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CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of U.S. patent application Ser. No. 11/817,292, filed Aug. 28, 2007, now U.S. Pat. No. 7,617,889 which is the U.S. National Phase Application of International Application No. PCT/US2006/042740, filed Oct. 31, 2006, which was published in English under PCT Article 21(2) and which claims the benefit of U.S. Provisional Application No. 60/733,860, filed Nov. 3, 2005. Each of the referenced applications is incorporated herein in its entirety. 
    
    
     FIELD 
     This application relates to drilling equipment, and in particular to an improved construction of a fluid-operated drilling tool. 
     BACKGROUND 
     Known types of fluid-operated drilling tools, particularly “down-the-hole” rock drilling tools, generally have one end connected to a source of pressurized fluid (referred to here as the proximal end) and an opposite distal or working end with a reciprocating bit that is controlled to strike material to be drilled or removed with high force. 
     In a conventional rock drilling tool, the source of pressurized fluid, which is typically compressed air or other gas, is connected to a backhead or top sub at the proximal end of the tool by a pressure fitting. A hollow wear sleeve is attached by a threaded connection to the backhead and extends distally to form the exterior surface or shank of the tool. Within the wear sleeve, there is a distributor with a check valve that selectively supplies pressurized fluid to move the piston. 
     Typically, the distributor is secured in place by its attachment to an interior surface of the wear sleeve. According to one known approach, the distributor is received within the bore of an inner cylinder, and the inner cylinder has a surrounding retaining ring that is received in a circumferential groove formed in the interior surface of the wear sleeve. Over time, it becomes necessary to remove the distributor, e.g., to repair or replace it, to replace the wear sleeve to which it is attached and/or to access other components within the wear sleeve, e.g., the piston. In conventional drilling tools, uncoupling the distributor from the wear sleeve is difficult. For example, it can be difficult to access the retaining ring and disengage it from the wear sleeve and/or the distributor. 
     In conventional drilling tools, some of the passageways for the pressurized fluid have reduced areas and/or other types of restrictions that decrease flow velocity and efficiency. Some of the passageways extend between coaxially positioned components, and some are formed at least in part by channels, grooves, openings, etc., formed in walls of the components. 
     In the operation of some drilling tools, such as a down-the-hole rock drilling tool, the tool is designed such that when the bit encounters a very low resistance during operation, such as when the bit encounters a void in the material being drilled, the bit is extended to a “drop open” position and further movement of the bits is stopped. In the way, the possibility for damage to the tool or to the operation is minimized. It would be advantageous to decrease the transition time for changing from a normal operating position to the drop open position. 
     In addition, the speed at which the tool transitions between other phases of operation is affected by the piston area. It would be advantageous to reduce the transition times between other phases of operation to improve overall efficiency. 
     SUMMARY 
     Described herein are embodiments of a backhead and drill assembly with a backhead that address some of the problems associated with current drilling tools. 
     According to one implementation, a portion of a drill assembly operated by a supply of compressed fluid comprises a backhead with an integrated piston, a hollow elongate wear sleeve and a piston. The backhead has a proximal end connectible to the supply, an axial bore and an open distal end having the integrated cylinder portion defined therein. The backhead has passages extending between the axial bore and outer surface of the backhead. The hollow elongate wear sleeve has a proximal end to which the backhead is coupled and into which the distal end of the backhead is received. The piston is housed by the wear sleeve and has a proximal end shaped to fit within the integrated cylinder portion of the backhead. The piston is slidably movable along the wear sleeve and the integrated cylinder portion in response to compressed fluid conveyed through the backhead. An intake flow path for an intake flow of compressed fluid in the drilling tool extends in a distal direction from the axial bore, through the passages in the backhead, through a space between the backhead and the wear sleeve and into an area between the piston and wear sleeve and into contact with the piston. Advantageously, the intake flow path is free from sharp bends. 
