Abstract:
An insert for thread repair having an interior thread and a specially formed exterior thread which both precludes upheaval between sides of the cracked material on opposite sides of the crack and draws the crack together, radially inward towards the center of the insert. The insert may include a driving head, either solid or having a central hollow and may include a frangible neck to dissociate the head from the insert.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to thread repair inserts and, in particular, to a thread repair insert and method for repairing threaded holes disposed in materials and for repairing threaded holes disposed in materials in which a crack has propagated therethrough such that the repaired threaded hole may receive a threaded fastener via the insert.  
         BACKGROUND OF THE INVENTION  
         [0002]    Materials often exhibit a variety of different failure mechanisms. On occasion, materials are loaded beyond a tensile strength of the material and the material fractures abruptly and completely. In these circumstances the material is usually non-repairable and must be replaced. However, often materials do not fail in an abrupt complete manner, but rather fail due to fatigue or localized stresses which exceed design criteria, causing cracks to form in the material. Often when these cracks initially form, the material is still functional for its desired purpose. For instance, a reciprocating machine may receive cracks in its structural material and yet continue to operate, albeit at perhaps a lesser efficiency. Cracks thus serve as indicators that a material is being over stressed and yet also provides an opportunity for remedial measures to be taken without requiring entire replacement of the affected material.  
           [0003]    Where the crack passes through a preexisting hole, however, such as through a stud hole, or worse, a spark plug hole, a severe problem exists.  
           [0004]    Crack repair pins are known in the art which are threaded into holes drilled and tapped in the cracks to prevent cracks from continuing to propagate. U.S. Pat. No. 5,379,505 (issued to the present inventor) provides one example of these crack repair pins. These crack repair pins have been effective in resealing materials such as cast iron casings which require that they maintain a somewhat pressurized environment without allowing fluids to escape therefrom.  
           [0005]    In addition, locks are known in the art which can draw cracks together somewhat and extend the life of the material which is cracked. One example of this can be found in the inventor&#39;s U.S. Pat. No. 4,662,806.  
           [0006]    However, neither of these solutions nor the prior art which applicant is aware address a threaded hole which needs repair to receive a threaded fastener and specifically, the repair of a threaded hole in which a crack has propagated therethrough. Where the threaded hole is a spark plug hole, the problem is quite severe because of the internal pressures within the combustion chamber.  
           [0007]    For example, a damaged threaded hole in which a threaded fastener is to be received in needs to be strengthened to a level which effectively allows the threaded fastener to be rigidly retained therein. In addition, the damaged threaded hole may need to be repaired to a level which allows parts to be coupled to one another to effectively provide a seal which will withstand a pressurized environment. Furthermore, the damaged threaded hole may be disposed in a material at a location which requires the walls of the hole to provide means to maintain a somewhat pressurized environment without allowing fluids to escape therefrom.  
           [0008]    The following prior art reflects the state of the art of which applicant is aware and is included herewith to discharge applicant&#39;s acknowledged duty to disclose relevant prior art. It is stipulated, however, that none of these references teach singly nor render obvious when considered in any conceivable combination the nexus of the instant invention as disclosed in greater detail hereinafter and as particularly claimed.  
                                                                                       PATENT NO.       ISSUE DATE   INVENTOR                           83,341       October 27, 1868   Foster           154,864       September 8, 1874   Harvey           250,728       December 13, 1881   Harvey           310,462       January 6, 1885   Patten           568,277       September 22, 1896   Rall           678,814       July 16, 1901   Riggs           899,916       September 29, 1908   Smith           1,345,425       July 6, 1920   Wells           1,408,793       March 7, 1922   Anderson, et al.           1,434,870       November 7, 1922   Brubaker, Jr.           1,539,628       May 26, 1925   Bayer           1,543,007       June 23, 1925   Hanson           1,963,542       June 19, 1934   Bergstrom           2,011,484       August 13, 1935   Harman           2,121,692       June 21, 1938   Hays           2,291,162       July 28, 1942   Kirby           2,300,310       October 27, 1942   Poeton           2,361,701       October 31, 1944   Michaels           2,506,233       May 2, 1950   Murphy           2,649,650       August 25, 1953   Javor           2,951,506       September 6, 1960   Diperstein           2,998,645       September 5, 1961   Diperstein           3,066,400       December 4, 1962   Forsythe           3,295,580       January 3, 1967   Waltermire           3,660,233       May 2, 1972   Dalke, et al.           4,074,950       February 21, 1978   Holmes           4,271,554       June 9, 1981   Grenell           4,599,781       July 15, 1986   Diperstein           4,662,806       May 5, 1987   Reed           4,824,279       April 25, 1989   Casazza           4,845,828       July 11, 1989   Reed           4,892,429       January 9, 1990   Giannuzzi           5,033,919       July 23, 1991   Choe           5,379,505       January 10, 1995   Reed            FOREIGN PATENT DOCUMENTS                UK 350,141       June 11, 1931   Arenz           IT 456,481       December 1950           AU 154,074       November 10, 1953   Metalock of Australia           SU 492,389       January 22, 1976   Ivanov Furniture           SU 975,270       November 28, 1982   Mosc Auto Mech Inst.                      
