  6    is an illustration of a schematic diagram of an exemplary version of an air gage system  32 , such as a non-contact air gage system  32   a , for example, a differential pressure gauge air gage system  32   b , and a gap measurement tool assembly  12  of the disclosure. As shown in  FIG.  6   , with the differential pressure gauge air gage system  32   b , the air flow  42  of air  36 , such as compressed air  36   a , flows from the air supply  34   a  in the air supply source  34 , through the filter  153 , and through the pressure regulator  154 . The pressure regulator  154  controls pressure (P)  167  (see  FIG.  6   ), such as pneumatic pressure, of the air flow  42  of the air  36 , such as the compressed air  36   a , and provides consistent pressure  167  of the air flow  42  of the air  36 , such as the compressed air  36   a , to the amplifier  136  (see  FIGS.  5 A- 5 B ). The pressure gauge  155  (see  FIG.  6   ) monitors and measures the pressure  167  of the air flow  42  of the air  36 , such as the compressed air  36   a.    
     As shown in  FIG.  6   , the air flow  42  splits into two channels  168 , including a reference channel  168   a , or base channel, that acts as a reference or base measurement, and including a measuring channel  168   b  that performs the measuring. As further shown in  FIG.  6   , air flow  42   a  flows through the equalizing jet  156 , such as the non-adjustable equalizing jet, to the reference channel  168   a . The air flow  42   a  in the reference channel  168   a  flows to the zone setting valve  160  and out of the differential pressure gauge air gage system  32   b , and this air flow  42   a  has a first pressure P 1   167   a  measured and monitored by the differential pressure gauge  158 . 
     As further shown in  FIG.  6   , air flow  42   b  flows through the master jet  157 , such as the non-adjustable master jet, to the measuring channel  168   b . The air flow  42   b  in the measuring channel  168   b  flows from the differential pressure gauge air gage system  32   b  to the gap measurement tool assembly  12  and out of gap  16 , and this air flow  42   b  has a second pressure P 2   167   b  measured and monitored by the differential pressure gauge  158 . 
     The differential pressure gauge  158  measures the difference between the second pressure P 2   167   b  of the air flow  42   b  flowing out of the gap measurement tool assembly  12  and out of gap  16  and the first pressure P 1   167   a  of the air flow  42   a  flowing out of the differential pressure gauge air gage system  32   b . Leakage of air  36 , such as compressed air  36   a , through the gap measurement tool assembly  12  and the gap  16  causes a corresponding change in the differential pressure  44  (see  FIG.  4   ) and is proportional to the gap size  28  (see  FIG.  4   ), such as gap width  30  (see  FIG.  4   ) of the gap  16 . 
       FIG.  6    further shows the gap measurement system  14  comprising the gap measurement tool assembly  12  receiving the air  36 , such as the compressed air  36   a , from the air gage system  32 , such as the non-contact air gage system  32   a , for example, the differential pressure gauge air gage system  32   b , and the air supply source  34 .  FIG.  6    further shows the gap measurement tool  10  with the first end  54 , the second end  56 , the hollow inner channel  76 , the first exterior annular groove  88  with the cross-hole  94  intersected by the hollow inner channel  76 , and the cross-hole  94  aligned with the gap  16 , such as the part interface gap  16   a .  FIG.  6    further shows the first seal element  106 , the second seal element  116 , and the third seal element  120  of the gap measurement tool assembly  12 .  FIG.  6    further shows the gap measurement tool assembly  12  partially inserted in the through hole  18 , such as the fastener through hole  18   a , through the mating parts  20 , such as the first mating part  20   a  and the second mating part  20   b.    