     The intake flow path may be configured so as not to extend through any apertures forcing the intake flow in a radially inward direction. The intake flow path may be configured so as not to require the intake flow to pass inwardly through any openings defined in a sidewall of the backhead. 
     The drill assembly may comprise a distributor positioned at least partially within the axial bore between the proximal end and the distal end, the distributor being removably secured to the backhead by a securing member accessible from an exterior surface of the backhead and including a check valve that is opened to allow the intake flow from the supply. The distributor can comprise a check valve having a sealing member, a biasing member that biases the sealing member to a closed position and a distally extending guide portion. The drill assembly may comprise a chuck coupled to a distal end of the wear sleeve and capable of receiving a drill bit and being movable in response to contact from the piston. 
     According to other embodiments, a portion of a drill assembly operated by a supply of compressed fluid comprises a backhead having a proximal end connectible to the supply, an axial bore and an open distal end having defined therein an integrated cylinder portion shaped to receive a piston member, and a distributor positioned at least partially within the axial bore between the proximal end and the distal end, the distributor being removably secured to the backhead by a securing member accessible from an exterior surface of the backhead. 
     The securing member can comprise a laterally extending pin inserted through at least one opening in the backhead. The securing member can comprise at least two laterally extending pins, each of the pins being inserted through one of a corresponding number of spaced-apart openings in the backhead. 
     The backhead can include an externally threaded portion to which a wear sleeve can be attached, and the securing member can comprise a laterally extending pin inserted through at least one opening in the backhead in the area of the threaded portion. 
     According to other embodiments, a portion of a drill assembly operated by a supply of compressed fluid comprises a backhead having a proximal end connectible to the supply, an axial bore and an open distal end having defined therein an integrated cylinder portion, the backhead having passages extending between the axial bore and outer surface of the backhead, a hollow elongate wear sleeve having a proximal end to which the backhead is coupled and into which the distal end of the backhead is received, and a piston housed by the wear sleeve and having a proximal end shaped to fit within the integrated cylinder portion of the backhead, the piston being slidably movable along the wear sleeve and the integrated cylinder portion in response to compressed fluid conveyed through the backhead. When the drill assembly is in a drop open position, a proximal end of the piston is spaced apart from the integrated cylinder portion in the distal direction and an open annular space is defined between a proximal end of the piston and the wear sleeve. 
     The piston can have an available piston area subject to pressure tending to move the piston in a distal direction that is about 5% to about 25% greater than the available piston area of a conventional drill assembly of the same outer diameter. In other embodiments, the piston can have an available piston area that is about 8% to about 10% greater than the available piston area of a conventional drill assembly of the same outer diameter. In still other embodiments, the piston can have an available piston area that is at least about 9% greater than the available piston area of a conventional drill assembly of the same outer diameter. 
     According to other embodiments, a portion of a drill assembly operated by a supply of compressed fluid comprises a backhead having a proximal end connectible to the supply, an axial bore and an open distal end having defined therein an integrated cylinder portion, the backhead having passages extending between the axial bore and outer surface of the backhead, a cylinder portion aligned with and in selective fluid communication with the backhead, a hollow elongate wear sleeve surrounding the cylinder portion and connected to the backhead, a piston housed by the wear sleeve and having a proximal end shaped to fit within the cylinder portion, the piston being slidably movable along the wear sleeve and the cylinder portion in response to compressed fluid conveyed through the backhead. When the drill assembly is in a drop open position, a proximal end of the piston is spaced apart from the cylinder portion and the wear sleeve. 