 
           [0009]    The other prior art listed above but not specifically described further catalog the prior art of which the applicant is aware. These references diverge even more starkly from the references specifically distinguished above.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention is distinguished over the known prior art in a multiplicity of ways. For one thing, the present invention provides a thread repair insert which not only repairs threads of a threaded hole but also draws together opposite sides of a crack which may have propagated through the threaded hole. In addition, the thread repair insert according to the present invention provides clamping means which allows the insert to lock into the material to preclude disengagement of the insert from the threaded hole. The clamping means, inter alia, precludes opposite sides of the crack from moving in a direction generally along an axis of the thread repair insert. Furthermore, the thread repair insert strengthens a damaged threaded hole to a level which effectively allows a threaded fastener to be rigidly retained therein and to provide a seal which will withstand a pressurized environment.  
           [0011]    In a preferred form, the thread repair insert of the present invention includes a drive head with a means to apply torque thereto, a neck below the head, a shoulder below the neck and a sleeve substantially below the shoulder. Preferably, a thread relief portion is interposed between the shoulder and the sleeve.  
           [0012]    The sleeve is provided with an interior thread and an exterior thread. The exterior thread of the sleeve is adapted to be threaded into a hole having a thread pitch which is formed complementally to the exterior thread of the sleeve. The interior threads are preferably adapted to receive a desired standard threaded fastener.  
           [0013]    The drive head of the insert can include any of a variety of torque-receiving configurations. For instance, the drive head can have multiple facets dimensioned to be addressed by facets of a wrench. In addition, slots may be formed complementally to a slotted screw driver or other similar structures. Thus, the drive head receives torque and causes the thread repair insert to rotate about a central, long axis thereof.  
           [0014]    The neck includes a break-off groove which is interposed between the drive head and the shoulder and preferably defines an area of the thread repair insert having a weakest tensile strength experiencing torsional loads. Thus, when torque is applied through the drive head at greater and greater magnitudes, the thread repair insert fractures at the break-off groove before fracturing in any other location.  
           [0015]    The shoulder is interposed between the neck and the exterior threads of the sleeve and includes a cusp exposed at a lower most portion of the shoulder. The cusp is formed by a greater diameter portion and-a lesser diameter portion, with the lesser diameter portion adjacent the exterior threads of the sleeve. Thus, as the thread repair insert is threaded into a complementally formed threaded hole, the cusp of the shoulder abuts against an outer surface of the hole and identifies a point at which the insert can translate no further along the central, long axis. In addition, when the cusp of the shoulder contacts the outer surface of the hole there is a wedging or clamping effect caused by the cusp of the insert and the inter engagement of the threads of the insert within the complementally threaded hole. Therefore, the clamping means precludes disengagement of the insert from the threaded hole and the upheaval of opposite sides of the cracked threaded hole. Furthermore, the thread repair insert strengthens and seals the cracked threaded hole such that a threaded fastener may be received and retained within said threaded hole via the thread repair insert.  
           [0016]    The sleeve is a substantially cylindrical construct having a top adjacent the cusp of the shoulder and a bottom defining a surface of the insert opposite the head. The sleeve has threads circumscribing an outer periphery and an inner periphery thereof. The exterior threads disposed on the sleeve include a crest defining a major diameter thereof and a root defining a minor diameter thereof. An upper surface extends from a bottom edge of the root to an upper edge of the crest. A lower surface extends from a top edge of the root to a lower edge of the crest. Specifically, the upper surface extends linearly upward from the bottom edge of the root to the upper edge of the crest. Similarly, the lower edge extends linearly upward from the top edge of the root to the lower edge of the crest. Thus, the upper edge of the crest is closer to the head than the adjacent bottom edge of the root.  
           [0017]    In mending the threaded hole, the hole is preferably tapped such that it is configured to have threads substantially complementally to the exterior threads of the thread repair insert. However, a major diameter of the tapped threaded hole may be slightly greater than a major diameter of the thread repair insert. The insert is threaded into the hole by applying torque to the head of the insert causing the insert to rotate and causing the upper surface of the exterior threads to slide along the complementally formed threads of the hole. Thus, drawing opposite sides of the threaded hole toward each other and hence opposite sides of the crack toward each other This torque continues to be applied until the cusp of the shoulder abuts the surface of the material adjacent the threaded hole. At that point, the thread repair insert ceases to be threaded into the material. However, the application of torque continues until a maximum torsional force for the break-off groove of the neck is reached causing the head to shear off at the break-off groove.  
           [0018]    In addition, multiple threaded holes may be located along the length of the crack and filled with crack repair stitching pins to further mend the crack. Furthermore, locks may be placed transverse to the crack which fit within complementally formed lock receiving holes to also further mend the crack.  
