Abstract:
A magazine assembly for a fastening tool. The magazine assembly slides on guide posts that are formed into the nose assembly of the fastening tool and is clamped to the fastening tool via a magazine clamp assembly that requires no tools to operate. The magazine clamp assembly may be partially released to permit the magazine assembly to be partially withdrawn from the nose assembly so that the nose assembly may be maintained without the complete removal of the magazine assembly. The construction of the nose assembly is such that when the magazine assembly is placed in a partially withdrawn state, a portion of the nose assembly mechanically inhibits actuation of the fastening tool trigger system.

Description:
PRIORITY &amp; CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/267,359, filed Feb. 8, 2001. Other features of the present invention are discussed and claimed in commonly assigned copending U.S. application Ser. No. 10/072,668 entitled Pneumatic Fastening Tool. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to a fastening tool for dispensing fasteners from a magazine assembly into a workpiece and more specifically to an improved magazine assembly for a fastening tool. 
     BACKGROUND OF THE INVENTION 
     A number of pneumatically operated devices have been developed for use in driving fasteners, such as staples and nails, into workpieces. These tools typically employ a magazine assembly for holding a plurality of the fasteners and feeding the fasteners into the nose of the tool prior to the installation of the fasteners into a workpiece. 
     Despite the wide spread use of such tools, several drawbacks have been noted. One such drawback concerns the dry-firing of the tool when an insufficient number of fasteners are contained in the magazine assembly. As is known in the art, the dry-firing of such tools tends to be harmful to the tool. 
     Another drawback relates to situations wherein one or more fasteners are jammed in the nose of the tool. In such situations, the magazine assembly is typically removed from the fastening tool so as to provide sufficient space to permit the operator to remove the jammed fasteners from the nose of the fastening tool. Often times, tools, such as pliers, are employed in this task, so that the amount of space that is required for servicing the nose of the tool can be significant. Unfortunately, the complete removal of the magazine assembly from the remainder of the tool is often times very time consuming and may also require the use of additional tools to physically disconnect the magazine assembly. 
     SUMMARY OF THE INVENTION 
     In one preferred form, the present invention provides a fastening tool for holding a plurality of fasteners and selectively setting a first one of the fasteners into a workpiece. The fastening tool includes a fastening tool portion and a magazine assembly. The fastening tool portion includes a handle, a clamp mechanism, an actuating trigger and a nose structure. The handle is configured to be gripped by an operator when using the fastening tool. The clamp mechanism is coupled to the handle and includes a clamp pin with a head portion and a body portion. The clamp pin is movable between an engaged condition and a disengaged condition. The actuating trigger is positionable in an unactuated condition, wherein the fastening tool portion is not be cycled to set a fastener. The actuating trigger is also positionable in an actuated condition, wherein the fastening tool portion is cycled to set the fastener. The nose structure includes a magazine flange, a nose body, and at least one elongated magazine guide post. The nose body is coupled to the magazine flange and extends generally forwardly therefrom. The nose body is configured to hold at least the first one of the fasteners and to guide the first one of the fasteners into the workpiece when the trigger is positioned in the actuated condition and the fastening tool is actuated to set the first one of the fasteners. The at least one elongated magazine guide post extends downwardly from the magazine flange. The magazine assembly has an upper surface and at least one guide port. The upper surface is configured to abut a bottom surface of the magazine flange. Each of the guide ports is sized to receive an associated one of the guide posts. The magazine assembly further includes a magazine housing, a coupling bracket, a follower structure and a follower spring. The magazine housing has a sidewall that at least partially defines a follower cavity. The coupling bracket is coupled to the magazine housing and includes a slotted coupling aperture having a first portion, which is sized larger than the head portion of the clamp pin, a second portion, which is sized to engage the head portion, and a slotted portion interconnecting the first and second portions of the slotted coupling aperture. The slotted portion of the slotted coupling aperture is sized larger than the body portion of the clamp pin and smaller than the head portion of the clamp pin. The follower structure has a follower body and a guide tab. The follower body is slidably disposed in the follower cavity, the guide tab is coupled to the follower body and configured to support the fasteners in the magazine assembly, and the follower spring is coupled to both the magazine body and the follower structure and biases the follower body upwardly in the follower cavity. The magazine assembly is positionable relative to the fastening tool portion in an uncoupled condition, wherein the magazine assembly is separated from the fastening tool portion. The magazine assembly is also positionable relative to the fastening tool portion in a coupled condition, wherein the magazine assembly is fixed to the fastening tool portion such that the at least one elongated magazine guide post is engaged to an associated guide port, the upper surface of the magazine assembly is abutted to the bottom surface of the magazine flange, the clamp pin is disposed in the second portion of the slotted coupling aperture and the clamp mechanism is positioned in the engaged position and generating a clamping force that is applied through the head portion of the clamp pin and against the coupling bracket to thereby secure the magazine assembly to the handle. The magazine assembly if further positionable relative to the fastening tool portion in a semi-coupled condition, wherein the at least one elongated magazine guide post is engaged to the associated guide port, the clamp mechanism is positioned in the disengaged position, and the clamp pin is disposed in the slotted portion of the slotted coupling aperture to thereby permit the magazine assembly to be slid relative to the fastening tool portion. The slotted portion is sized to limit sliding movement of the magazine assembly so that the at least one elongated magazine guide post does not disengage the associated guide port. 
     In another preferred form, the present invention provides a fastening tool for holding a plurality of fasteners and selectively setting a first one of the fasteners into a workpiece. The fastening tool includes a fastening tool portion and a magazine assembly. The fastening tool portion has an actuating trigger, a nose structure, a contact trip and a trigger lever. The actuating trigger is positionable in an unactuated condition, wherein the fastening tool portion cannot be cycled to set the first one of the fasteners, and an actuated condition, wherein the fastening tool portion can be cycled to set the first one of the fasteners. The nose structure includes a magazine flange and a nose body. The magazine flange includes a lock-out aperture. The nose body is coupled to the magazine flange and extends generally forwardly therefrom. The nose body is configured to hold at least the first one of the fasteners and to guide the first one of the fasteners into the workpiece when the trigger is positioned in the actuated condition to thereby actuate the fastening tool portion to set the first one of the fasteners. The contact trip is coupled to the nose structure and is slidable between an extended position and a retracted position. The contact trip is biased into the extended position and slides into the retracted position or rearwardly therefrom in response to contact with the workpiece. The trigger lever is coupled to the contact trip for movement therewith. The trigger lever interacts with the actuating trigger such that the actuating trigger is positionable in the actuated condition only when the contact trip is positioned in or rearward of the retracted position to thereby push the trigger lever into engagement with the actuating trigger. The magazine assembly holds at least a portion of the plurality of fasteners. The magazine assembly has an upper surface that is configured to abut a bottom surface of the magazine flange. The magazine assembly further includes a magazine housing, a follower structure and a follower spring. The magazine housing has a sidewall that at least partially defines a follower cavity. The follower structure has a follower body, a guide tab and a lock-out dog. The follower body is slidably disposed in the follower cavity, the guide tab is coupled to the follower body and configured to support the plurality of fasteners in the magazine assembly, and the lock-out dog is coupled to the follower body and extends upwardly therefrom. The follower spring is coupled to both the magazine body and the follower structure and biases the follower body upwardly in the follower cavity. The lock-out dog is sized to extend through the lock-out aperture only when the quantity of fasteners in the magazine assembly is less than a predetermined quantity and contacts at least one of the contact trip and the trigger lever and inhibits the trigger lever from interacting with the actuating trigger to position the actuating trigger in the actuated condition. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a left side view of a tool constructed in accordance with the teachings of a preferred embodiment of the present invention; 
     FIG. 2 is a right side view of the tool of FIG. 1; 
     FIG. 3 is an exploded perspective view of the tool of FIG. 1; 
     FIG. 4 is a sectional view of the tool of FIG. 1 taken through its longitudinal axis; 
     FIG. 4 a  is a section view taken along the line  4   a — 4   a  of FIG. 4; 
     FIG. 5 is a top view of the tool of FIG. 1; 
     FIG. 6 is a sectional view taken along the line  6 — 6  of FIG. 5; 
     FIG. 7 is an enlarged portion of FIG. 4 illustrating the nose assembly in greater detail; 
     FIG. 8 is a front view of a portion of the tool of FIG. 1 illustrating the nose body and the contact tip in greater detail; 
     FIG. 9 is a sectional view taken along the line  9 — 9  of FIG. 2; 
     FIG. 9 a  is sectional view of a portion of the magazine clamp assembly illustrating the spring collar in greater detail; 
     FIG. 9 b  is a sectional view of a portion of the magazine clamp assembly illustrating the clamp pin in greater detail; 
     FIG. 10 is an enlarged portion of FIG. 4 illustrating the trigger assembly in greater detail; 
     FIG. 11 is an exploded view of the tool of FIG. 1; 
     FIG. 12 is an enlarged portion of FIG. 4 illustrating the rear of tool in greater detail; 
     FIG. 13 is a sectional view of a portion of the exhaust manifold illustrating the construction of the exhaust ports in greater detail; 
     FIG. 14 is an enlarged portion of FIG. 4 illustrating the engine assembly in greater detail; 
     FIG. 15 is an enlarged portion of FIG. 11 illustrating the engine assembly in greater detail; 
     FIG. 16 is a sectional view of the sleeve taken along its longitudinal axis; 
     FIG. 17 is a sectional view taken along the line  17 — 17  of FIG. 16; 
     FIG. 18 is a sectional view similar to that of FIG. 10 but illustrating the trigger assembly in an actuated condition; 
     FIG. 19 is an exploded perspective view of the magazine assembly; 
     FIG. 20 is a sectional view taken along the line  20 — 20  of FIG.  1  and illustrating the construction of the magazine body assembly; 
     FIG. 21 is a rear view of a portion of the magazine body assembly; 
     FIG. 22 is a side view of a portion of the magazine body assembly illustrating the L-shaped pin aperture in greater detail; 
     FIG. 23 is a top view of a guide structure; 
     FIG. 24 is a front view of the bracket structure; 
     FIG. 25 is a rear view of a portion of the bracket structure; 
     FIG. 26 is a side view of a portion of the bracket structure; 
     FIG. 27 is a side view of the follower structure; 
     FIG. 28 is a top view of a portion of the follower structure illustrating the construction of a portion of the follower body, the follower guide and the actuating lever; 
     FIG. 29 is a view of a portion of the follower structure illustrating the configuration of the forward leg of the follower body; 
     FIG. 30 is a view of a portion of the follower structure illustrating the configuration of the rearward leg of the follower body; 
     FIG. 31 is a front view of a portion of the follower structure; 
     FIG. 32 is a partial view of the follower structure from a side opposite the side which is illustrated in FIG. 27; 
     FIG. 33 is a side view of the follower spring; 
     FIG. 34 is a side view of the magazine end cap assembly; 
     FIG. 35 is a sectional view of a portion of the end cap structure taken along the line  35 — 35  in FIG. 34; 
     FIG. 36 is a sectional view of a portion of the end cap structure taken along the line  36 — 36  in FIG. 35; 
     FIG. 37 is a top view of a portion of the end cap structure; 
     FIG. 38 is a front view of the cam follower; 
     FIG. 39 is a partial side view of the cam follower; 
     FIG. 40 is an enlarged portion of the cam follower illustrated in FIG. 38; 
     FIG. 41 is a partial side view of the cam follower illustrating the follower hook in greater detail; 
     FIG. 42 is a partial section view illustrating the position of the cam follower on the pivot structure just prior to contact between the loading cam and the follower hook; 
     FIG. 43 is a partial section view similar to that of FIG. 42 but illustrating the cam follower when the follower hook is contacting the first loading cam portion; 
     FIG. 44 is a side view of the follower structure engaged to the magazine end cap assembly; 
     FIG. 45 is a section view taken along the line  45 — 45  illustrating the follower hook disposed within the capture aperture; 
     FIG. 46 is a side view of a portion of a tool constructed in accordance with the teachings of the an alternate embodiment of the present invention illustrating the magazine assembly removed from the tool; and 
     FIG. 47 is a side view similar to that of FIG. 46 but illustrating the magazine assembly coupled to the tool. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1 of the drawings, a fastening tool constructed in accordance with the teachings of the present invention is generally indicated by reference numeral  10 . Fastening tool  10  is illustrated to include a detachable magazine assembly  20  and a fastening tool portion  30 . The fastening tool portion  30  includes a nose assembly  40 , a housing assembly  42 , a cap assembly  44 , an engine assembly  46  and a trigger assembly  48 . 