     According to other embodiments, a portion of a drill assembly operated by a supply of compressed fluid comprises a backhead having a proximal end connectible to the supply, an axial bore and an open distal end having defined therein an integrated cylinder portion, the backhead having passages extending between the axial bore and outer surface of the backhead, a hollow elongate wear sleeve having a proximal end to which the backhead is coupled and into which the distal end of the backhead is received, and a piston housed by the wear sleeve and having a proximal end shaped to fit within the integrated cylinder portion of the backhead, the piston being slidably movable along the wear sleeve and the integrated cylinder portion in response to compressed fluid conveyed through the backhead. An intake flow path for an intake flow of compressed fluid in the drilling tool extends in a distal direction from the axial bore, through the passages in the backhead, through a space between the backhead and the wear sleeve and into an area between the piston and wear sleeve and into contact with the piston, and a filling flow path extends in the proximal direction from the area between the piston and the wear sleeve, along the piston and between the piston and the backhead into a space proximal of the proximal end of the piston. Advantageously, a separation is maintained between the intake flow path and the filling flow path in the area between the piston and the wear sleeve. 
     The distal end of the backhead can have a circumferential wall configured to guide the filling flow flowing in the proximal direction along an inner surface of the wall and configured to guide the intake flow flowing in the distal direction along an outer surface of the wall, the intake flow and the filling flow being separated from each other by the wall. 
     The foregoing and other features and advantages will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of a fluid-operated drilling tool showing a new backhead, a distributor, a piston, a wear sleeve, a chuck, and a bit and bit retaining rings. 
         FIG. 2  is an enlarged exploded perspective view of the backhead and distributor assembly of  FIG. 1 . 
         FIG. 3  is a sectioned view, in elevation, of the backhead and distributor assembly attached to the wear sleeve and showing a portion of the piston within an inner end of the backhead. 
         FIG. 4  is a perspective view of the backhead and distributor assembly of  FIG. 1 . 
         FIGS. 5A and 5B  are sectioned views, in elevation, of a conventional fluid-operated drilling tool and a similar tool with the new backhead of  FIG. 1 , respectively. 
         FIG. 6  is a perspective view of the backhead and distributor assembly similar to  FIG. 2 , except showing a securing member in the form of two pins. 
         FIG. 7  is a sectioned view of the backhead and distributor assembly of  FIG. 6  as assembled showing the positions of the two pins. 
         FIG. 8A  and  FIG. 8B  are sectioned views, in elevation, of a conventional fluid-operated drilling tool and a new drilling tool according to this application, respectively, showing the pistons in an impact position. 
         FIG. 9A  and  FIG. 9B  are sectioned views similar to  FIG. 8A  and  FIG. 8B , respectively, except showing the tools in a drop open position. 
         FIGS. 10A and 10B  are sectioned views similar to  FIG. 8A  and  FIG. 8B , respectively except showing the tools with the pistons in a top position. 
         FIG. 11A  and  FIG. 11B  are enlarged views of portions of  FIG. 8A  and  FIG. 8B , respectively. 
         FIG. 12A  and  FIG. 12B  are enlarged views of portions of  FIGS. 9A and 9B , respectively. 
         FIG. 13A  and  FIG. 13B  are enlarged views of portions of  FIG. 10A  and  FIG. 10B , respectively. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is an exploded perspective view showing an embodiment of a fluid-operated drilling tool  10 . The major components of the drilling tool  10  are a backhead and distributor assembly  12  at a proximal end of the tool, a wear sleeve  14 , an axially movable piston  16 , and at the distal or working end of the tool, a chuck  18 , a bit  20  and bit retaining rings  22 . In operation, pressurized fluid supplied to the backhead and distributor assembly  12  is used to selectively drive the piston  16  to reciprocatingly translate and to strike the bit  20 , thus causing the bit to exert an impact force on any adjacent material to be drilled. 
     The backhead assembly  12  includes a backhead  24  and a distributor or check valve assembly  26 . The backhead  24  is an elongate member having an exposed proximal end  28  with a connection  30  for attachment to a source of pressurized fluid. The backhead  24  also has a tool receiving portion  32  shaped to receive a tool, e.g., a wrench, to assist in installing and removing the backhead. Adjacent the tool receiving portion  32  is a threaded portion  34 . In the illustrated embodiment, an outer diameter of the backhead is stepped down at a shoulder  33  immediately adjacent the threaded portion  34 . 