         OBJECTS OF THE INVENTION  
         [0019]    A primary object of the present invention is to provide a new and novel thread repair insert for repairing a threaded hole.  
           [0020]    A further object of the present invention is to provide the thread repair insert as characterized above which includes a drive head which is driveable by a commonly available torque-applying instrumentality.  
           [0021]    Another further object of the present invention is to provide the thread repair insert as characterized above which further includes a sleeve having interior and exterior threads disposed thereon.  
           [0022]    Another further object of the present invention is to provide a thread repair insert as characterized above which includes a shoulder interposed between said drive head and said sleeve.  
           [0023]    Another further object of the present invention is to provide the thread repair insert as characterized above wherein the shoulder includes means for opposing central axis translation of the thread repair insert when threaded in a threaded hole.  
           [0024]    Another further object of the present invention is to provide a thread repair insert as characterized above wherein the central axis translation opposition means and the exterior threads of the sleeve create opposing forces which preclude the thread repair insert from disengaging from the threaded hole.  
           [0025]    Another further object of the present invention is to provide the thread repair insert as characterized above which includes a break-off groove located at a transition area between a top surface of the shoulder and the drive head.  
           [0026]    Another further object of the present invention is to provide a thread repair insert as characterized above which is designed to enter a complementally formed hole a finite distance and in which the drive head severs off of the thread repair insert at the break-off groove location prior to the exterior threads of the sleeve becoming damaged.  
           [0027]    Another further object of the present invention is to provide a thread repair insert as characterized above which includes means for drawing together opposite sides of a crack which has propagated through a threaded hole.  
           [0028]    Another further object of the present invention is to provide a thread repair insert as characterized above which strengthens and seals a cracked threaded hole such that a threaded fastener may be received within said threaded hole via the thread repair insert.  
           [0029]    Another further object of the present invention is to provide a thread repair insert as characterized above wherein the exterior threads angle upwards towards the head of the insert.  
           [0030]    Another further object of the present invention is to provide a thread repair insert wherein the exterior threads are slightly spaced from each other and maintain a minimum thickness between a root and a crest thereof, providing a durable thread.  
           [0031]    Another further object of the present invention is to provide a thread repair insert as characterized above which is simple and inexpensive to manufacture and yet durable in construction.  
           [0032]    Another further object of the present invention is to provide a thread repair insert as characterized above for cold crack repair which quickly mends cracks which have propagated within a threaded hole in a material and for receiving a threaded fastener therein.  
           [0033]    These and other objects will be made manifest when considering the following detailed specification when taken in conjunction with the appended drawing figures.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    [0034]FIG. 1 is an elevational view of a thread repair insert according to the present invention which is shown partially disposed into a threaded hole being repaired to receive a threaded fastener, the hole is shown in section.  
         [0035]    [0035]FIG. 2 is a partial sectional view of the thread repair insert of FIG. 1 showing detail of exterior threads.  
         [0036]    [0036]FIG. 3 is a cross-sectional view of the thread repair insert shown in FIG. 1.  
         [0037]    [0037]FIG. 4 is a sectional view of the threaded hole shown in FIG. 1.  
         [0038]    [0038]FIG. 5 is an elevational view of a tapping bit configured to form the threads shown in the threaded hole of FIG. 4.  
         [0039]    [0039]FIG. 6 is a detail view of a portion of the teeth of the tap shown in FIG. 5.  
         [0040]    [0040]FIG. 7 is a bottom view of that which is shown in FIG. 5.  
         [0041]    [0041]FIG. 8 is an elevational view of a conventional stud withdrawn from a bore which is to be mended according to the present invention.  
         [0042]    [0042]FIG. 9 is a sectional view of the bore shown in FIG. 8, before the bore is tapped.  
         [0043]    [0043]FIG. 10 is an isometric view of a bit utilized to form a counter-bore.  
         [0044]    [0044]FIG. 11 is an isometric view of the thread repair insert entering the bore shown in FIG. 10.  
         [0045]    [0045]FIG. 12 is an isometric view of that which is shown in FIG. 11 with the thread repair insert coupled within the bore and a head of the insert sheared off.  
         [0046]    [0046]FIG. 13 is an isometric view of that which is shown in FIG. 12 with a portion of a shoulder of the thread repair insert ground flush with a surface and ready to receive a threaded fastener.  
         [0047]    [0047]FIG. 14 is a flow chart revealing the steps according to the present invention involved in the method of repairing a threaded hole to receive a threaded fastener.  
         [0048]    [0048]FIG. 15 is a partial sectional view of the thread repair insert showing detail of a threaded blank disposed within the insert.  
         [0049]    [0049]FIG. 16 is an elevational view of the threaded blank shown in FIG. 15.  