     Nose Assembly 
     With reference to FIGS. 1 through 9, the nose assembly  40  is illustrated to include a nose structure  50 , a contact trip  52 , a trigger lever  54  and a contact trip-return spring  56 . The nose structure  50  includes a nose body  60 , a pair of magazine stabilizing tabs  62 , a magazine flange  64 , a pair of magazine guide posts  66 , a mounting base  68 , a spring post  70  and a pair of contact trip guides  72 . The nose body  60  is generally U-shaped, with the legs  80  of the “U” being inwardly offset to form a semi-circular blade cavity  82 . The inwardly offset legs  80  of the nose body  60  also serve as a guide surface  84  for guiding the lower front portion  86  of the contact trip  52 . The contact trip guides  72  are coupled to the top of the nose body  60  and form a guide surface for guiding the portion  88  of the contact trip  52  that extends over the nose body  60 . 
     The magazine stabilizing tabs  62  are situated on opposite sides of the nose body  60  and are spaced apart by a predetermined distance. The magazine flange  64  is a generally flat structure that is coupled to the bottom of the nose body  60  and that includes a lock-out dog aperture  90 . The magazine guide posts  66 , which are cylindrically shaped in the particular embodiment illustrated, extend downwardly and rearwardly from the magazine flange  64 . The magazine stabilizing tabs  62 , magazine flange  64  and magazine guide posts  66  are discussed in greater detail, below. 
     The mounting base  68  is coupled to the magazine flange  64  and the nose body  60  and includes a pair of mounting apertures  94 , a nose seal groove  96  and a nose guide  98 . The nose guide  98  is generally cylindrically shaped and includes an internal cavity  100  that having a cross-section that is configured to receive the fastener F and which may include a fastener stop  102  which is configured to prevent the fasteners F from traveling rearwardly toward the engine assembly  46 . In the embodiment illustrated, the internal cavity  100  is generally semi-circular in shape but which includes a key-shaped fastener stop  102 . The nose seal groove  96  is formed around the outer perimeter of the nose guide  98  and is sized to receive a nose seal  104 , which is an O-ring seal in the particular embodiment illustrated. The spring post  70  is coupled to the top of the mounting base  68  and includes a boss  108  that is sized to fit within the contact trip-return spring  56 . 
     The contact trip  52  is fit over and slides on the nose body  60 , being guided thereon by the inwardly offset legs  80  of the nose body  60  and the contact trip guides  72 . Preferably, the effective length of the contact trip  52  is adjustable so as to permit the tool operator to vary the depth at which the tool  10  sets the fasteners F. A spring protrusion  110 , which is sized to engage the inside diameter of the contact trip-return spring  56 , is formed in the rear of the contact trip  52 . The contact trip-return spring  56  is set over the boss  108  on the spring post  70  and the spring protrusion  110  on the contact trip  52  and exerts a spring force that biases the contact trip  52  away from the spring post  70 . Forward motion of the contact trip  52  is checked by a contract trip stop  114  that is formed onto a side of the nose body  60  and which contacts the contact trip  52  at a predetermined point. 
     The trigger lever  54  is fixedly coupled to the contact trip  52  at a first end  120  and extends rearwardly from the nose structure  50  where a second end  122  engages the trigger assembly  48  in a conventional manner that is well known in the art. Briefly, the trigger assembly  48  includes a primary trigger  126 , a secondary trigger  128  and a trigger valve  130  that selectively controls the flow of compressed air to the engine assembly  46 . The primary trigger  126  is pivotably mounted to the housing assembly  42  and movable in response to the tool operator&#39;s finger. Movement of the primary trigger  126  will not, in and of itself, alter the state of the trigger valve  130 . Rather, the second end  122  of the trigger lever  54  must also move rearwardly and into contact with the secondary trigger  128  before the state of the trigger valve  130  is changed to permit compressed air to flow to the engine assembly  46 . A stop member  134 , which is configured to interact with the magazine assembly  20  in a matter that will be discussed in greater detail below, is coupled to the trigger lever  54  below the magazine flange  64  and extends inwardly toward the nose body  60 . In the particular embodiment illustrated, the stop member  134  is die-punched into the trigger lever  54  and is offset inwardly therefrom toward the nose body  60 . 
     Housing Assembly 
     Housing assembly  42  includes a unitarily formed housing  150 , a piston bumper  152 , a magazine clamp assembly  154  and a housing seal  156 , which is illustrated to be an O-ring seal in the example provided. The housing  150  includes a housing body  160 , a trigger housing  162 , a nose housing  164  and a handle portion  166 . The housing body  160  is a container-like structure having a front base  170  and an outwardly tapering sidewall  172  that cooperate to form a housing cavity  174 . The outwardly tapering sidewall  172  terminates at the rear of the housing body  160  at a rear housing face  176 , which in the particular embodiment illustrated, includes a housing seal groove  178  that is configured to receive the housing seal  156 . A guide bore  180  is formed into the inside face  182  of the housing cavity  174  and terminates at its forward end at a guide stop  184 . A nose guide aperture  188  is formed through the front base  170  of the housing body  160 . 
     The nose housing  164  is coupled to the front base  170  of the housing body  160  and extends forwardly therefrom. The nose housing  164  includes an upper shroud  200 , a pair of sidewalls  202  and a pair of spaced apart bosses  204 , each of which having a threaded aperture  206 . The upper shroud  200 , sidewalls  202  and spaced apart bosses  204  cooperate to locate the nose assembly  40  to the housing  150  and the nose guide  98  is inserted into the nose guide aperture  188 . Threaded fasteners  210  are placed through each of the mounting apertures  94  in the mounting base  68  and threadably engaged to the threaded apertures  206  in the spaced apart bosses  204  to fixedly but removably couple the nose assembly  40  to the housing  150 . The axis  212  of the threaded fasteners  210  is skewed toward the rear of the tool  10 , causing the threaded fasteners  210  to exert a clamping force that pushes the nose assembly  40  downwardly onto the spaced apart bosses  204  and rearwardly against the front face of the front base  170  to thereby compress the nose seal  104  and sealingly engage the nose structure  50  to the housing body  160 . The upper shroud covers the spring post  70 , the contact trip-return spring  56  and a portion of the rear of the contact trip  52  to prevent foreign objects from lodging between the rear of the contact trip  52  and the spring post  70 . 
     The handle portion  166  is preferably non-circular in shape and contoured to comfortably fit the hand of a tool operator. The distal end  250  of the handle portion  166  is enlarged so as to render the handle portion  166  less prone to slipping out of the tool operator&#39;s hand. With additional reference to FIG. 4 a , a clamp boss  252  is coupled to the forward face of the distal end  250  of the handle portion  166 . The clamp boss  252  includes a clamp boss base  254  that extends toward the front of the tool  10 , a clamp boss sidewall  256  that wraps around the perimeter of the clamp boss base  254  and an annular intermediate clamp boss wall  258  that cooperates with a portion of the clamp boss sidewall  256  to form a circular spring cavity  260 . The clamp boss base  254  and the clamp boss sidewall  256  cooperate to form a clamp cavity  262  into which the magazine clamp assembly  154  is disposed. A pair of U-shaped pin apertures  264 , which will be discussed in further detail below, are formed into an end of the clamp boss sidewall  256 . 