     Now referring to  FIG.  7   ,  FIG.  7    is an illustration of a graph  170  with a plot line  172  showing a correlation between gap size  28 , such as gap width  30  (see  FIGS.  4 ,  5 A- 5 B ), and back pressure  40 , air flow  42 , and differential pressure  44 , depending on which is measured with the gap measurement system  14  (see  FIGS.  4 ,  5 A- 5 B ). As shown in  FIG.  7   , the graph  170  shows gap size  28  along the x-axis, and shows back pressure (BP)  40 , air flow (AF)  42 , and differential pressure (DP)  44  along the y-axis. The plot line  172  shows a linear range  174  representing a linear relationship established by using two mating master blocks  21  (see  FIGS.  3 A- 3 C ) with gaps  16  that are precisely machined to establish the linear equation used. The plot line  172  further shows a first master point  176  obtained using one set of mating master blocks  21  having one known gap size  28   a , and further shows a second master point  178  obtained using another set of mating master blocks  21  having another known gap size  28 . Once the gap measurement system  14  (see  FIGS.  4   ,  5 A- 5 B) is properly mastered or calibrated, an operator or user can quickly measure gaps  16  (see  FIGS.  4 ,  5 A- 5 B ) between mating parts  20  (see  FIGS.  4 ,  5 A- 5 B ) in a structure  24  (see  FIGS.  4 ,  5 A- 5 B ), such as an aircraft structure  26  (see  FIGS.  4 ,  5 A ). 
     Now referring to  FIG.  8   ,  FIG.  8    is an illustration of a flow diagram of an exemplary version of a method  180  of the disclosure. In another version of the disclosure, there is provided the method  180  for measuring a gap  16  (see  FIGS.  4 ,  5 A- 5 B ) at a through hole  18  (see  FIGS.  4 ,  5 A- 5 B ), such as a fastener through hole  18   a  (see  FIGS.  4 ,  5 A- 5 B ), between mating parts  20  (see  FIGS.  4 ,  5 A- 5 B ) of a structure  24  (see  FIGS.  4 ,  5 A- 5 B ), such as an aircraft structure  26  (see  FIG.  4   ). The structure  24  may comprise a fully assembled structure  24   a  (see  FIG.  5 A ), or a partially assembled structure  24   b  (see  FIG.  5 B ). 
     The blocks in  FIG.  8    represent operations and/or portions thereof, or elements, and lines connecting the various blocks do not imply any particular order or dependency of the operations or portions thereof, or elements.  FIG.  8    and the disclosure of the steps of the method  180  set forth herein should not be interpreted as necessarily determining a sequence in which the steps are to be performed. Rather, although one illustrative order is indicated, it is to be understood that the sequence of the steps may be modified when appropriate. Accordingly, certain operations may be performed in a different order or simultaneously. 
     As shown in  FIG.  8   , the method  180  comprises the step of providing  182  a gap measurement tool assembly  12  (see  FIGS.  2 A,  4 ,  5 A- 5 B ). As discussed in detail above, the gap measurement tool assembly  12  comprises a gap measurement tool  10  (see  FIGS.  1 A- 1 G,  4   ) comprising a first end  54  (see  FIGS.  1 A,  4   ), such as a proximal end  54   a  (see  FIG.  1 A ), a second end  56  (see  FIGS.  1 A,  4   ), such as a distal end  56   a  (see  FIG.  1 A ), and a body  58  (see  FIGS.  1 A,  4   ) formed between the first end  54  and the second end  56 . The body  58  comprises a hollow inner channel  76  (see  FIGS.  1 D,  4   ), an exterior annular crevice  86  (see  FIGS.  1 A,  4   ), a first exterior annular groove  88  (see  FIGS.  1 A,  4   ) positioned distal to the exterior annular crevice  86  and having a cross-hole  94  (see  FIGS.  1 D,  4   ) intersected by the hollow inner channel  76 , and a second exterior annular groove  102  (see  FIGS.  1 A,  4   ) positioned distal to the first exterior annular groove  88 . 