         [0050]    [0050]FIG. 17 is a top view of the sleeve of the insert resizing a hole.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0051]    Considering the drawings, wherein like reference numerals denote like parts throughout the various drawing figures, reference numeral  10  is directed to the thread repair insert according to the present invention.  
         [0052]    In essence, and referring to FIGS. 1 through 4, the thread repair insert  10  includes a drive head  20  at an uppermost portion thereof, a neck  30  below the drive head  20 , a shoulder  40  below the neck  30  and a sleeve  60  extending below the shoulder  40 . The sleeve  60  includes an exterior threaded portion  62  which extends from an upper portion  64  to a bottom portion  66 . The exterior threaded portion  62  includes a plurality of threads  68  thereon which include an upper surface  72  and a lower surface  74  which extend upward toward the head  20  from a minor diameter  69  to a major diameter  70 . Thus, a crest  76  of each thread  68  is closer to the head  20  than a portion of the thread  68  between adjacent roots  82  at the minor diameter  69 .  
         [0053]    The thread repair insert  10  further includes an interior bore  100  extending from a first end  102  to a second end  104 . A chamfer  106  is provided on a top surface  22  of the head  20  and provides a transition area between the top surface  22  of the head  20  and the first end  102  of the interior bore  100 . Preferably, the interior bore  100  includes a cylindrical side wall  110  which extends between the first end  102  of the interior bore  100  to an area adjacent a top surface  42  of the shoulder  40 . The interior bore  100  then transitions to an in-taper  112  which preferably tapers inwardly from the area adjacent the top surface  42  of the shoulder  40  to a start of interior threads  114 . The interior threads  114  preferably extend from in-taper  112  to the second end  104  of the interior bore  100 . Preferably, the interior threads  114  are of a common type which include an inner diameter  116  and an outer diameter  118 . Each thread includes an upper side  120  and a lower side  122 .  
         [0054]    In mending a threaded hole H in which a common threaded fastener  150  is to be received, the threaded hole H is tapped using a tapping bit  200  (FIGS. 5 through 7) which causes threads T to be formed in the threaded hole H which are substantially complementally to the exterior threads  68  of the thread repair insert  10 . The threads T of the threaded hole H thus are closer to a surface S of a material M at a major diameter A of the threads T than at a minor diameter B of the threads T.  
         [0055]    The drive head  20  may be coupled to a torque applying instrument which can thread the thread repair insert  10  into the threaded hole H. The shoulder  40  includes a cusp  50  exposed at a lowermost portion thereof. Thus, when the thread repair insert  10  has been threaded entirely into the threaded hole H, the cusp  50  of the shoulder  40  abuts against the material M. This abutment, inter alia, prevents the pin  10  from translating along a central long axis  12  any deeper into the hole H.  
         [0056]    Upon rotation of the thread repair insert  10 , the upper surface  72  of the exterior threads  68  engage the threads T of the hole H forcing a first curved wall G of the hole H on one first side D of the crack C toward a second curved wall I of the hole H on a second side E of the crack C. Thus, the first side D and the second side E of the crack C are drawn toward one another.  
         [0057]    The neck  30  includes a crease or break-off groove  32  which fractures, shearing the head  20  off of the thread repair insert  10  before a magnitude of torque applied to the insert  10  reaches a maximum torque allowed to be applied to the exterior threads  68 . Thus, the head  20  shears off at the break-off groove  32  prior to torsion ally over stressing the exterior threads  68 . Head  22  is then discarded.  
         [0058]    More specifically, and referring in detail to FIGS. 1 through 4, the thread repair insert  10  and associated hole H are shown in detail. The thread repair insert  10  includes the head  20  at an uppermost end thereof. The head  20  preferably includes multiple facets  24  arranged to allow a torque applying device to effectively engage the head  20 . Preferably, the facets  24  are arranged in a hexagonal pattern to be engaged by a variety of commonly available torque applying tools. Alternatively, various other facet arrangements could be utilized or slots could be provided in the top surface  22  of the head  20  for receiving a screwdriver or other similarly shaped tool. In addition, an internal hex head may be formed on an internal sidewall  110  of the head  20  (FIG. 3). The head  20  is preferably radially symmetrical about a central axis  12  passing through the thread repair insert  10 .  
         [0059]    The neck  30  joins the head  20  to the shoulder  40 . The neck  30  is preferably arranged as a single frustum with a greater diameter base  34  adjacent the head  20  and a lesser diameter crease or break-off groove  32  adjacent the shoulder  40 . The break-off groove  32  and base  34  thus define parallel planes of the frustum of the neck  30 . The break-off groove  32  is preferably designed to have a cross-sectional area which is less than a cross-sectional area of almost any other portion of the thread repair insert  10 . Thus, when torsional loads increase, the thread repair insert  10  is most likely to fracture at the break-off groove  32  than at any other location along the thread repair insert  10 .  