     The handle portion  166  intersects both the housing body  160  and the trigger housing  162  and includes an air inlet cavity  270  which extends through the distal end  250  of the handle portion  166  to receive a supply of compressed air. The air inlet cavity  270  extends through the handle portion  166  and into both the housing cavity  174  and the trigger housing  162  to permit the compressed air to be directed through the tool  10  in a predetermined manner that will be described in detail, below. 
     In the example provided, the magazine clamp assembly  154  is illustrated to include a clamp pin  300 , a compression spring  302 , a spring collar  304 , an actuating cam  306  and a coupling pin  308 . The clamp pin  300  includes a head portion  322 , a first body section  324 , which is coupled to the head portion  322 , and a second body section  326  that is coupled to the opposite end of the first body section  324 . The first body section  324  is generally cylindrically shaped and includes a pair of parallel flats  328 . The second body section  326  is generally cylindrically shaped but has an outer diameter that is smaller than that of the first body section  324 . The head portion  322  includes a frusto-conical abutting face  330 . 
     The spring collar  304  includes a first annular portion  340  having a diameter that is sized to fit within the compression spring  302 , and a second annular portion  342  that is relatively larger in diameter than the compression spring  302  and which has a flat contact surface  344 . A pin aperture  346  is formed through the spring collar  304  that is sized to receive the second body section  326  of the clamp pin  300 . 
     The actuating cam  306  has a base portion  350  and a leg portion  352  which are arranged relative to one another in an L-shape. The end of the base portion  350  opposite the intersection point  354  between the base and leg portions  350  and  352  includes a coupling pin aperture (not specifically shown) which is sized to engage the coupling pin  308 . The leg portion  352  of the actuating cam  306  is arcuate in shape and includes a plurality of gripping protrusions  356  or is otherwise textured on its inside surface so as to improve the tool operator&#39;s ability to move the actuating cam  306  in a desired direction. A slot  358 , which is sized to engage the second body segment  326  of the clamp pin  300  in a slip-fit manner, is formed into the actuating cam  306  through the base portion  350  and a portion of the leg portion  352 . 
     The clamp pin  300  extends through a pin aperture  360  formed into the clamp boss base  254  of the clamp boss  252  such that the second body section  326  extends into the spring cavity  260 . The compression spring  302  is positioned over the second body section  326  and into the spring cavity  260 . The spring collar  304  is placed over the second body section  326  such that the first annular portion  340  is disposed inside the compression spring  302 . The base portion  350  of the actuating cam  306  is positioned into contact with the flat contact surface  344  such that the second body segment  326  extends into the portion of the slot  358  that is formed into the base portion  350  of the actuating cam  306 . The coupling pin  308 , which is a roll-pin in the example illustrated, is positioned into one of the U-shaped pin apertures  264  and driven through the base portion  350  of the actuating cam  306  and into engagement with a pin aperture  364  in the second body segment  326  of the clamp pin  300 . Accordingly, the coupling pin  308  pivotably couples the actuating cam  306  to the clamp pin  300 . Rotation of the actuating cam  306  about the coupling pin  308  places the intersection point  354  into contact with the flat contact surface  344 , causing the spring collar  304  to compress the compression spring  302  and transmit a clamping force to the head portion  322  of the clamp pin  300 . When the actuating cam  306  has been pivoted sufficiently so as to place the leg portion  352  into contact with the flat contact surface  344 , the force exerted by the compression spring  302  urges the spring collar  304  against the leg portion  352  to releasably lock the actuating cam  306  in place. The clamp cavity  262  protects the actuating cam  306  from being contacted during the operation of the tool  10 , thereby guarding against the inadvertent unlocking or releasing of the actuating cam  306 . 
     In FIG. 10, the trigger housing  162  is configured to receive the trigger assembly  48  and includes a supply port  370 , which is coupled to the air inlet cavity  270  to provide the trigger assembly  48  with a source of compressed air. A biasing port  372  extends from the trigger housing  162  through the guide bore  180  in the housing cavity  174  that permits the trigger assembly  48  to direct air to or exhaust air from the housing cavity  174 . 
     As shown in FIGS. 7 and 11, the piston bumper  152  is a unitarily formed molded elastomeric structure. In the particular example illustrated, the piston bumper  152  has a cylindrical body portion  390  and an annular lip  392 . The cylindrical body portion  390  preferably includes a first annular bumper portion  396  and a second annular bumper portion  398  that is generally larger in diameter than the first annular bumper portion  396  and which is disposed between the first annular bumper portion  396  and the annular lip  392 . The annular lip  392  extends radially outwardly of the body portion  390  and includes a front abutting face  400  that is configured to abut the inside surface  402  of the housing body  160  and sealingly engage the front base  170  of the housing body  160 . The annular lip  392  also includes a rear abutting face  404  having a first annular lip portion  406  and a second annular lip portion  408  that lies radially outwardly of and recessed forwardly relative to the first annular lip portion  406 . The rear abutting face  404  and a cylindrically-shaped driver blade aperture  410  that extends through the center of the piston bumper  152  will be described in detail, below. 
     Cap Assembly 
     With reference to FIGS. 11 and 12, the cap assembly  44  includes a cap housing  420 , an exhaust manifold  422  and a top bumper  424 . The cap housing  420  includes an outer cap wall  430  that is generally flat at the rear of the tool  10 , but folds over on its sides to form a cup-like container having a generally flat forward face  432  that is configured to engage the housing seal  156  to permit the cap housing  420  to be sealingly coupled to the rear of the housing  150 . 
     The cap housing  420  also includes a plurality of foot tabs  434 , a plurality of strengthening gussets (not specifically shown), an annular exhaust port wall  438 , an exhaust button  440  and a cylindrical locating hub  442  having a threaded aperture  444  formed therethrough. The foot tabs  434  extend forwardly from the flat portion of the outer cap wall  430  beyond the front face  432  by a predetermined distance. The outside diameter of the foot tabs  434  is sized such that the foot tabs  434  fit within the housing cavity  174 . The foot tabs  434  will be discussed in greater detail, below. The strengthening gussets are employed to couple both the foot tabs  434  or the outer cap wall  430  to the annular exhaust port wall  438 , which extends forwardly from the flat rear portion  446  of the outer cap wall  430 . The exhaust button  440  is an annular member that also extends forwardly from the flat rear portion  446  of the outer cap wall  430  but which is spaced apart from the annular exhaust port wall  438  and the locating hub  442 . A plurality of primary exhaust ports  450  are formed through the exhaust button  440  and a plurality of secondary exhaust ports  452  are formed through the portion of the outer cap wall  430  between the annular exhaust port wall  438  and the exhaust button  440 . 
     The exhaust manifold  422  is preferably unitarily formed from a molded from a plastic material and includes a center hub  460 , an annular spacing wall  462  and an annular manifold wall  464 . The center hub  460  is configured to fit between the exhaust button  440  and the locating hub  442  and includes a hub aperture  468  that is configured to engage the locating hub  442  in a slip fit manner. The annular spacing wall  462  is coupled to the forward-most portion of the center hub  460  and is spaced apart from the exhaust button  440 . The annular manifold wall  464  is coupled to the outer perimeter of the annular spacing wall  462  and includes a plurality of circumferentially extending exhaust slots  470  that are spaced around the circumference of the annular manifold wall  464 . The exhaust slots  470  are generally U-shaped and as best shown in FIG. 13, have a rear edge  472  that tapers rearwardly and inwardly toward the center hub  460 . 
     Returning to FIGS. 11 and 12, the top bumper  424  preferably includes a dampening member  480  that is molded from an elastomeric material, such as urethane, and a structural member  482 , such as a washer, that is molded into the dampening member  480 . The dampening member  480  is a cup-shaped structure that is sized to fit within the center hub  460  of the exhaust manifold  422 . The dampening member  480  includes an annular wall  484  that extends forwardly from the base  486  of the dampening member  480 . A ridge  488  is formed into the forward end of the annular wall  484 , thereby creating a groove  490  between the base  486  of the dampening member  480  and the ridge  488 . A plurality of slits  492  are formed into the annular wall  484 , creating a plurality of wall segments  494  that are flexibly coupled to the base  486 . A threaded fastener  496  is threadably engaged to the threaded aperture  444  in the locating hub  442  to fixedly but removably couple the top bumper  424  to the cap housing  420 . The structural member  482  is employed so as to permit the clamping force that is exerted by the threaded fastener  496  to be transmitted through the top bumper  424  without crushing the base  486  of the dampening member  480 . A portion of the clamping force is transmitted through the base  486  of the dampening member  480  and into the center hub  460  of the exhaust manifold  422  to maintain the exhaust manifold  422  in a stationary position relative to the cap housing  420 . 
     Engine Assembly 
     Engine assembly  46  is shown to include a cylinder assembly  500 , a piston assembly  502 , a rod or driver blade  504 . The cylinder assembly  500  includes a hollow, cylindrical, and unitarily constructed sleeve  510 , an inner exhaust port seal  512 , an outer exhaust port seal  514 , a cap flange seal  516 , rear and front guide seals  518  and  520 , a guide assembly  522 , a compensating valve  524 , a rear spring flange  526 , a spring  528 , a front spring flange  530  and a front spring flange seal  532 . In the particular embodiment illustrated, inner exhaust port seal  512 , outer exhaust port seal  514 , rear and front guide seals  518  and  520  and front spring flange seal  532  are conventional, commercially available O-ring seals. The cap flange seal  516  is a molded elastomeric seal having an outside surface with a generally flat seal face  540  and first and second radially inwardly extending flanges  542  and  544 , respectively, that are spaced apart from one another to form an engagement groove  546  therebetween. 