     The gap measurement tool assembly  12  further comprises a plurality of seal elements  105  (see  FIG.  4   ) comprising the first seal element  106  (see  FIGS.  2 A- 2 C,  4   ) fitted around the exterior annular crevice  86 . The plurality of seal elements  105  of the gap measurement tool assembly  12  further comprises the second seal element  116  (see  FIGS.  2 A- 2 C,  4   ) fitted around the second exterior annular groove  102 . The plurality of seal elements  105  of the gap measurement tool assembly  12  further comprises the third seal element  120  (see  FIGS.  2 A- 2 C,  4   ) inserted into a portion  76   a  (see  FIG.  2 C ) of the hollow inner channel  76  (see  FIG.  2 C ) at the second end  56  (see  FIG.  2 C ) of the gap measurement tool  10  (see  FIG.  2 C ). 
     As shown in  FIG.  8   , the method  180  further comprises the step of coupling  184  the gap measurement tool assembly  12  to an air gage system  32  (see  FIGS.  4 ,  5 A- 5 B ) and an air supply source  34  (see  FIGS.  4 ,  5 A- 5 B ) coupled to the air gage system  32 . The air supply source  34  contains and supplies air  36  (see  FIGS.  4 ,  5 A- 5 B ), such as compressed air  36   a  (see  FIGS.  4 ,  5 A- 5 B ). The gap measurement tool assembly  12 , the air gage system  32 , and the air supply source  34  comprise, and obtain, the gap measurement system  14  (see  FIGS.  4 ,  5 A- 5 B ). 
     As shown in  FIG.  8   , the method  180  further comprises the step of inserting  186  the gap measurement tool assembly  12  into the through hole  18  (see  FIGS.  4 ,  5 A- 5 B ), such as the fastener through hole  18   a  (see  FIGS.  4 ,  5 A- 5 B ), and through the mating parts  20  (see  FIGS.  4 ,  5 A- 5 B ), such as the first mating part  20   a  (see  FIGS.  4 ,  5 A- 5 B ) and the second mating part  20   b  (see  FIGS.  4 ,  5 A- 5 B ), so that the cross-hole  94  (see  FIGS.  4 ,  5 A- 5 B ) of the gap measurement tool  10  is aligned with the gap  16  (see  FIGS.  4 ,  5 A- 5 B ), such as the part interface gap  16   a  (see  FIGS.  4 ,  5 A- 5 B ), at the through hole  18  between the mating parts  20 . The entry side  110  (see  FIGS.  4 ,  5 A- 5 B ) and the exit side  118  (see  FIGS.  4 ,  5 A- 5 B ) of the through hole  18  are sealed with the first seal element  106  and the second seal element  116 , respectively. 
     The step of inserting  186  the gap measurement tool assembly  12  into the through hole  18  and through the mating parts  20 , further comprises sealing, with the first seal element  106  (see  FIGS.  5 A- 5 B ), the circumference  108   a  (see  FIGS.  5 A- 5 B ) of the through hole  18  (see  FIGS.  5 A- 5 B ) at the entry side  110  (see  FIGS.  5 A- 5 B ), or entry point, of the through hole  18 . The step of inserting  186  the gap measurement tool assembly  12  into the through hole  18  and through the mating parts  20 , further comprises, sealing, with the second seal element  116  (see  FIGS.  5 A- 5 B ), the circumference  108   b  (see  FIGS.  5 A- 5 B ) of the through hole  18  at, or near, the exit side  118  (see  FIGS.  5 A- 5 B ) of the through hole  18 . The step of inserting  186  the gap measurement tool assembly  12  into the through hole  18  and through the mating parts  20 , further comprises, sealing, with the third seal element  120  (see  FIGS.  5 A- 5 B ), the second end  56  (see  FIGS.  5 A- 5 B ) of the gap measurement tool  10 . The step of inserting  186  the gap measurement tool assembly  12  into the through hole  18  and through the mating parts  20 , further comprises, inserting  186  the gap measurement tool assembly  12  into the through hole  18  comprising the fastener through hole  18   a  (see  FIG.  4   ), and through the mating parts  20  comprising aircraft mating parts  22  (see  FIG.  4   ) of the structure  24  comprising the aircraft structure  26  (see  FIG.  4   ), such as the fuselage section  26   a  (see  FIG.  4   ), the tail section  26   b  (see  FIG.  4   ), or another suitable aircraft structure. 