         [0060]    In addition, the break-off groove  32  is preferably provided with a cross-sectional area which gives the break-off groove  32  a maximum torsional load characteristic which is less than a torsional load necessary to cause damage to the exterior threads  68  of the exterior threaded portion  62  of the sleeve  60 . In this way, if torsional loads on the thread repair insert  10  begin to approach a level which may cause damage to the exterior threads  68  of the exterior threaded portion  62 , the break-off groove  32  of the neck  30  will fracture before a damaging torsional load is reached which will damage the threads  68 .  
         [0061]    The shoulder  40  is interposed substantially between the neck  30  and the sleeve  60 . The shoulder  40  includes a top surface  42 , a cylindrical side portion  44  and a cusp  50  exposed at a lowermost portion of the shoulder  40 . The top surface  42  of the shoulder  40  is adjacent the break-off groove  32  and transitions into the cylindrical side portion  44 . The cusp  50  is located at the lowermost portion of the shoulder  40  and includes an upslope  52  which preferably diverges inwardly and upwardly toward the central axis  12  and then transitions into a downslope  54  which preferably diverges inwardly and downwardly-toward the central axis  12 . The downslope  54  of the cusp  50  transitions into the top  64  of the exterior threaded portion  62  in which the helically wound exterior threads  68  terminate.  
         [0062]    The cusp  50  of the shoulder  40  abuts against a surface S of a material M surrounding the threaded hole H when the thread repair insert  10  is threaded into the threaded hole H a sufficient distance along the central axis  12 . Thus, this identifies one point at which the thread repair insert  10  can translate no further along the central axis  12 . Should the bottom of the insert  10  reach a blind core of the thread hole H, the same phenomena of freezing the insert  10  into the hole H will occur. As one consequence, referring to FIGS. 11 through 13, when the cusp  50  of the shoulder  40  abuts against the surface S adjacent the threaded hole H there is a wedging or clamping effect along the arrows F 1  and F 2 . This clamping effect is caused by the abutment of the cusp  50  with the surface S and the inter engagement of the threads  68  of the insert  10  with the complementally formed threads T of the threaded hole H. Thus, opposing forces are developed along arrows F 1  and F 2  which preclude the exterior threads  68  from disengaging from the complementally formed threads T of the threaded hole H. More importantly, the compressive effect of forces F 1 , F 2  prevent “upheaval” with respect to opposite sides of the cracks. In other words, side D (FIG. 12) will not “shift” or move (e.g. into another plane) relative to side E. The opposing forces arrows F 1  and F 2  also precludes the thread repair insert  10  from rotating when a standard threaded fastener  150  is being disposed therein.  
         [0063]    The abutment of the cusp  50  with the surface S also causes the exterior threads  68  of the thread repair insert  10  to further draw the first side D and the second side E of the crack C toward each other, rather than the exterior threads  68  drawing the thread repair insert  10  further into the threaded hole H along the central axis  12 . Thus, the cusp  50  of the shoulder  40  further acts to redirect forces radially inwardly, i.e. towards the center of the thread repair insert  10  and the threaded hole H along arrows F 3 , F 4  of FIG. 11.  
         [0064]    The exterior threaded portion  62  of the sleeve  60  is of a substantially cylindrical construct which extends from a location adjacent the downslope  54  of the cusp  50  of the shoulder  40  to the bottom  66  of the sleeve  60 . The bottom  66  of the sleeve  60  defines an opposite end of the thread repair insert  10  from the head  20 . The exterior threaded portion  62  is oriented about the central axis  12  with the central axis  12  passing through a geometric center of the sleeve  60 . The sleeve  60  has the exterior threads  68  formed about the cylindrical surface thereof.  
         [0065]    The exterior threads  68  are preferably one continuous helically wound thread which begins at the bottom  66  and spirals up to a top  64 . While this single thread design is preferred, other arrangements including a compound series of threads which wind helically together from the bottom  66  to the top  64  could also be utilized.  
         [0066]    Referring to FIG. 2, the exterior threads  68  include the crest  76  defining the major diameter  70  of the exterior threads  68 . The exterior threads  68  have an upper surface  72  which extends from a bottom edge  86  of a root  82  to an upper edge  78  of the crest  76 . The exterior threads  68  also include a lower surface  74  which extends from a top edge  84  of the root  82  to a lower edge  80  of the crest  76 . Both the upper surface  72  and the lower surface  74  angle upward toward the head  20  as the surfaces  72 ,  74  extend from the root  82  to the crest  76 . Both the crest  76  and the root  82  exhibit a substantially constant distance from the central axis  12  between the upper edge  78  and the lower edge  80  and between the top edge  84  and the bottom edge  86 .  
         [0067]    In section, the surfaces  72 ,  74  extend linearly from the root  82  to the crest  76 . However, as this contour is rotated helically about the sleeve  60  along with the threads  68 , the upper surface  72  and lower surface  74  take on a curved surface appearance. This appearance is similar to that which would be formed by a linear section of the surface of a cone with a tip of the cone oriented downward and the cone rotated and translated upward along a central axis thereof. The upper surface  72  and the lower surface  74  thus have a curved surface in three dimensions similar to that of a cone, but a linear character when viewed in section.  