     With additional reference to FIG. 16, the sleeve  510  is shown to include a first sleeve body portion  550 , an annular sleeve flange  552 , a second sleeve body portion  554  having a maximum outer diameter that is generally the same as that of the first sleeve body portion  550  and a third sleeve body portion  556  having a maximum outer diameter that is generally larger than that of the first sleeve body portion  550 . The first sleeve body portion  550  includes a first U-shaped seal groove  560 , which is sized to receive the front spring flange seal  532 , a plurality of circumferentially-spaced front exhausting ports  562 , a spring flange groove  564 , which is sized to receive the rear spring flange  526 , a valve groove  566 , which is discussed in greater detail, below, and a second U-shaped seal groove  568 , which is sized to receive the front guide seal  520 . 
     The valve groove  566  has a first U-shaped portion  570 , a second U-shaped portion  572  and a plurality of valve apertures  574 . The first U-shaped portion  570  is sized to receive the compensating valve  524 , which in the particular embodiment illustrated, is a flat elastomeric band  580 . The second U-shaped portion  572  is disposed within the first U-shaped portion  570 , but has a diameter that is somewhat smaller than that of the first U-shaped portion  570  so as to define an annular ring that extends around the circumference of the first U-shaped portion  570 . In the particular embodiment illustrated, the diameter of the second U-shaped portion  572  is about 0.010 inches to about 0.030 inches smaller in diameter than the first U-shaped portion  570 . The valve apertures  574  are illustrated to be relatively small diameter holes that are located within the second U-shaped portion  572  and which are drilled through the sleeve  510 . The valve apertures  574  will be discussed in greater detail, below, as will the set of front exhausting ports  562  that are located between the first U-shaped seal groove  560  and the spring flange groove  564 . 
     The annular sleeve flange  552  extends radially outwardly from the first sleeve body portion  550  of the sleeve  510  and separates the first and second sleeve body portions  550  and  554  from one another. A third U-shaped seal groove  584 , which is sized to receive the rear guide seal  518  is formed into the outer surface of the annular sleeve flange  552 . 
     The majority of the second sleeve body portion  554  of the sleeve  510  is of approximately the same outer diameter as the first sleeve body portion  550 . The rear end of the second sleeve body portion  554 , however, includes a flange portion  590  that extends radially outwardly to form a seal lip  592  and a fourth U-shaped seal groove  594  prior to its connection with the third sleeve body portion  556 . The seal lip  592  is configured to engage the engagement groove  546  formed into the cap flange seal  516  and abut the first and second radially inwardly extending flanges  542  and  544 . The fourth U-shaped seal groove  594  is configured to receive a portion of the first radially inwardly extending flange  542 . 
     The third sleeve body portion  556  is fixedly coupled to the end of the second sleeve body portion  554  and is larger in diameter than the outer diameter of the first sleeve body portion  550 . A fifth U-shaped seal groove  600  is formed into the outer surface of the third sleeve body portion  556  and is sized to receive the outer exhaust port seal  514 . A plurality of circumferentially extending rear exhaust slots  604  are disposed around the perimeter of the third sleeve body portion  556 . The rear exhaust slots  604  are located between the fourth and fifth U-shaped seal grooves  594  and  600 . A sixth U-shaped seal groove  608 , which is configured to receive the inner exhaust port seal  512 , is formed into the inner diameter of the third sleeve body portion  556 . 
     The hollow cavity  610  that is formed through the sleeve  510  has a first cavity portion  612  that is generally of a constant diameter over the portion of its length that includes the first and second sleeve body portions  550  and  554  and the annular sleeve flange  552 . The hollow cavity  610  also has a second cavity portion  614  having a larger diameter than that of the first cavity portion  612 . 
     In FIG. 14, the guide assembly  522  is shown to include a guide  650  and first and second housing seals  652  and  654 , which in the particular embodiment illustrated, are O-ring seals. The guide  650  is a molded plastic component, having a stepped-diameter body portion  660 , a plurality of longitudinally extending legs  662 , a locating tab  664  and a plurality of stop tabs  668 . The stepped-diameter body portion  660  includes a flange bore  670 , which is sized to receive the annular sleeve flange  552  and sealingly engage the rear guide seal  518 , a body bore  672 , which is sized to receive the first sleeve body portion  550  and sealingly engage the front guide seal  520 , and an abutting flange  676  that forms the transition between the flange bore  670  and the body bore  672 . 
     The longitudinally extending legs  662  extend away from the stepped-diameter body portion  660  and are spaced apart circumferentially in equal amounts. The locating tab  664  is positioned on the same side of the stepped-diameter body portion  660  as the longitudinally extending legs  662  between two of the longitudinally extending legs  662 . The locating tab  664  is employed to signify the presence of an air gallery  680  and locate the guide assembly  522  relative to the housing assembly  42 . The air gallery  680  is configured to permit air to flow through the stepped-diameter body portion  660  from a point between the first and second housing seals  652  and  654  through the stepped-diameter body portion  660  and out the abutting flange  676 . 
     The rear and front guide seals  518  and  520  and the elastomeric band  580  that forms a portion of the compensating valve  524  are initially installed to the sleeve  510 . Thereafter, the guide assembly  522  is positioned over the first sleeve body portion  550  and pushed onto the sleeve  510  such that the flange bore  670  and body bore  672  are sealingly engaged to the rear and front guide seals  518  and  520 , respectively, and the abutting flange  676  abuts the annular sleeve flange  552 . 
     The rear spring flange  526  is next installed to the sleeve  510 . The rear spring flange  526  is a plastic collar that is split on one side to permit the ends of the rear spring flange  526  to be spread apart so that it may be loaded onto the first sleeve body portion  550  of the sleeve  510  and into the spring flange groove  564 . The rear spring flange  526  has a cylindrically shaped body portion  690  and a flange portion  692  that extends radially-outwardly from the body portion  590  in a manner that provides the rear spring flange  526  with a L-shaped cross-section. The rear spring flange  526  is located to the spring flange groove  564  such that the flange portion  692  is nearest the annular sleeve flange  552 . 
     The front spring flange  530  is a plastic collar having a tapering outside diameter  596  and a generally flat rear face  698 . The inside surface  700  of the front spring flange  530  is generally cylindrical, but includes an annular protrusion  702  that extends radially inwardly of the remainder of the inside surface  700  and which engages the first sleeve body portion  550  of the sleeve  510  in a slip-fit manner. 
     The spring  528  is a conventional compression spring having both ends ground flat. The spring  528  is disposed over the first sleeve body portion  550  of the sleeve  510  such that its rear end abuts the flange portion  692  of the rear spring flange  526 . Thereafter, the front spring flange  530  is positioned such that its rear face  698  contacts the second end of the spring  528 . The front spring flange  530  is pushed toward the annular sleeve flange  552  to compress the spring  528  a sufficient distance to permit the front spring flange seal  532  to be inserted into the first U-shaped seal groove  560 . Thereafter, the front spring flange  530  is moved toward the front of the sleeve  510  such that the front spring flange seal  532  is sealingly engaged with the inside surface  700  of the front spring flange  530 . The rear side of the front spring flange seal  532  contacts the annular protrusion  702  to limit the forward travel of the front spring flange  530  prior to the installation of the engine assembly  46  to the housing assembly  42 . Forward motion of the guide assembly  522  along the sleeve  510  is checked by contact between the stop tabs  668  and the rear surface of the flange portion  692  of the rear spring flange  526  to thereby prevent the guide  650  from becoming disengaged from the rear and front guide seals  518  and  520 . Construction in this manner is highly advantageous in that it permits the entire cylinder assembly  500  to be pre-assembled outside of the housing assembly  42  in a relatively easy and cost efficient manner. 
     The piston assembly  502  includes a piston  720  and a ring  722 . In the example provided, the piston  720  is shown to include a first piston portion  730  and a second piston portion  732 . The first piston portion  730  in an annular member that is smaller in diameter than the first cavity portion  612  of the hollow cavity  610  in the sleeve  510 . A U-shaped annular ring groove  734  is formed around the circumference of the first piston portion  730  that is sized to receive the ring  722 . In the embodiment illustrated, the ring  722  is shown to be fabricated from a plastic material and have a rectangular cross-section. The ring  722  is split to permit its ends of the ring  722  to be spread apart so that it may be loaded around the first piston portion  730  and into the ring groove  734 . The second piston portion  732  is an annular member that is smaller in diameter than the first piston portion  730 . The second piston portion  732  is coupled to the rear end of the first piston portion  730  and includes a pair of wrench flats  740  and a locking protrusion  744 , both of which will be discussed in more detail, below. A generous fillet radius  746  is employed at the intersection between the first and second piston portions  730  and  732  so as to reduce the concentration of stress within the piston  720 . 
     The construction of the driver blade  504  is largely conventional and as such, a detailed discussion of it is neither required nor within the scope of this disclosure. Briefly, the driver blade  504  is shown to include a coupling portion  760  and a driver body  762 . In the example provided, the coupling portion  760  includes a collar  764  and a threaded portion  766  which are formed into the rear end of the driver blade  504 . The wrench flats  740  on the second piston portion  732  are employed to facilitate relative rotation between the driver blade  504  and the piston  720  to permit the threaded portion  766  to threadably engage a threaded aperture  768  that is formed through the piston  720  and to permit the collar  764  to engage the front surface  770  of the piston  720  to generate a clamping force that fixedly but removably couples the piston  720  and the driver blade  504  together. Coupling of the piston  720  and the driver blade  504  via a threaded connection is presently preferred so as to permit the servicing and replacement of the driver blade  504 , since this portion of the tool  10  is essentially perishable. Those skilled in the art will understand, however, that other coupling mechanisms, such as press-fitting, shrink fitting, welding, or any other mechanical coupling method may also be employed. 