     As shown in  FIG.  8   , the method  180  further comprises the step of passing  188  the air  36  (see  FIGS.  4 ,  5 A- 5 B ), such as the compressed air  36   a  (see  FIGS.  4 ,  5 A- 5 B ), through the hollow inner channel  76  (see  FIGS.  4 ,  5 A- 5 B ) of the gap measurement tool  10  (see  FIGS.  4 ,  5 A- 5 B ) and into the gap  16  (see  FIGS.  4 ,  5 A- 5 B ), such as the part interface gap  16   a  (see  FIGS.  4 ,  5 A- 5 B ). 
     As shown in  FIG.  8   , the method  180  further comprises the step of using  190  the air gage system  32  to take a measurement  38  (see  FIG.  4   ) of one of, a back pressure  40  (see  FIG.  4   ), an air flow  42  (see  FIG.  4   ), or a differential pressure  44  (see  FIG.  4   ). 
     As shown in  FIG.  8   , the method  180  further comprises the step of correlating  192  the measurement  38  (see  FIG.  4   ) of one of, the back pressure  40 , the air flow  42 , or the differential pressure  44 , to a predetermined gap measurement  46  (see  FIG.  4   ), to determine a gap size  28  (see  FIG.  4   ) of the gap  16 , such as a part interface gap  16   a , at the through hole  18  between the mating parts  20  of the structure  24 . 
     Before the step of inserting  186  the gap measurement tool assembly  12  into the through hole  18  and through the mating parts  20 , the method  180  may include the step of calibrating the gap measurement system  14  (see  FIGS.  3 D- 3 E,  4   ) using mating master blocks  21  (see  FIGS.  3 A- 3 E ) that are precisely machined and have gaps  16  (see  FIGS.  3 C- 3 E ) that are precisely machined and have known gap sizes  28   a  (see  FIGS.  3 C- 3 D ), such as known gap widths  30   a  (see  FIGS.  3 C- 3 D ), to obtain a range  47  (see  FIG.  4   ) of predetermined gap measurements  46  (see  FIG.  4   ). The gap measurement system  14  may be calibrated by inserting the gap measurement tool assembly  12  into the through hole  18  (see  FIG.  3 C ), such as a master block through hole  18   b  (see  FIG.  3 C ), between mating master blocks  21 , passing the air  36 , such as compressed air  36   a , through the hollow inner channel  76  of the gap measurement tool  10  and into the gap  16  (see  FIGS.  3 D- 3 E ), such as the maximum gap  16   b  (see  FIG.  3 D ), or the minimum gap  16   c  (see  FIG.  3 E ), having a known gap size  28   a  (see  FIGS.  3 D- 3 E ) between the mating master blocks  21 , using the air gage system  32  (see  FIGS.  3 D- 3 E ) to take a measurement  38  (see  FIG.  4   ) of one of, a back pressure  40  (see  FIG.  4   ), an air flow  42  (see  FIG.  4   ), or a differential pressure  44  (see  FIG.  4   ), and preparing the range  47  (see  FIG.  4   ) of predetermined gap measurements  46  (see  FIG.  4   ). The amount of air  36 , such as compressed air  36   a , passed into the gap  16  is measured and the back pressure  40 , air flow  42 , or differential pressure  44 , is correlated to the gap size  28  (see  FIG.  4   ) the gap  16  (see  FIG.  4   ). Further, a pressure signal of the back pressure  40  measured may be digitally converted, using the amplifier  136 , to an equivalent gap and mapped by using sets of mating master blocks  21 , such as the first set  21   c  (see  FIG.  3 D ) and the second set  21   d  (see  FIG.  3 E ) representing two defined gaps  16 , such as the maximum gap  16   b  (see  FIG.  3 D ) and the minimum gap  16   c  (see  FIG.  3 E ), respectively. The amplifier  136  (see  FIGS.  5 A- 5 B ) displays on the flat panel display  142  (see  FIGS.  5 A- 5 B ) the gap size  28 . After the appropriate pressure correlations are made using the mating master blocks  21 , the gap measurement system  14  can be used to quickly measure the gap  16  in the through hole  18  in a fraction of the time of known methods, such as those using known feeler gauges. After proper calibration, the gap measurement tool assembly  12  of the gap measurement system  14  can be quickly inserted into a through hole  18  between mating parts  20 , such as aircraft mating parts  22 , and a gap measurement  48  of the gap size  28  of the gap  16  is made. 