         [0068]    The upper surface  72  extends from the root  82  to the crest  76  at an upper surface angle ut diverging from a reference plane  14  orthogonal to the central axis  12 . The upper surface angle ou is preferably approximately 20 degrees but could be any angle between 0 degrees and 90 degrees. The lower surface  74  extends from the root  82  to the crest  76  at a lower surface angle β with respect to the reference plane  14 . The lower surface angle β is preferably approximately 40 degrees but could also vary between 0 degrees and 90 degrees.  
         [0069]    Preferably, the upper surface angle ox is less than the lower surface angle β such that a thickness of the exterior thread  68  at the crest  76  is less than a thickness of the threads  68  between adjacent roots  82 . In this way, the threads  68  are provided with greater thickness, and hence greater strength, adjacent the minor diameter  69  than at the major diameter  70  and are thus more capable of bearing the loads experienced within the threaded hole H. Alternatively, both angles could be parallel.  
         [0070]    Referring to FIG. 4, details of the threaded hole H are shown. The threaded hole H is preferably substantially complementally in form to the exterior threads  68  disposed on the sleeve  60  of the thread repair insert  10 . The threaded hole H includes threads T which include a major diameter A and a minor diameter B. Each thread T includes a lower surface L and an upper surface U. In addition, the threaded hole H includes a first curved wall G on a first side D of the crack C and a second curved wall I on the second side E of the crack C. Thus, for example, the threaded hole H is substantially bisected by the crack C. However, when mending a threaded hole in which a crack C has propagated therethrough, the distribution of the crack C on either side of the thread repair insert  10  may not be a symmetrical one.  
         [0071]    The threads T of the threaded hole H are shaped to have surfaces U, L which conform to the upper surface  72  and the lower surface  74  of the exterior threads  68  of the thread repair insert  10 . However, the major diameter A of the threaded hole H is preferably slightly greater than the major diameter  70  of the exterior threads  68  and the minor diameter B of the threads T is preferably slightly greater than a minor diameter  69  of the exterior threads  68 .  
         [0072]    This slight disparity provides a tolerance between the thread repair insert  10  and the threaded hole H for ease of fitting the thread repair insert  10  into the threaded hole H. Furthermore, this provides a finite amount of travel between the first curved wall G and the second curved wall I of the threaded hole H when the thread repair insert  10  is tightened therein, causing the first side D and the second side E of the crack C to be drawn toward each other. Thus, the major diameter A and minor diameter B of the threaded hole H are preferably initially greater than the major diameter  70  and the minor diameter  69  of the exterior threads  68 . After tightening the thread repair insert  10  into the threaded hole H, this difference between the threaded hole H and the exterior threads  68  of the sleeve  60  is reduced or substantially eliminated. Note that the size difference between the threaded hole H and the exterior threads  68  may be unnecessary.  
         [0073]    The thread repair insert  10  further includes an interior bore  100  having a first end  102  and a second end  104 . Chamfer  106  is provided on the top surface  22  of the head  20  and transitions therefrom to the first end  102  of the interior bore  100 . The interior bore  100  then transitions from the first end  102  to taper  112  via a cylindrical sidewall  110 . The taper  112  transitions from a greater diameter adjacent the break-off groove  32  of the neck  30  to a lesser diameter adjacent an interior threaded portion  113 . The interior threaded portion  113  includes interior threads  114  disposed thereon. The interior threads  114  are preferably of a conventional type and include an inner diameter  116  and an outer diameter  118 . In addition, the interior threads  114  are provided with an upper side  120  and a lower side  122 .  
         [0074]    [0074]FIGS. 5 through 7 show details of the tapping bit  200 , preferably used to form the threaded hole H. The tapping bit  200  is preferably a substantially cylindrical construct having a torque input head  202  at one end thereof and a bottom  204  on an opposite end thereof. A stop  206  defines a transitional region between a lesser diameter portion  208  and a greater diameter portion  210  of the tapping bit  200 . The stop  206  is preferably located a distance from the bottom  204  equal to a desired depth of the threaded hole H to be formed by the tapping bit  200 .  
         [0075]    A plurality of teeth  220  are oriented proximate the bottom  204  which extend helically around the tapping bit  200  at an angle approximately similar to an angle of the threads T within the threaded hole H. The teeth  220  include a upper side  222 , a lower side  224 , an outer side  226  and an inner side  228 . The teeth  220  are interrupted by a cutout groove  230  which precludes the teeth  220  from forming one continuous tooth extending along the tapping bit  200 . The cutout groove  230  allows filings cut out of the hole H by the tapping bit  200  to escape from the teeth  220  and thus allow the teeth  220  to cut the threads T into the hole H without obstruction.  