     The backhead  24  has an open distal end  36  defining one end of an axial bore  38 . The distributor  26  is fit within the bore  38  and is coupled to the backhead  24 , e.g., by a securing member accessible from an exterior of the backhead, such as a pin  40 , as described below in more detail. The distributor has an elongated guide  42  that extends distally. 
     The piston  16  has a proximal end  44  slidably received in the bore  38  and a distal end  46  slidably received within the wear sleeve  14 . The wear sleeve  14  is removably connected at its proximal end to the backhead and distributor assembly  12 , such as by the threaded portion  34 . The wear sleeve  14  extends distally in the drilling direction, and the chuck  18  is attached at its distal end. The chuck  18  receives the bit  20 , which can be held in place by the bit retaining rings  22 . 
     Referring to  FIG. 2 , the distributor  26  has a check valve  48  with a cap-shaped sealing member  50 , a biasing member  52  and stationary member  54  from which the elongated guide  42  extends. The stationary member  54  has a transverse bore  56  sized to receive the pin  40  and a circumferential groove  58  for a seal. 
     Referring to  FIG. 3 , the axial bore  38  has an inlet bore segment  60  extending from the proximal end  28  of the backhead  24  that widens into a chamber  62 . At its distal end, the chamber  62  narrows slightly into a necked-down portion defining a check valve receiving area  64  that receives the stationary member  54  of the distributor  26  as shown. At the distal end of the check valve receiving area  64 , the bore  38  is widest and the inner surface thereof defines a cylinder portion  65  within which the proximal end of the piston  16  is slidably received. The stationary member  54  and a seal in the groove  58  seal off the chamber  62  from the bore  38  when the valve is in normal operation. Thus, the check valve receiving area  64  separates the cylinder portion  65  from an inlet area  67  extending proximally of the check valve receiving area  64 . 
     The backhead  24  has at least one through passage  68  which connects the chamber  60  with an axially extending annular space  70  in the wear tube  14 . In a representative embodiment as shown in  FIGS. 3 and 4 , there are multiple circumferentially-spaced fluid through passages  68 . Adjacent the distal end  36 , the backhead  24  has circumferentially spaced external grooves  72  that also serve as flow passages between the backhead and the surrounding wear sleeve  14 . As best shown in  FIG. 3 , the backhead  24  and wear sleeve  14  can be shaped such that the backhead has a close fit with the wear sleeve adjacent the distal end  36 , and is spaced from the wear sleeve along at least a segment of its length in the area of the annular space  70 . 
     In one embodiment, as best shown in  FIG. 4 , the securing member is the pin  40  and the backhead  24  has a transverse opening  66  sized to receive the pin  40 . In the  FIG. 4  embodiment, unthreading the backhead  24  exposes the pin  40  and thus allows the distributor  26  to be removed from the backhead  24 . Of course, it is also possible to use a securing member of a type other than a pin. 
     In another embodiment, as best shown in  FIG. 6 , the securing member is a pair of pins  41   a  and  41   b,  each of which can be inserted into a respective one of the openings  67   a ,  67   b  in the backhead and distributor to removably secure the distributor in place relative to the backhead. The openings  67   a ,  67   b  are parallel and spaced from each other, extending in a direction transverse to the backhead. As best shown in  FIG. 7 , the pins  41   a ,  41   b  engage opposite sides of a groove  69  formed in the backhead. Of course, it would be possible to use additional pins or elements, and/or to use elements extending only partially through the backhead. 
       FIG. 5A  shows a conventional fluid-operated drilling tool  110  having a backhead  124 , an inner cylinder component  113  separate from the backhead  124  and a distributor (or check valve assembly)  126  coupled to the wear sleeve  114 . The backhead  124  is connected to the wear sleeve  114  by a threaded connection. Because the inner cylinder component  113  is also a separate component, it must also be coupled to the wear sleeve  114 . As shown in  FIG. 5A , the inner cylinder component  113  is coupled to the wear sleeve  114  by retaining members  115  that expand to fit within a circumferential groove  117  formed in the wear sleeve at a position spaced from its proximal end. 