     The method  180  may further comprise, after the step of coupling  184  the gap measurement tool assembly  12  to the air gage system  32  and the air supply source  34 , to obtain the gap measurement system  14 , the step of mounting the gap measurement system  14  to a robot  125  (see  FIGS.  4 ,  5 B ), to obtain an automated gap measurement system  14   a  (see  FIGS.  4 ,  5 B ) that automatically measures the gap  16  at the through hole  18  between the mating parts  20 . In one version, the gap measurement system  14  is the automated gap measurement system  14   b  (see  FIGS.  4 ,  5 B ). In another version, the gap measurement system  14  is a portable gap measurement system  14   a  (see  FIGS.  4 ,  5 A ) configured to be independently carried, or transported, for example, by an operator or user, to the mating parts  20  having the gap  16  to be measured at the through hole  18  between the mating parts  20 . 
     Now referring to  FIG.  9   ,  FIG.  9    is an illustration of a perspective view of a vehicle  200 , such as an aircraft  200   a , incorporating aircraft mating parts  22  that may be measured for gaps  16  (see  FIGS.  4 ,  5 A- 5 B ), such as part interface gaps  16   a  (see  FIGS.  4 ,  5 A- 5 B ), using exemplary versions of the gap measurement tool assembly  12  (see  FIGS.  2 A- 2 C,  4   ), the gap measurement system  14  (see  FIGS.  4 ,  5 A- 5 B ), and the method  180  of the disclosure. As shown in  FIG.  9   , the vehicle  200 , such as the aircraft  200   a , includes a fuselage  202 , a nose  204 , wings  206 , engines  208 , and an empennage  210 . As shown in  FIG.  9   , the empennage  210  comprises a vertical stabilizer  212  and horizontal stabilizers  214 . In one illustrative version, as shown in  FIG.  9   , the aircraft mating parts  22  comprise a first aircraft mating part  22   a  and a second aircraft mating part  22   b  joined together with one or more fasteners  19  (see  FIGS.  5 A- 5 B ), such as one or more permanent fasteners  19   a  (see  FIG.  5 A ), for example, one or more rivets, bolts, screws, or other suitable fasteners, to form the aircraft structure  26 , for example, the fuselage section  26   a  (see  FIG.  4   ). The vehicle  200  may also include rotorcraft, spacecraft, watercraft, and other suitable vehicles. 
     Now referring to  FIGS.  10  and  11   ,  FIG.  10    is an illustration of a flow diagram of an exemplary aircraft manufacturing and service method  300 , and  FIG.  11    is an illustration of an exemplary block diagram of an aircraft  316 . Referring to  FIGS.  10  and  11   , versions of the disclosure may be described in the context of the aircraft manufacturing and service method  300  as shown in  FIG.  10   , and the aircraft  316  as shown in  FIG.  11   . 