     The driver body  762  is sized to fit in the blade cavity  82  and is shown to include a keyway  774 , a slide surface  776 , a loading groove  778  and a tip portion  780 . The keyway  774  is illustrated to be a cut that is formed into the surface of the driver body  762  along its longitudinal axis. The fastener stop  102  that is formed into the internal cavity  100  in the nose guide  98  is disposed within the keyway  782  to guard against a situation wherein fasteners F feed rearwardly into the tool  10 . The slide surface  776  is generally flat and provides the driver body  762  with a relatively large surface that will consistently slide over the fasteners F that are loaded into the magazine assembly  20 . The tip portion  780  is formed at the front end of the driver body  762  and is operable for contacting the fasteners F and driving them into a workpiece. The loading groove  778  is cylindrically shaped and is formed along an axis that is skewed to the longitudinal axis of the driver blade  504  such that it intersects both the tip portion  780  and the slide surface  776 . The loading groove  778  is tapered such that it is deepest at the front of the driver blade  504 . The loading groove  778  ensures that only one fastener F is sheared from the remaining fasteners F in the magazine assembly  20 . The loading groove  778  also permits the fasteners F in the magazine assembly  20  to move upwardly toward the nose body  60  of the tool  10  prior to the time at which the driver blade  504  has stroked back to its rear-most (i.e., retracted) position to thereby minimize the lag time between the point at which the driver blade  504  has moved to its retracted position and the point at which the driver blade  504  can be moved forwardly to drive another fastener F. 
     With additional reference to FIGS. 16 and 17, the driver blade  504  and the piston assembly  502 , once coupled to one another, are inserted into the second cavity portion  614  of the hollow cavity  610  in the sleeve  510 . The diameter of the second cavity portion  614  is larger than the diameter of the piston assembly  502  (with the ring  722  in an expanded condition). A chamfer  790  is employed at the front of the second cavity portion  614  to facilitate the transition to the smaller-diameter first cavity portion  612 . With the exertion of light force onto the rear of the piston assembly  502 , the piston assembly  502  is moved forwardly in the hollow cavity  610  and into contact with the chamfer  790 . The chamfer  790  is operable for compressing the ring  722  to permit the piston assembly  502  to travel into the first cavity portion  612 . 
     Once assembled, the engine assembly  46  is placed into the housing cavity  174  such that the locating tab  664  is aligned to a tab slot  800  formed into the housing cavity  174  and the driver blade  504  is inserted through the driver blade aperture  410  in the piston bumper  152  and into the internal cavity  100  in the nose guide  98 . The engine assembly  46  is pushed forwardly into the housing cavity  174  to engage the guide assembly  522  against the guide stop  184 . In this position, the first and second housing seals  652  and  654  sealingly engage the guide bore  180  that is formed into the inside surface  182  of the outwardly tapering sidewall  172 . The first and second annular bumper portions  396  and  398  extend through the front face  810  of the sleeve  510  and into the hollow cavity  610 . The front face  820  of the front spring flange  530  sealingly contacts the second annular lip portion  408  on the piston bumper  152 . The cap assembly  44  is thereafter placed onto the rear end of the housing assembly  42  such that each of the longitudinally extending legs  662  contacts one of the foot tabs  434 . The foot tabs  434  cooperate with the longitudinally extending legs  662  to prevent the guide assembly  522  from moving along the longitudinal axis of the tool  10 . The sleeve  510 , however, is slidable within the guide assembly  522 , as will be discussed in greater detail, below. 
     Alternatively, the piston assembly  502  and driver blade  504  may be inserted into the housing cavity  174  such that the driver blade  504  is inserted through the driver blade aperture  410  in the piston bumper  152  and into the internal cavity  100  in the nose guide  98 . The cylinder assembly  500  is then loaded into the housing cavity  174  in the manner discussed above. A lead L formed into the front face  810  of the sleeve  510  that permits the ring  722  to be compressed so that the piston assembly  502  can travel rearwardly into the first cavity portion  612  of the hollow cavity  610  in the sleeve  510 . 
     Engine Operation 
     With reference to FIGS. 10,  14  and  16 , when the tool  10  has been coupled to a source of compressed air, the trigger assembly  48  maintains the trigger valve  130  in an unactuated state wherein compressed air is directed from the supply port  370  to the biasing port  372  where it enters the air gallery  680  at a point between the first and second housing seals  652  and  654 . Compressed air flows through the stepped-diameter body portion  660  and exits from the abutting flange  676  where it enters a sleeve return chamber  850  that is defined by the forward face  852  of the annular sleeve flange  552 , the rear guide seal  518 , the flange bore  670 , the body bore  672 , the front guide seal  520  and the first sleeve body portion  550  of the sleeve  510 . As the guide  650  is not movable within the housing  150 , the pressure of the air that is in the sleeve return chamber  850  is exerted against the front face  852  of the annular sleeve flange  552  to bias the sleeve  510  in a rearward direction. 
     The air inlet cavity  270  also provides compressed air to a sleeve extend chamber  860  that is defined by the rearward face  862  of the annular sleeve flange  552 , the rear guide seal  518 , the guide  650 , the second housing seal  654 , the portion of the outwardly tapering sidewall  172  that is situated rearwardly of the second housing seal  654 , the outer portion of the cap housing  420  that includes the annular exhaust port wall  438 , the cap flange seal  516  and the second sleeve body portion  554  of the sleeve  510 . Compressed air in the sleeve extend chamber  860  directs force to both the rearward face  862  of the annular sleeve flange  552  and the front face  864  of the flange portion  590  of the second sleeve body portion  554  of the sleeve  510 . 
     The forces that act on the annular sleeve flange  552  and the front face  864  of the flange portion  590 , in cooperation with the force that is exerted by the spring  528 , bias the sleeve  510  in a rearward direction into its retracted position such that the flat seal face  540  of the cap flange seal  516  sealingly engages the front face  866  of the annular exhaust port wall  438 . 
     With reference to FIGS. 10 and 12, when the sleeve  510  is in the retracted position, a primary exhaust chamber  870  is defined by the cap flange seal  516 , the inside surface  872  of the annular exhaust port wall  438 , the outer exhaust port seal  514 , the third sleeve body portion  556  of the sleeve  510 , the inner exhaust port seal  512 , the exhaust manifold  422 , the second sleeve body portion  554  of the sleeve  510 , the piston assembly  502  and the driver blade  504 . The position of the sleeve  510  relative to the cap assembly  44  is such that the air that is in the primary exhaust chamber  870  is permitted to flow between the third sleeve body portion  556  and exhaust manifold  422 , through the exhaust slots  470  in the exhaust manifold  422  and out the primary exhaust ports  450  in the exhaust button  440  where this air is vented to atmosphere. 
     With the sleeve  510  in the retracted position, a secondary exhaust chamber  880  is formed by the annular exhaust port wall  438 , the outer exhaust port seal  514 , the third sleeve body portion  556  of the sleeve  510 , the inner exhaust port seal  512 , the exhaust manifold  422 , the exhaust button  440  and the portion of the outer cap wall  430  between the annular exhaust port wall  438  and the exhaust button  440 . Air that is in the secondary exhaust chamber  880  is vented to the atmosphere through the primary exhaust ports  450  in the exhaust button  440  and through the secondary exhaust ports  452  in the portion of the outer cap wall  430  between the annular exhaust port wall  438  and the exhaust button  440 . 
     With reference to FIGS. 12,  14  and  18 , when the trigger assembly  48  is actuated to change the state of the trigger valve  130  to an actuated state, air in the sleeve return chamber  850  is vented through the trigger assembly  48  to the atmosphere. Consequently, the force that is exerted onto the rear face  862  of the annular sleeve flange  552  causes the sleeve  510  to slide forwardly relative to the housing assembly  42 . When the sleeve  510  slides in a forward direction, the seal between the cap flange seal  516  and the front face  866  of the annular exhaust port wall  438  is broken, permitting compressed air to flow through the rear exhaust slots  604  in the third sleeve body portion  556  of the sleeve  510 . As the area of the front surface  900  of the rear exhaust slots  604  is larger than the area of its rear surface  902 , the pressure of the air flowing through the rear exhaust slots  604  also tends to push the sleeve  510  in a forward direction. The piston bumper  152  checks forward travel of the sleeve  510 . More specifically, forward travel of the sleeve  510  is checked when the front face  810  of the sleeve  510  contacts the first annular lip portion  406  of the piston bumper  152 . 
     Simultaneous with the forward motion of the sleeve  510 , the inner exhaust port seal  512  slides forwardly by an equal amount to sealingly engage the outer circumference  910  of the exhaust manifold  422  at a point forward of the exhaust slots  470  to thereby prevent air from flowing to the atmosphere through the exhaust slots  470 . Pressure acts on the rear surface  920  of the piston assembly  502  to disengage the locking protrusion  744  in the second piston portion  732  from the groove  490  in the top bumper  424 . The pressure acts on the piston assembly  502  to drive the piston assembly  502  and the driver blade  504  forwardly through the first cavity portion  612  of the hollow cavity  610  in the sleeve  510 . Air in the first cavity portion  612  is compressed by the forward motion of the piston assembly  502 , causing it to be expelled from the hollow cavity  610  through the internal cavity  100  in the nose guide  98 , as well as through the front exhausting ports  562  and into a frontal air chamber  940 . The frontal air chamber  940  is defined by the first sleeve body portion  550  of the sleeve  510 , the front guide seal  520 , the guide  650 , the first housing seal  652 , the outwardly tapering wall  172  of the housing body  160 , the second annular lip portion  408  of the annular lip  392  in the piston bumper  152 , the front spring flange  530  and the front spring flange seal  532 . 