     With the conventional tool  110 , removing the distributor  126  can be very difficult. The distributor  126  might need to be removed in order to repair or service it, to use it in a new wear sleeve  114 , to replace or service the piston  116 , etc. To remove the distributor  126 , the backhead  124  is unscrewed from the wear sleeve  114 . A tool is then inserted into the wear sleeve  114  in an effort to contact the retaining members  115  and disengage them from the groove  117 . This operation is often very difficult to execute, especially in conditions encountered in the field. With small versions of the tool  110 , a user can sometimes succeed in disengaging the distributor  126  by inverting the wear sleeve  114  and hitting its proximal end against a hard surface. With larger versions of the tool  110 , it is not possible to maneuver the wear sleeve in this way. 
     By comparison, the tool  10  with the new backhead and integrated cylinder as shown in  FIG. 5B  allows comparatively easy disassembly. The backhead assembly  12  is unscrewed from the wear sleeve  14 , and the distributor  26  can be removed from the backhead  24  by removing the securing member, e.g., removing the pin  40 . With the backhead assembly  12  unscrewed from the wear sleeve  14 , the piston  16  is easily accessible and can be slid out of the wear sleeve  14 . 
     The wear sleeve  14  does not require any complicated machining to form a groove or other undercut retaining feature similar to the groove  117 , and thus is simpler and cheaper to produce. Without these features, the walls of the wear sleeve can be made thinner. Stated differently, for a given external diameter, such as for the 4-inch tools  10  and  110 , the wear sleeve  14  can accommodate a piston  16  having an area at least about 5% greater than the piston  116 , as is described below in greater detail. 
     The new backhead assembly  12  with the integrated distributor  26  conserves operating length in the axial direction. Thus, the tool  10  can have a shorter length than the conventional tool  110  with the same or comparable operating capabilities. As a result, the tool  10  can save costs and is easier to handle. 
     In the following description, a comparison of the flow passageways and piston areas between the conventional drilling tool  110  and the drilling tool  10  is described. 
       FIG. 8A  and  FIG. 8B  are section views in elevation showing the conventional drilling tool and a drilling tool according to an embodiment of this application, respectively, in the impact position, i.e., when the piston has contacted the bit, which in turn exerts an impact on any material with which the bit is in contact.  FIGS. 9A and 9B  are similar to  FIGS. 8A and 8B , but shown the respective drilling tools in a drop open position, when the tools have been brought to rest, such as, e.g., if a void is encountered while drilling.  FIGS. 10A and 10B  are similar to  FIGS. 8A and 8B , but show the respective drilling tools in a position when the piston is at the top of its stroke, i.e., withdrawn in the distal direction. 
       FIG. 11A  is an enlarged section view of a portion of the conventional drilling tool  110  shown in  FIG. 8A  (i.e., in the impact position). As seen in  FIG. 11A , the intake of compressed operating fluid, which occurs at one or more times during a complete operating cycle, forces the fluid to follow a flow path  180  through two substantial changes in direction. As a result, the flow&#39;s velocity is decreased and thus the time required to complete the intake is lengthened. Specifically, the intake flow path  180  has an upper segment  182  beginning in the passageway  192  between the wear sleeve  114  and the inner cylinder component  113 . Where the flow leaves the passageway  192 , the flow path  180  turns abruptly inward at a first sharp bend  186  and continues through the aperture  194  formed in a wall of the inner cylinder  113  along an intermediate segment  184 . After traveling through the aperture  194 , the flow encounters the solid wall of the piston  116 , so it makes another abrupt turn at a second sharp bend  188 . The flow path  180  then continues in a downward direction along a lower segment  190  in the direction of the arrow, which travels through an inner passageway  196  formed between an inner side of the inner cylinder  113  and the outer wall of the piston  116 , before leading into a region  198  between the piston  116  and the wear sleeve  114 . 