         [0076]    The sides  222 ,  224 ,  226  and  228  are dimensioned in a manner similar to the threads T to be formed in the hole H. As noted above, the threads  68  of the thread repair insert  10  may be slightly smaller in minor diameter  69  and major diameter  70  than the threads T disposed within the threaded hole H and thus, the teeth  220  are preferably slightly larger in dimension than surfaces  72  and  74  of the thread repair insert  10 . However, the shape of the teeth  220  forming the threads T of the threaded hole H may be substantially similar in size and shape.  
         [0077]    The upper sides  222  of each of the teeth  220  are oriented at an upper side angle ρ from a reference plane  240  orthogonal to a long axis  242  of the tapping bit  200 . The lower sides  224  of each of the teeth  220  are angled at a lower side angle σ with respect to a reference plane  244 . The lower side angle σ and upper side angle ρ are preferably similar to the lower surface angle β and the upper surface angle α, respectively.  
         [0078]    A portion of the teeth  220  directly adjacent the bottom  204  are preferably beveled at a bevel  250  having a bevel angle γ of approximately preferably 60 degrees from a reference plane  252  orthogonal to the long axis  242 . This bevel  250  allows the teeth  220  to cut the thread T into the hole H in progressively greater amounts as the tapping bit  200  initially enters the hole H. While the tapping bit  200  is preferably utilized to form the threads T within the hole H, other tapping bits and other methods of forming and finishing the threads may be utilized to form the complementally threads T within the hole H.  
         [0079]    As an alternative to the tapping bit  200 , a series of taps may be used to most effectively form the threads T within the hole H. Initially, a roughing tap may be passed axially into the hole H to form a first rough shape of the threads T. The roughing tap may then be followed by a finishing tap which completes the formation of the threads T within the hole H. If the hole H is formed as a blind bore to receive the thread repair insert  10 , a bottom tap may be utilized to fully form the threads T to within approximately one thread T of a bottom of the hole H. By utilizing the three taps in series, a more precisely and defined threaded hole H may be formed with less damage to each of the respective taps.  
         [0080]    In forming the threads T within the threaded hole H, the tapping bit  200  is oriented overlying hole H and is coupled to an appropriate torque applying device such as a drill. The tapping bit  200  is then lowered into the hole H with the bottom  204  entering the hole H. As the tapping bit  200  is lowered within the hole H, the threads T are cut into the first curved wall G and the second curved wall I of the threaded hole H.  
         [0081]    Referring to FIG. 10, the threaded hole H may be provided with a counter bore  270  at an upper end of the hole H adjacent the surface S of the material M. A spot facing bit  280  having a particularly formed cutting surface is utilized to form the counter bore  270 . Particularly, the spot facing bit  280  includes an angled edge  282  having an angle which conforms to the slope of a thread relief  272  and a floor forming edge  284  having an angle which conforms substantially with the angle of the floor  274  of the counter bore  270 . A pilot portion  286  of the spot facing bit  280  below the angled edge  282  is preferably not greater in diameter than a diameter of the hole H. Thus, when the spot facing bit  280  is lowered axially into the hole H, the angled edge  282  and the floor forming edge  284  form the counter bore  270 . Depth of the counter bore  270  is preferably controlled by some form of drill stop  290  or other jig to preclude the counter bore  270  from passing too deeply into the hole H.  
         [0082]    Preferably, the floor forming edge  284  and the angled edge  292  are angled to form a cusp in the floor  274  which complements the geometry of the cusp  50  disposed on the lower surface of the shoulder  40 . Thus, the cusp  50  of the shoulder  40  is received within the floor  274  of the counter bore  270  and abuts thereagainst. Therefore, the opposing forces F 1  and F 2  are brought about by the cusp  50  of the shoulder  40  abutting the floor  274  of the counter bore  270  and the upper surface  72  of the exterior threads  68  against the upper surface U of the threaded hole H. Furthermore, the counter bore  270  may be formed before or after the hole H is tapped.  
         [0083]    In use and operation, and for example, the thread repair insert  10  is utilized for cold crack repair of a threaded hole H in which a threaded fastener is to be received via the insert  10 . For example, and referring to FIGS. 8 and 14, a standard threaded hole H is shown in which a crack has propagated therethrough. Preferably, the first step in repairing the standard threaded hole H′ is to prepare it for receiving the thread repair insert  10 . This is accomplished by removing or drilling out the previous standard threaded fastener which may still be retained within the threaded hole H′. Once the standard threaded fastener has been removed or a hole H′ is provided, the tapping bit  200  (FIG. 5) is preferably used to form the threads T of the hole H as is detailed in FIG. 4.  