     The piston bumper  152  checks the forward motion of the sleeve  510 . Thereafter, the piston assembly  502  pushes the driver blade  504  forwardly so that the tip portion  780  drives a fastener F into a workpiece (not shown). With the piston bumper  152  also checks the forward motion of the piston assembly  502  and effectively seals against the front surface  770  of the piston assembly  502  to seal the frontal air chamber  940 . In this condition, the piston assembly  502  is positioned forwardly of the valve apertures  574  in the first sleeve body portion  550  of the sleeve  510 . Accordingly, if the pressure of the air in the portion of the hollow cavity  610  that is rearward of the piston assembly  502  is greater than the pressure of the air in the frontal air chamber  940 , the compensating valve  524  permits air to flow through the sleeve  510  and into the frontal air chamber  940  so as to balance the air pressure that is acting on the front and rear surfaces  770  and  920  of the piston assembly  502 . The compensating valve  524 , however, is a one-way valve that does not permit air to flow from the frontal air chamber  940  through the valve apertures  574  and into the hollow cavity  610 . 
     Referring back to FIGS. 10,  12 ,  14  and  16 , when the state of the trigger valve  130  is changed to its unactuated state, compressed air is once again routed to the sleeve return chamber  850  where it applies a force against the front face  852  of the annular sleeve flange  552 . The balance of the forces on the sleeve  510  is such that the sleeve  510  is pushed in a rearward direction until the cap flange seal  516  sealingly engages the front face  866  of the annular exhaust port wall  438 . Air in the primary and secondary exhaust chambers  870  and  880  is then vented to the atmosphere in the manner discussed above. 
     The piston assembly  502 , immediately prior to the exhausting of the air in the primary and secondary exhaust chambers  870  and  880 , was such that it remained in sealed engagement with the piston bumper  152 . When the air in the primary exhaust chamber  870  is vented to the atmosphere, however, the pressure in the frontal air chamber  940  generates a force on the front surface  770  of the piston assembly  502  that exceeds the force that is acting on its rear face  920 . As mentioned above, the compensating valve  524  is a one-way valve that prevents air from flowing through the valve apertures  574  and into the hollow cavity  610  and as such, the pressure of the air to the rear of the piston assembly  502  is less than the pressure of the air in the frontal air chamber  940 . Accordingly, the pressure acting on the front surface  770  of the piston assembly  502  drives the piston assembly  502  rearwardly until the locking protrusion  744  in the second piston portion  732  engages the groove  490  in the top bumper  424 . 
     Those skilled in the art will understand that while the above-described configuration of the engine assembly  46  results in a relatively lighter-weight tool as compared with pneumatic fastening devices that employ a conventional head valve, the reduction in the weight of the tool  10  does not come at the expense of increased recoil that is felt by the tool operator. In this regard, the felt force that is exerted onto the cap assembly  44  when a fastener F is driven into a workpiece is counteracted by the felt force that is exerted by the sliding of the sleeve  510  in a forward direction. 
     Magazine Assembly 
     The magazine assembly  20  is shown to include a magazine body assembly  1000 , a follower structure  1002 , a follower spring  1004  and a magazine endcap assembly  1006 . The magazine body assembly  1000  includes a magazine housing  1010 , a pair of guide structures  1012   a  and  1012   b  and a coupling bracket  1014 . In the example illustrated, the magazine housing  1010  is extruded from a lightweight material, such as aluminum and includes a wall member  1020  that defines a fastener head portion  1022 , a follower housing portion  1024 , a pair of guide housing portions  1026  and a fastener body portion  1028 . 
     The fastener head portion  1022  is generally rectangular in shape, defining a fastener head chamber  1030  that is open at its top and bottom ends so as to permit the head portion H of the fasteners F to travel through the fastener head portion  1022 . The fastener head portion  1022  is also open along a portion of one of its sides  1032  so as to permit the follower structure  1002  to travel upwardly within the magazine housing  1010 . With additional reference to FIG. 21, a threaded fastener  1034  is threadably engaged to the wall member  1020 , forming a contact surface  1036  that checks the upward travel of the follower structure  1002 . 
     As shown in FIGS. 19,  20  and  22 , the follower housing portion  1024  is coupled to the forward side of the fastener head portion  1022  and defines a generally rectangular follower cavity  1040  that is sized to receive the follower structure  1002  and the follower spring  1004 . A slot  1042  is formed into the rear surface  1044  of the follower housing portion  1024 . The slot  1042  interconnects the follower cavity  1040  to the fastener head chamber  1030 . An L-shaped pin aperture  1050  is formed into a side of the follower housing portion  1024 . The L-shaped pin aperture  1050  includes a relatively narrow first portion  1052  that extends generally parallel the longitudinal axis of the follower housing portion  1024  and a second portion  1054  that is skewed to the first portion  1052 . The L-shaped pin aperture  1050  will be discussed in greater detail, below. 
     In FIGS. 19 and 20, each guide housing portion  1026  is shown to include a pair of spaced apart and arcuate protrusions  1060   a  and  1060   b  that are coupled to the wall member  1020 . The arcuate protrusions  1060   a  and  1060   b  cooperate with the wall member  1020  to define a guide structure cavity  1062  that extends over the length of the magazine housing  1010  and which is configured to receive one of the guide structures  1012   a  and  1012   b . In the particular embodiment illustrated, the guide structure cavity  1062  includes a first cavity portion  1064  that is generally cylindrically shaped and located proximate the follower housing portion  1024 , and a second cavity portion  1066  that is shaped as a generally flat void that is generally tangent to the cylindrically shaped first cavity portion  1064 . 
     The fastener body portion  1028  is generally U-shaped, being coupled to the forward portion of the pair of guide housing portions  1026 . The fastener body portion  1028  includes a U-shaped fastener body cavity  1070  that is configured to receive the body B of the fasteners F. A plurality of oval windows  1072  are formed into the sides  1074  of the fastener body portion  1028  which permit the tool operator to monitor the quantity of fasteners F that are housed in the magazine assembly  20 , as well as to reduce the overall weight of the magazine assembly  20 . 
     As guide structures  1012   a  and  1012   b  are generally identical in construction, reference numerals may occasionally be shown on only of the guide structure  1012   a  and  1012   b . Those skilled in the art will understand, however, that guide structure  1012   b  is a mirror image of guide structure  1012   a . In the embodiment illustrated in FIGS. 19,  20  and  23 , each of the guide structures  1012   a  and  1012   b  includes a cylindrically-shaped guide port  1100 , first and second retention tabs  1102  and  1104 , respectively, an intermediate member  1106  and an end member  1108 . The guide port  1100  is generally hollow, having an outside diameter that is sized to slip fit into the first cavity portion  1064  of an associated one of the guide housing portions  1026  and an inside diameter that is to engage an associated one of the magazine guide posts  66 . The first retention tab  1102  is coupled to the guide port  1100  on one side and to the intermediate member  1106  on the opposite side. The second retention tab  1104  is coupled to the intermediate member  1106  on the side opposite the first retention tab  1102 . The intermediate member  1106  is sized to fit between the arcuate protrusions  1060   a  and  1060   b  in the guide housing portion  1026  as well as to space the first and second retention tabs  1102  and  1104  apart from one another by a predetermined distance that permits the first and second retention tabs  1102  and  1104  to engage the arcuate protrusions  1060   a  and  1060   b  when the guide structures  1012   a  and  1012   b  are inserted into the guide structure cavities  1062 . The inner surface  1110  of the second retention tab  1104  extends inwardly further toward the centerline  1112  of the magazine housing  1010  than the inside surfaces of the U-shaped fastener body cavity  1070  so as to form a wear surface  1114  against which the body B of the fastener F is permitted to rub. The end member  1108  is coupled to the end of the guide structures  1012   a  and  1012   b  opposite the end to which the guide port  1100  is coupled. The end member  1108  is configured to abut the ends of the arcuate protrusions  1060   a  and  1060   b  so as to prevent the guide structures  1012   a  and  1012   b  from moving upwardly out of the top of the magazine housing  1010 . 
     In FIGS. 24 and 25, the coupling bracket  1014  is shown to have a pair of threaded bushings  1200  and a bracket structure  1202  having a pair of mounting flanges  1204  and a U-shaped body portion  1206  that is coupled to one of the mounting flanges  1204  at each of its opposite ends. Each of the threaded bushings  1200  is coupled to one of the mounting flanges  1204 . The mounting flanges  1204  abut the side of the follower housing portion  1024  and threaded fasteners  1210  (FIG. 2) are employed to engage the threaded bushings  1200  to fixedly but removably couple the coupling bracket  1014  to the magazine housing  1010 . 
     The U-shaped body portion  1206  includes a base  1220  and a plurality of legs  1222 , with each of the legs  1222  coupling a side of the base  1220  to an associated one of the mounting flanges  1204 . The base  1220  includes a slotted pin aperture  1230  that includes a circular portion  1232 , a slotted portion  1234  that is spaced apart from the circular portion  1232 , and a necked-down slotted portion  1236  having a width that is smaller than that of the slotted portion  1234  and which interconnects the circular and slotted portions  1232  and  1234 . The circular portion  1232  is sized to receive the head portion  322  of the clamp pin  300 , the slotted portion  1234  is sized to slidingly receive the first body section  324  of the clamp pin  300 , and the necked-down slotted portion  1236  is sized to receive the second body section  326  of the clamp pin  300  but not the first body section  324 . With specific reference to FIG. 25, the back side of the base  1220  is illustrated in pertinent detail. The end of the slotted portion  1234  is shown to include a conical detent  1238  which is configured to confront the frusto-conical abutting face  330  of the head portion  322  of the clamp pin  300 . 