     During pre-production, exemplary aircraft manufacturing and service method  300  may include specification and design  302  of the aircraft  316  and material procurement  304 . During manufacturing, component and subassembly manufacturing  306  and system integration  308  of the aircraft  316  takes place. Thereafter, the aircraft  316  may go through certification and delivery  310  in order to be placed in service  312 . While in service  312  by a customer, the aircraft  316  may be scheduled for routine maintenance and service  314  (which may also include modification, reconfiguration, refurbishment, and other suitable services). 
     Each of the processes of the aircraft manufacturing and service method  300  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include, without limitation, any number of aircraft manufacturers and major-system subcontractors. A third party may include, without limitation, any number of vendors, subcontractors, and suppliers. An operator may include an airline, leasing company, military entity, service organization, and other suitable operators. 
     As shown in  FIG.  11   , the aircraft  316  produced by the exemplary aircraft manufacturing and service method  300  may include an airframe  318  with a plurality of systems  320  and an interior  322 . Examples of the plurality of systems  320  may include one or more of a propulsion system  324 , an electrical system  326 , a hydraulic system  328 , and an environmental system  330 . Any number of other systems may be included. Although an aerospace example is shown, the principles of the disclosure may be applied to other industries, such as the automotive industry. 
     Methods and systems embodied herein may be employed during any one or more of the stages of the aircraft manufacturing and service method  300 . For example, components or subassemblies corresponding to component and subassembly manufacturing  306  may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft  316  is in service  312 . Also, one or more apparatus embodiments, method embodiments, or a combination thereof, may be utilized during component and subassembly manufacturing  306  and system integration  308 , for example, by substantially expediting assembly of or reducing the cost of the aircraft  316 . Similarly, one or more of apparatus embodiments, method embodiments, or a combination thereof, may be utilized while the aircraft  316  is in service  312 , for example and without limitation, to maintenance and service  314 . 
     Disclosed versions of the gap measurement tool assembly  12  (see  FIGS.  2 A- 2 C ), the gap measurement system  14  (see  FIGS.  4 ,  5 A- 5 B ), and the method  180  (see  FIG.  8   ) allow for the measuring of gaps  16  (see  FIGS.  4 ,  5 A- 5 B ), such as part interface gaps  16   a  (see  FIGS.  4 ,  5 A- 5 B ), between mating parts  20  (see  FIGS.  4 ,  5 A- 5 B ), such as aircraft mating parts  22  (see  FIGS.  4 ,  5 A ), of a structure  24  (see  FIGS.  4 ,  5 A- 5 B ), such as an aircraft structure  26  (see  FIGS.  4 ,  5 A ), for example, large aircraft assemblies such as a fuselage section  26   a  (see  FIGS.  4 ,  5 A ), a tail section  26   b  (see  FIG.  4   ), or another suitable aircraft structure, and greatly reduce the time to measure thousands of gaps  16  in aircraft sections and assemblies. In particular, disclosed versions of the gap measurement tool assembly  12  (see  FIGS.  2 A- 2 C ), the gap measurement system  14  (see  FIGS.  4 ,  5 A- 5 B ), and the method  180  (see  FIG.  8   ) allow for conducting gap analysis at join sections for a cylindrical object such as a fuselage  202  (see  FIG.  9   ). Disclosed versions of the gap measurement tool assembly  12  (see  FIGS.  2 A- 2 C ), the gap measurement system  14  (see  FIGS.  4 ,  5 A- 5 B ), and the method  180  (see  FIG.  8   ) may also be used in other applications where gaps  16  are measured between mating parts  20 . 
     In addition, disclosed versions of the gap measurement tool assembly  12  (see  FIGS.  2 A- 2 C ), the gap measurement system  14  (see  FIGS.  4 ,  5 A- 5 B ), and the method  180  (see  FIG.  8   ), provide for measuring of gaps  16  between mating parts  20  where the measuring is fast, simple, stable, a one-step process  49  (see  FIG.  4   ) with a single measurement  38   a  (see  FIG.  4   ), may be automated with an automated gap measurement system  14   b  (see  FIG.  4   ) mounted to a robot  125  (see  FIG.  4   ), and is accurate, robust, and repeatable. Further, the disclosed gap measurement tool assembly  12  (see  FIGS.  2 A- 2 C ) and gap measurement system  14  (see  FIGS.  4 ,  5 A- 5 B ) provide a lightweight solution, including a portable gap measurement system  14   a  (see  FIG.  5 A ) option, that avoids ergonomic challenges for operators or users, as compared to known gap check systems. 