     As shown in the drawing, the intake flow must travel through two substantial bends, each of which is approximately 90 degrees along the mean flow path. As a result, velocity decreases substantially and momentum and energy are lost. Although only a single intake flow path  180  is represented for the portion of the conventional drilling tool  110  shown in  FIG. 11A , it should be noted that the conventional drilling tool has four equally spaced apertures  194 , and thus there are four corresponding intake flows following corresponding intake flow paths  180 . 
       FIG. 11B  is an enlarged view of a portion of the drilling tool  10  according to this application, taken from  FIG. 8B . As seen in  FIG. 11B , a comparable intake flow path  80  begins in the area of one of the grooves  72  formed in the backhead  24  and extends in a generally straight direction downward into a region  96  between the piston  16  and the wear sleeve  14 . 
     The flow path  80  as shown in  FIG. 11B , which corresponds to one of the grooves  72  appears as a single path. In fact, the flow path  80  comprises the area of all of the grooves  72  positioned around the entire circumference of the backhead  24 . Thus, the many grooves  72  of the drilling tool  10  comprise a greater flow area than the four apertures  194  in the conventional drilling tool  10 . 
     Because the flow path  80  is substantially free of sharp bends, loss of energy due to friction and decreases in velocity are reduced. Stated differently, the flow path  80  is much more energy efficient than the flow path  180  in the conventional drilling tool  110 . In addition, the flow path  80  does not force the intake flow through any apertures or other bounded openings at sharp angles to the flow&#39;s primary direction. Also, the intake flow passes along walls (e.g., the outer periphery of the backhead  24 ) rather than through them (compare the conventional drilling tool  110 , where the intake flow must pass through the inner cylinder  113 ). 
       FIG. 12A  is an enlarged section view of a portion of the conventional drilling tool  110  shown in  FIG. 9A  (drop open position). Similarly,  FIG. 12B  is an enlarged view of a portion of the drilling tool  10  according to this application, taken from  FIG. 9B . 
       FIG. 12A  shows the piston area, A C , against which the pressurized operating fluid can act to move the piston  116  in the conventional drilling tool  110 . As shown in  FIG. 12B , the piston area A N  in the drilling tool  10  is greater than the piston area A C . 
     The available piston area includes the area of the upper surface of the piston and other areas exposed to the pressure, which equate to the annular area bounded on the outside by the piston&#39;s outer diameter and on the inside by the piston&#39;s axial bore. In some embodiments, the area A N  exceeds the area A C  by about 5% to even about 25%. As an example, a 4-inch diameter drilling tool may have a piston area A C  of about 8.36 in 2 , whereas a 4-inch diameter drilling tool according to an embodiment of this application has a piston area A N  of about 9.13 in 2 , which is about 9.3% greater. 
     The greater available piston area in the drilling tool  10  allows the pressure acting on the piston  16  to move the piston more quickly, thus increasing the power of the piston. 
     In the drilling tool  10 , when bit is positioned in the drop open position, as best seen in  FIG. 9B , the upper end of the piston is spaced away any surrounding surfaces. Because the entire circumferential area around the piston  16  is open, this area fills with pressurized fluid quickly and thus pushes the piston  16  downward to the full drop open position shown in  FIG. 9B  faster. 
     By way of contrast, in the drilling tool  110 , the piston  116  remains in contact with the surrounding inner cylinder  113 . Thus, pressurized air tending to push the piston  116  downward into the full drop open position shown in  FIG. 9A  must be forced through the smaller area of the four apertures  194 . 
     In addition, as best seen by comparing  FIG. 9A  and  FIG. 9B , because the drilling tool  10  is designed to function without the upper end of the piston  16  being in contact with any surrounding structure in the drop open position, the piston  16  can be made shorter in length than the piston  116  of the conventional drilling tool  110  of a comparable overall outer diameter. 