     With reference to FIGS. 19,  20  and  27  through  32 , the follower structure  1002  is illustrated to have a follower body  1300 , a front guide tab  1302 , a lock-out dog  1304 , a loading cam  1306 , a follower guide  1308  and an actuating lever  1310 . The follower body  1300  is generally U-shaped, having a base  1320  and a pair of follower legs  1322   a  and  1322   b . The lock-out dog  1304  extends upwardly from the base  1320  in a direction opposite that of the follower legs  1322   a  and  1322   b . The front guide tab  1302  is also coupled to the base  1320  but extends upwardly and forwardly therefrom in the same plane as the base  1320 . Accordingly, when the follower structure  1002  is installed to the magazine housing  1010 , the front guide tab  1302  extends forwardly from the follower housing portion  1024 , past the pair of guide housing portions  1026  and into the fastener body portion  1028  where the U-shaped tip portion  1330  of the front guide tab  1302  supports the body B of the fasteners F. 
     The loading cam  1306  is formed into follower leg  1322   a  and includes a first loading cam portion  1350 , a second loading cam portion  1352  and a third loading cam portion  1354 . The first loading cam portion  1350  is a tapered ramp that extends outwardly and upwardly from the distal end of the follower leg  1322   a . The second loading cam portion  1352  includes an oval follower capturing portion  1360 , a downwardly and forwardly extending intermediate portion  1362  and a forwardly and upwardly extending catch portion  1364  and a catch aperture  1368  that is formed at the lower-most portion of the catch portion  1364 . The follower capturing portion  1360  and the intermediate portion  1362  are formed into a first side of the follower leg  1322   a  at a first depth, and the catch portion  1364  is formed into the first side of the follower leg  1322   a  at a second depth that is greater than the first depth. The third loading cam portion  1354  is a generally flat portion of the front surface  1370  of the follower leg  1322   a.    
     The follower guide  1308  is formed onto the outside surface of follower leg  1322   b . The follower guide  1308  includes a V-shaped flange  1380 , an end member  1382  and a connector portion  1384  that couples the V-shaped flange  1380  and the end member  1382 . The connector portion  1384  is configured to fit into the slot  1042  in the follower housing portion  1024  such that the V-shaped flange  1380  and the end member  1382  confront the rear inside surface  1044  and the rear outside surface  1388 , respectively, of the follower housing portion  1024 . 
     The actuating lever  1310  extends outwardly from the end member  1382  and thereafter bends inwardly toward the follower legs  1322   a  and  1322   b . The distal end of the actuating lever  1310  forms an engagement surface  1390  that is configured for receiving an input from the tool operator&#39;s thumb. A protrusion  1392  that is configured to contact the contact surface  1036  in the fastener head portion  1022  is also formed onto the actuating lever  1310 . 
     With reference to FIGS. 19,  20 ,  29 ,  30  and  33 , the follower spring  1004  is illustrated to include a spring hook  1400 , a coiled, flat band spring  1402 , a cylindrically-shaped spring roller body  1404  and a spring roller pin  1406 . The spring roller pin  1406  extends through and rotatably supports the spring roller body  1404 . The band spring  1402  is a type of torsion spring, being coupled to and wound around the spring roller body  1404 . The free end of the band spring  1402  is coupled to the spring hook  1400 . Each end of the spring roller pin  1406  is set into a generally U-shaped spring roller slot  1410  that is formed into each inside surface of the follower legs  1322   a  and  1322   b  to couple the follower spring  1004  to the follower structure  1002 . 
     When the follower structure  1002  is disposed within the follower housing portion  1024 , the band spring  1402  is unwound to permit the C-shaped spring hook  1400  to be engaged to the side of the follower housing portion  1024  opposite the side in which the L-shaped pin aperture  1050  is formed. The torsion exerted by the band spring  1402  is converted to a force that is exerted through the spring roller pin  1406  to the follower structure  1002 , thereby biasing the follower structure  1002  in an upward direction toward the spring hook  1400 . 
     In the particular embodiment illustrated in FIGS. 1,  19  and  35  through  45 , the magazine endcap assembly  1006  includes a molded end cap structure  1600 , a crush tube  1602 , a pivot structure  1604 , a cam follower  1606 , a cam follower spring  1608  and a thrust member  1610 . The end cap structure  1600  is configured to mate against the bottom of the magazine housing  1010  to close off the follower housing portion  1024  and the fastener body portion  1028 . 
     The end cap structure  1600  includes a bushing trunnion  1620  for receiving the crush tube  1602 , a fastener trunnion  1622  for receiving a fastener  1623   a  (FIG. 1) that couples the nose  1623   b  of the end cap structure  1600  to the fastener body portion  1028  and a pair of pivot trunnions  1624  for receiving the pivot structure  1604 , which is illustrated to be a threaded fastener  1626  that is secured to the end cap structure  1600  via a threaded nut  1628  in the example provided. The crush tube  1602 , which is retained by the bushing trunnion  1620 , prevents the end cap structure  1600  form being overstressed as well as the follower housing portion  1024  from being deformed as a result of the clamping force that is exerted by the threaded fastener  1630  (FIG. 1) that couples the end cap structure  1600  to the follower housing portion  1024 . 
     The end cap structure  1600  also includes a follower directing wall  1640 , a thrust flange  1642  and a spring flange  1644 . The follower directing wall  1640  extends upwardly from the base  1646  of the end cap structure  1600  and includes a ramped portion  1650 , which tapers outwardly and downwardly from the top end  1652  of the follower directing wall  1640 , and a generally flat portion  1654  that interconnects the ramped portion  1650  to the base  1646  of the end cap structure  1600 . The spring flange  1644  is located proximate one of the pivot trunnions  1624 , extending upwardly from the base  1646  of the end cap structure  1600  behind one of the pivot trunnions  1624 . The thrust flange  1642  is located between the spring flange  1644  and the follower directing wall  1640  and includes a first U-shaped aperture  1660  that is configured to receive the pivot structure  1604  and a second U-shaped aperture  1662  that is configured to receive the hollow thrust member  1610 . 
     In the particular embodiment illustrated, the cam follower  1606  includes a lever  1670  and a follower hook  1672 . The lever  1670  includes a slotted pivot aperture  1680  that is sized to receive and rotate as well as pivot in a lateral (side-to-side) direction on a portion of the pivot structure  1604 . The lever  1670  extends beyond the slotted pivot aperture  1680  to form a spring follower hook  1672  that can be employed during the assembly of the magazine endcap assembly  1006 . The follower hook  1672  includes a cylindrical body portion  1690  that is coupled to the distal end of the lever  1670  and a leg member  1692  that is coupled to the outer end of the body portion  1690  and which extends downwardly from the body portion  1690  generally parallel to the lever  1670 . The outside face  1694  of the leg member  1692  is heavily chamfered such that the leg member  1692  terminates at a rounded tip portion  1696 . The intersection between the body portion  1690  and the leg member  1692  is undercut by a radius  1698 . 
     The cam follower spring  1608  is illustrated to be a combination compression and torsion spring having a spring body  1700  that wraps around a portion of the pivot structure  1604 , a bent end  1702  for contacting the front face of the lever  1670  and a straight end  1704  for contacting the spring flange  1644 . The cam follower spring  1608  is operable for exerting a rotational biasing force onto the cam follower  1606  which biases the cam follower  1606  toward the rear of the tool  10 . The cam follower spring  1608  is also operable for exerting a lateral force onto the cam follower  1606  which biases the cam follower  1606  toward the thrust member  1610 . 
     The pivot structure  1604  is positioned through the pivot trunnion  1624  that is adjacent the spring flange  1644 . The cam follower spring  1608  is positioned over a portion of the pivot structure  1604  such that the straight end  1704  is in contact with the spring flange  1644 . The cam follower  1606  is positioned into the end cap structure  1600  such that the lever  1670  will contact the thrust member  1610  and the follower hook  1672  will be proximate the follower directing wall  1640 . The spring follower hook  1672  of the cam follower  1606  is employed to lift the bent end  1702  of the cam follower spring  1608  onto the lever  1670 . The pivot structure  1604  is then pushed through the slotted pivot aperture  1680 . The hollow thrust member  1610 , which is a washer in the embodiment illustrated, is positioned in the second U-shaped aperture  1662  in the thrust flange  1642  and the pivot structure  1604  is pushed entirely through the end cap structure  1600  and secured in place with the threaded nut  1628 . 
     With additional reference to FIGS. 27,  31  and  32 , when fasteners F are to be loaded into the magazine assembly  20 , the tool operator presses the engagement surface  1390  of the actuating lever  1310  to move the follower structure  1002  downward toward the end cap structure  1600 . The ramped portion  1650  of the follower directing wall  1640  directs the follower leg  1322   a  of the follower structure  1002  toward the cam follower  1606  and the flat portion  1654  of the follower directing wall  1640  ensure that proper contact is established and maintained between the loading cam  1306  and the cam follower  1606 . 