     Moreover, disclosed versions of the gap measurement tool assembly  12  (see  FIGS.  2 A- 2 C ), the gap measurement system  14  (see  FIGS.  4 ,  5 A- 5 B ), and the method  180  (see  FIG.  8   ), avoid manually gap checking mating parts with a known feeler gage or a known modified feel gage having a 90 degree bend, avoid multiple gap checks with successively larger and larger feeler gages until the largest size that fits in the gap is established, avoid a two-step gap checking process, and avoid reliance on operator “feel” when gap checking. Further, disclosed versions of the gap measurement tool assembly  12  (see  FIGS.  2 A- 2 C ), the gap measurement system  14  (see  FIGS.  4 ,  5 A- 5 B ), and the method  180  (see  FIG.  8   ) provide the ability to take gap measurements  48  (see  FIG.  4   ) of gaps  16  between mating parts  20 , or adjoining parts, where it may be difficult to take a gap measurement  48  because of geometry. 
     In addition, disclosed versions of the gap measurement tool assembly  12  (see  FIGS.  2 A- 2 C ), the gap measurement system  14  (see  FIGS.  4 ,  5 A- 5 B ), and the method  180  (see  FIG.  8   ) use an air gage system  32  (see  FIGS.  4 ,  5 A- 5 B ), such as a non-contact air gage system  32   a  (see  FIGS.  5 A- 5 B ), and air supply source  34  (see  FIGS.  4 ,  5 A- 5 B ) with air  36  (see  FIGS.  4 ,  5 A- 5 B ), such as compressed air  36   a  (see  FIGS.  4 ,  5 A- 5 B ), to measure one of, back pressure  40  (see  FIG.  4   ), air flow  42  (see  FIG.  4   ), or differential pressure  44  (see  FIG.  4   ), and correlate the measurement  38  (see  FIG.  4   ) to a predetermined gap measurement  46  (see  FIG.  4   ), to determine the gap size  28  (see  FIG.  4   ), such as the gap width  30  (see  FIG.  4   ) of the gap  16 , such as the part interface gap  16   a , at the through hole  18 , such as the fastener through hole  18   a , between the mating parts  20  of the structure  24 . 
     When the gap measurement tool  10 , or mandrel, is inserted into the through hole  18 , the cross-hole  94  (see  FIGS.  5 A- 5 B ) aligns, or lines up with, the gap  16  between the mating parts  20  and at the through hole  18  through the mating parts  20  or part stack-up. The gap measurement tool  10 , or mandrel, comprises the hollow inner channel  76  (see  FIGS.  5 A- 5 B ) with the third seal element  120  (see  FIGS.  5 A- 5 B ), such as a plug seal  120   a  (see  FIG.  4   ) at the second end  56  (see  FIGS.  5 A- 5 B ), such as the distal end  56   a , to seal the hollow inner channel  76 . The gap measurement tool  10 , or mandrel, further comprises the second seal element  116  (see  FIGS.  5 A- 5 B , such as the O-ring seal  116   a  (see  FIG.  4   ), on the second exterior annular groove  102  (see  FIG.  1 E ) near the second end  56 , such as the distal end  56   a , to secure the gap measurement tool  10 , or mandrel, in the through hole  18 , with the cross-hole  94  (see  FIGS.  5 A- 5 B ), or circular opening, on either side of the gap measurement tool  10 , or mandrel, that extends from the hollow inner channel  76  to the exterior  74   a  (see  FIG.  1 E ) of the body  58  (see  FIG.  1 E ) of the gap measurement tool  10 , or mandrel, located at a position that is approximately 180 degrees apart on opposite sides of the gap measurement tool  10 , or mandrel. 