         [0084]    Preferably, the threaded hole H includes a counter bore  270  formed at a transition area between the hole H and the surface S of the material M. The counter bore  270  preferably includes a cylindrical side portion  276  dimensioned similarly to the cylindrical side surface  44  of the shoulder  40 . The cylindrical side portion  276  of the counter bore extends from the surface S down to the floor  274  which is preferably cusped. The floor is formed by the floor forming edge  284  and the angled edge  282  of the spot facing bit  280 . The floor  274  preferably substantially complements the geometry of the cusp  50  of the shoulder  40 .  
         [0085]    The spot facing tool is important in that, inter alia, it determines the amount of the shoulder  40  which remains after grinding or machining the shoulder substantially flush with the surface S of the material M. In addition, the spot facing tool provides the shoulder with a precision fit and seat. Furthermore, the spot facing tool may provide a relief cut  272  for a first exterior thread of the thread repair insert  10 . The relief cut  272  may be provided as a result of the possibility of the first exterior thread of the insert  10  being partially incomplete when manufactured. Thus, the relief cut prevents the first exterior thread from spreading the crack prior to the exterior threads having a chance to seat within the threaded hole H.  
         [0086]    Referring now to FIGS. 11 through 14, details of the utilization of the thread repair insert  10  are shown. After the threads T are formed within the threaded hole H and the counter bore  270  has been formed, the-thread repair insert  10  is disposed within the threaded hole H. The thread repair insert  10  is disposed within the threaded hole H by applying torque to the drive head  20  causing rotation of the thread repair insert  10  about arrow F (FIG. 11). This rotation causes the upper surface  72  of the exterior threads  68  to slide along the complementally formed threads T of the hole H. Thus, drawing opposite sides D, E of the threaded hole radically toward each other along arrows F 3 , F 4  and hence opposite sides of the crack toward each other.  
         [0087]    Rotation continues about arrow F until the shoulder  40  abuts against the floor  274  of the counter bore  270  and thus, providing the clamping effect (forces F 1 , F 2 ) discussed supra. As additional rotational forces are applied about arrow F, opposite sides of the crack C are drawn further together as discussed hereinabove with utilization of the thread repair insert  10 . Once sufficient torque is applied that the neck  30  can no longer resist deformation, the head  20  is severed therefrom at the break-off groove  32  (please see FIG. 12). At this time, a portion of the shoulder  40  may be ground or machined flush until coplanar with the surface S of the material M.  
         [0088]    Referring to FIG. 13, the standard threaded fastener  150  or the like, may be disposed within the thread repair insert  10 . Preferably, the threaded fastener  150  is provided with threads which are substantially complementally in form to the interior threads  114  of the thread repair insert  10 .  
         [0089]    Referring to FIGS. 15 and 16, a blank  300  may be disposed within the thread repair insert  10  prior to the insert  10  being threaded into the threaded hole H. The blank  300  includes threads  302  disposed thereon which are preferably complementally to the interior threads  114  of the insert  10 . One purpose for providing the blank  300  may be to keep the interior threads clean or to retain the integrity of the interior threads  114  when applying a torque to the drive head  20  which will cause the head  20  to shear at the break-off groove  32 . Preferably, a top  304  of the threaded blank  300  is at a location which is below the shoulder  40  of the thread repair insert  10  such that the threaded blank  300  does not interfere with the shearing of the drive head  20  from the shoulder  40  or with the shoulder being ground or machined flush with the surface S. The blank  300  preferably includes a torque receiving means  306  such that the blank  300  may be easily inserted in and removed from the interior bore  100 . The torque receiving means  306  may be in the form to receive a standard ratchet or a typical screwdriver or the like. After the repair has been made, blank  300  can be removed and a stud or other insert such as a spark plug can be properly installed. Therefore, the thread repair insert may be provided with an open or blind interior bore  10  by providing an opened or closed second end  104  of the interior bore  100 .  
         [0090]    Furthermore, and referring to FIGS. 8 and 15, the hole H′ is preferably oversized from a first diameter once capable of receiving the threaded fastener  150  to a second, greater diameter threaded hole H. The diameter D of the second, greater diameter threaded hole H can be determined from the following formula:  
           D=d+ 2 T    
         [0091]    where D is the diameter of the second, greater diameter threaded hole H,  
         [0092]    d is the diameter of a member received within the sleeve  62  of the insert  10 , and  
         [0093]    T is the thickness of the sleeve  62 .  
         [0094]    For example, D represents a major diameter of the threads  220  of the tapping bit  200 , d represents a major diameter of a threaded portion of the threaded fastener  150  and T represents the thickness of the sleeve  62  measured from a major diameter  70  of the external threads  68  to an outer diameter  118  of the interior threads  114  of the interior bore  100 .  
         [0095]    Thus, the sleeve  62  may be threaded into the hole H for re-sizing the hole from the second, greater diameter D to the first diameter d for receiving the threaded fastener  150 .  
         [0096]    Moreover, having thus described the invention, it should be apparent that numerous structural modifications and adaptations may be resorted to without departing from the scope and fair meaning of the instant invention as set forth hereinabove and as described hereinbelow by the claims.