       FIG. 13A  is an enlarged section view of a portion of the conventional drilling tool  110  shown in  FIG. 10A , where the piston  116  is in its uppermost position and just prior to commencing a downward stroke. As shown in  FIG. 13A , the intake flow follows the same flow path  180  that includes the two sharp bends  186 ,  188  as shown and as described above in connection with  FIG. 11A . 
     In addition, as the intake flow travels through the aperture  194  and then flows in a downward direction along the lower segment  190  within the inner passageway  196 , it encounters a volume of pressurized air in an area  121  ( FIG. 10A ) surrounding the piston  116 . At the position shown in  FIG. 13A , the piston  116  has just moved upward (i.e., from the position shown in  FIG. 11A ) such that its upper end is no longer sealed against the inner cylinder  113 , thereby creating an upper opening  119  (i.e., the space between the outer surface of the piston and the inner relieved surface of the inner cylinder  113 , which has a larger diameter) in the passageway  196 . The opening  119  thus connects the lower portion of the passageway  196  with the space above the upper surface of the piston  116 , which is at a lower pressure. Once the opening  119  is established, the higher pressure fluid in the area  121  seeks to expand, thus exerting an upward pressure in the direction of the opening  119 . At the same time, however, a portion of the intake flow from the aperture  194  is seeking to flow downwardly through the passageway  196  (a portion of the intake flow also flows upwardly as shown). Thus, the intake flow, particularly along the lower segment  190 , must overcome the oppositely directed pressure  210  from the area  121 . As shown by the arrows, this confrontation takes place in a highly constricted area. Because of this confrontation, the intake flow along the intake flow path  180  experiences greater energy losses and its resulting velocity is lower. 
       FIG. 13B  is an enlarged section view of a portion of the drilling tool  10  according to this application, taken from  FIG. 10A . In contrast to the conventional drilling tool  110 , the intake flow path  92  and the filling flow path  200  are arranged for increased efficiency. First, the number of sharp bends in the intake flow path is reduced (in the case of the flow path  92 , there are no sharp bends). Second, the intake flow path  92  and the filling flow path  200  are configured so that they are spaced apart from each other rather than directed along nearly the same axis. Third, the area where the intake flow and the filling flow first encounter each other (i.e., where they are first no longer separated from each other by a wall), has a much larger cross section to promote separation between the flows. 
     Comparing  FIG. 13B  to  FIG. 13A , it can be seen that the cross-sectional area of the inner passage way  96  in the area where the flows  92  and  200  pass each other is much greater than the area of the inner passageway  196  adjacent the aperture  194 . In addition, as described above, there only four such flow path areas as shown in  FIG. 13A , whereas there are a much greater number of the flow paths shown in  FIG. 13B . Therefore, compared to the conventional drilling tool, the flow  92  in the drilling tool  10  flows with much less energy loss due to conflict with the flow  200 , and vice versa. 
     In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples and should not be taken as limiting in scope. Rather, the scope is defined by the following claims.

Summary:
A portion of a drill assembly operated by a supply of compressed fluid comprises a backhead, a cylinder portion, a hollow elongate wear sleeve and a piston. The backhead has a proximal end connectable to the supply, an axial bore and an open distal end having an integrated cylinder portion defined therein. The backhead has passages extending between the axial bore and the outer surface of the backhead. The cylinder portion is aligned with and in selective fluid communication with the backhead. The hollow elongate wear sleeve surrounds the cylinder portion and is connected to the backhead. The piston is housed by the wear sleeve and has a proximal end shaped to fit within the cylinder portion. The piston is slideably movable along the wear sleeve and the cylinder portion in response to compressed fluid conveyed through the backhead. When the drill assembly is in a drop open position, the proximal end of the piston is radially spaced apart from a nearest surrounding surface.