     When the first loading cam portion  1350  of the loading cam  1306  contacts the leg member  1692  of the follower hook  1672  on the cam follower  1606 , the ramp of the first loading cam portion  1350  pushes the follower hook  1672  in a side-to-side motion along the axis of the pivot structure  1604  in the direction of Arrow R (FIG.  43 ), permitting the leg member  1692  to travel over the first loading cam portion  1350  and into the oval follower capturing portion  1360  of the second loading cam portion  1352  of the loading cam  1306 . With the leg member  1692  being positioned in the oval follower capturing portion  1360 , the follower structure  1002  cannot be moved further down the magazine housing  1010 . When pressure on the engagement surface  1390  of the actuating lever  1310  is released, the force generated by the follower spring  1004  is employed to lift the follower structure  1002  within the magazine housing  1010  so as to simultaneously cause the cam follower  1606  to pivot about the axis of the pivot structure  1604 , thereby permitting the leg member  1692  to travel through the intermediate portion  1362  and into the catch portion  1364  of the second loading cam portion  1352  of the loading cam  1306 . When the leg member  1692  is positioned in the catch portion  1364  of the loading cam  1306 , the leg member  1692  extends through the catch aperture  1368  and around the follower leg  1322   a  of the follower structure  1002  thereby securely coupling the cam follower  1606  to the follower structure  1002  and inhibiting upward travel of the follower structure  1002  within the magazine housing  1010 . In this condition, fasteners F may be readily loaded into the magazine assembly  20 . 
     If the magazine assembly  20  is not already coupled to the fastening tool portion  30 , this operation is performed next. This is accomplished by positioning the top end of the magazine assembly  20  relative to the nose assembly  40  such that the holes in the guide ports  1100  are proximate an associated one of the magazine guide posts  66 , the stop member  134  on the trigger lever  54  is positioned directly above the first portion  1052  of the L-shaped pin aperture  1050 , and the head portion  322  of the clamp pin  300  is engaged to the circular portion  1232  of the slotted pin aperture  1230  in the base  1220  of the bracket structure  1202 . The actuating cam  306  is then pushed toward the clamp boss  252  to compress the compression spring  302  and extend the clamp pin  300  in an outward direction so that the second body section  326  of the clamp pin  300  extends through the slotted pin aperture  1230 . With the clamp pin  300  in this condition, the magazine assembly  20  is slid upwardly until the clamp pin  300  is fully positioned into the slotted portion  1234  of the slotted pin aperture  1230 . Simultaneously, the guide ports  1100  are slid further onto the magazine guide posts  66  so that the top of the magazine assembly  20  cannot pivot relative to the nose assembly  40  and the stop member  134  on the trigger lever  54  is disposed in the second portion  1054  of the L-shaped pin aperture  1050 . 
     Thereafter, the tool operator releases the actuating cam  306 , causing the compression spring  302  to retract the clamp pin  300  somewhat so that the first body section  324  of the clamp pin  300  is disposed within the slotted portion  1234  of the slotted pin aperture  1230 . In this condition, the parallel flats  328  that are formed onto the first body section  324  abut the parallel sides of the slotted portion  1234  of the slotted pin aperture  1230 , thereby permitting the magazine assembly  20  to be slid along an axis defined by the magazine guide posts  66  and the slotted portion  1234  of the slotted pin aperture  1230 . The magazine assembly  20  is pushed upwardly into contact with the magazine flange  64  that is formed into the nose structure  50 . The actuating cam  306  is then pivoted to place the leg portion  352  in contact with the flat contact surface  344 . More specifically, the frusto-conical abutting face  330  of the head portion  322  of the clamp pin  300  engages the conical detent  1238  that is formed into the end of the slotted portion  1234  to both locate the magazine assembly  20  relative to the tool portion  30  as well as to mechanically lock the clamp pin  300  to the coupling bracket  1014 . 
     In this condition, the compression spring  302  exerts a clamping force that is transmitted through the clamp pin  300  to fixedly but removably couple the coupling bracket  1014  to the clamp boss  252 . The magazine stabilizing tabs  62  extend downwardly from the magazine flange  64  and abut the opposite sides of the fastener body portion  1028  of the magazine housing  1010  to inhibit excessive rotation of the magazine assembly  20  relative to the nose assembly  40 . 
     With the magazine assembly  20  attached, the fasteners F are fed into the magazine assembly  20  such that the body B of the fasteners F enter the follower cavity  1040  via the slot  1042 . Typically, the fasteners F are collated (usually at an angle of 20° or 31°) in “sticks”, which permits the magazine assembly  20  to be loaded relatively rapidly. 
     The follower structure  1002  is released from the cam follower  1606  by pressing downwardly on the engagement surface  1390  of the actuating lever  1310 . The body portion  1690  of the follower hook  1672  rides on the upper surface of the forwardly and upwardly extending catch portion  1364 , causing the cam follower  1606  to rotate forwardly. The simultaneous downward movement of the follower structure  1002  and the forward rotation of the cam follower  1606  continues until the leg member  1692  slips out of the catch portion  1364  and the body portion  1690  of the follower hook  1672  slides onto the third loading cam portion  1354  of the loading cam  1306 . As the leg member  1692  of the follower hook  1672  is not contacting the side of the leg  1322   a  of the follower structure  1002 , the follower spring  1004  exerts a force against the lever  1670  that pushes the follower hook  1672  in a side-to-side motion so that the lever  1670  abuts the thrust member  1610 . With the body  1690  of the follower hook  1672  engaged against the third loading cam portion  1354  of the loading cam  1306 , the body  1690  of the follower hook  1672  prevents the cam follower  1606  from engaging the follower structure  1002  and the upward motion of the follower structure  1002  is controlled by the follower spring  1004 . The upward movement of the follower structure  1002  brings the tip portion  1330  of the front guide tab  1302  into contact with the bottom-most fastener F in the magazine assembly  20  which urges the fasteners F upwardly and into the nose assembly  40 . The force exerted by the follower structure  1002  onto the fasteners F, along with the configuration of the fastener head portion  1022 , ensures that fasteners F will not slip rearwardly out of the magazine assembly  20  during the operation of the tool  10 . 
     As discussed above, the tool operator must push the contact trip  52  against the workpiece to cause the trigger lever  54  to push the secondary trigger  128  in to contact with the trigger valve  130  to permit the state of the trigger valve  130  to be changed. With the magazine assembly  20  fully engaged against the magazine flange  64 , the stop member  134  on the trigger lever  54  is free to move in a direction parallel to the longitudinal axis of the tool  10  (i.e., rearwardly-forwardly) within the second portion  1054  of the L-shaped pin aperture  1050 . 
     In the event of a “jam” condition wherein fasteners F have not fed properly through the nose assembly  40 , the tool operator need only rotate the actuating cam  306  such that its base portion  350  is abutted against the flat contact surface  344  to release the clamping force that is exerted through the clamp pin  300 . The magazine assembly  20  may then be slid downwardly from the magazine flange  64  to permit the tool operator to service the nose assembly  40 . The magazine assembly  20 , however, is constrained by the magazine guide posts  66  and the clamp pin  300  so that it can only move in a predetermined linear direction. The predetermined linear direction is cooperatively defined by the magazine guide posts  66 , which remain engaged in the holes  1800  in the guide ports  1100 , and the first body section  324  of the clamp pin  300 , which remains engaged in the slotted portion  1234  of the slotted pin aperture  1230 . Downward movement of the magazine assembly  20  is checked when the first body section  324  of the clamp pin  300  contacts the necked-down slotted portion  1236  of the slotted pin aperture  1230 . Accordingly, the nose assembly  40  may be serviced without completely removing the magazine assembly  20  from the magazine flange  64 . Furthermore, when the magazine assembly  20  is moved downwardly into this condition, the stop member  134  is moved out of the second portion  1054  of the L-shaped pin aperture  1050  and into the first portion  1052  of the L-shaped pin aperture  1050 . With the stop member  134  located in this manner, rearward motion of the contact trip  52  relative to the nose body  60  is limited such that the stop member  134  contacts the rearward edge  1820  of the first portion  1052  of the L-shaped pin aperture  1050 , thereby preventing the trigger lever  54  from pushing the secondary trigger  128  sufficiently rearward so that the state of the trigger valve  130  cannot be changed (i.e., actuated). Accordingly, the stop member  134  and the L-shaped pin aperture  1050  cooperate to selectively prevent the trigger valve  130  from being actuated depending upon the position of the magazine assembly  20  relative to the magazine flange  64 . 
     Those skilled in the art will understand that as fasteners F are dispensed from the tool  10 , the follower spring  1004  will force the follower structure  1002  in an upwardly direction so as to continue to feed fasteners F into the nose body  60 . When the magazine assembly  20  is empty of fasteners F, the follower structure  1002  will be raised within the magazine housing  1010  to a point wherein the lock-out dog  1304  extends through the lock-out dog aperture  90  that is formed into the magazine flange  64  so that it inhibits sufficient rearward motion of the contact trip  52  so as to prevent the trigger lever  54  from changing the state of the trigger valve  130 . Accordingly, the lock-out dog  1304  inhibits the tool  10  from cycling when the magazine assembly  20  is empty of fasteners F and coupled to the magazine flange  64 . 
     In an alternate embodiment of the present invention illustrated in FIGS. 46 and 47, the nose assembly  40  includes a pivoting lock-out tab  2000  that is rotatably coupled to the nose structure  50  and pivotable between a first position, which is illustrated in FIG. 47, that permits the contact trip  52  to move rearwardly a sufficient amount that permits the trigger lever  54  to change the state of the trigger valve  130 , and a second position, which is shown in FIG. 46, that inhibits rearward motion of the contact trip  52  by an amount wherein the trigger lever  54  cannot change the state of the trigger valve  130 . As illustrated in FIG. 47, when the magazine assembly  20  abuts the magazine flange  64 , the top surface  2010  of the magazine housing  1010  contacts the lock-out tab  2000  and rotates it into the first position. When the magazine assembly  20  is not abutted against the magazine flange  64  as illustrated in FIG. 46, however, the lock-out tab  2000  is rotated by a torsion spring (not specifically shown) into the second position to prevent the tool  10  from being cycled. 
     While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the foregoing description and the appended claims.