     Air  36 , such as compressed air  36   a , is passed from the air supply source  34  and the air gage system  32 , into and through the gap measurement tool  10 , or mandrel, inserted into the through hole  18 , and air leakage through the gap measurement tool  10 , or mandrel, and through any gap  16 , such as an interior gap, at the through hole  18  causes a corresponding change in the back pressure  40 , the air flow  42 , or the differential pressure  44 , and is proportional to the gap size  28 , such as the gap width  30 , of the gap  16 . The back pressure  40 , the air flow  42 , and/or the differential pressure  44  generated through the air gage  130  (see  FIGS.  5 A- 5 B ) or differential pressure gauge  158  (see  FIG.  6   ), is measured with the air gage system  32 . Disclosed versions of the gap measurement tool assembly  12  (see  FIGS.  2 A- 2 C ), the gap measurement system  14  (see  FIGS.  4 ,  5 A- 5 B ), and the method  180  (see  FIG.  8   ) use air pressure and a sensitive air gage  130  or differential pressure gauge  158  to measure the amount of air  36 , such as compressed air  36   a , that passes through the gap  16 , such as the inner gap. In addition, the air gage system  32  and gap measurement tool assembly  12  have no moving parts, which provide for a long-lasting system. Further, the air gage system  32  is self-cleaning and the air gage  130  or differential pressure gauge  158  that is inserted into the gap measurement tool assembly  12  is resistant to dust and debris which is blown out with the air  36 , such as the compressed air  36   a.    
     In addition, known air gaging techniques typically measure a mean surface of a part or a diameter of a hole in a part, and no air gaging assembly, system, or method is believed to measure a gap at a through hole between mating parts. 
     Moreover, the gap measurement system  14  (see  FIGS.  4 ,  5 A- 5 B ) may be calibrated or mastered using mating master blocks  21  (see  FIGS.  3 A- 3 E ), such as a first set  21   c  (see  FIG.  3 D ) and a second set  21   d  (see  FIG.  3 E ), each with a first mating master block  21   a  (see  FIG.  3 A ) and a second mating master block  21   b  (see  FIG.  3 B ), having precisely machined gaps, to obtain a range  47  (see  FIG.  4   ) of predetermined gap measurements  46  (see  FIG.  4   ) with known gap sizes  28   a  (see  FIGS.  3 C- 3 D ), such as known gap widths  30   a  (see  FIGS.  3 C- 3 D ). 
     To calibrate the gap measurement system  14 , the gap measurement tool assembly  12 , and in particular, the gap measurement tool  10 , or mandrel, may be inserted into the first set  21   c  of mating master blocks  21  having a maximum gap  16   b  (see  FIG.  3 D ) with a known gap size  28   a  (see  FIG.  3 D ), and inserted into the second set  21   d  of mating master blocks  21  having a minimum gap  16   c  (see  FIG.  3 E ), such as no gap or a very small gap, and one of the back pressure  40 , air flow  42 , or differential pressure  44 , is measured with the air gage system  32  coupled to the gap measurement tool assembly  12 . Once the gap measurement system  14  is properly calibrated or mastered, an operator or use can quickly measure gaps  16  at through holes  18  through mating parts  20  in a structure  24 , such as the aircraft structure  26 . 
     Many modifications and other versions of the disclosure will come to mind to one skilled in the art to which this disclosure pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. The versions described herein are meant to be illustrative and are not intended to be limiting or exhaustive. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. Functionally equivalent methods and apparatuses within the scope of the disclosure, in addition to those enumerated herein, are possible from the foregoing descriptions. Such modifications and variations are intended to fall within the scope of the appended claims. The present disclosure is to be limited only by the terms of the appended claims, along with the full scope of equivalents to which such claims are entitled.