PRIORITY 
     The present application claims the benefit of U.S. Provisional Application Ser. No. 61/048,523, filed Apr. 28, 2008, which is expressly incorporated herein by reference in its entirety. 
    
    
     THE FIELD OF THE INVENTION 
     The present invention relates to a tool for servicing large diesel engines used in road tractors (semi trucks). More specifically, the present invention relates to a clip for securing the power piston located in the exhaust valve rocker arm and used for engine braking purposes in modern Volvo® and similar large diesel engines. 
     BACKGROUND 
     The large Volvo® and similar diesel engines used in road tractors (herein referred to simply as a diesel engine) utilize an engine brake to aid in slowing the vehicle. The engine brake in modern diesel engines utilizes a brake rocker arm with additional cam shaft lobes to selectively open and close the exhaust valves. The brake rocker arm interacts with the exhaust valve rocker arm via a pump piston and power piston to operate the engine brake. During normal operation, the pump piston and power piston are provided with low pressure engine oil and are thus in a collapsed state. To effect operation of the engine brake, an oil control valve increases the pressure of oil provided to the pump piston and power piston such that the brake rocker arm pressing on the pump piston causes expansion of the power piston and thus causes the exhaust valves to open. 
     One problem with the present engine brake system is that the various parts associated with the power piston are not captively held inside of the exhaust rocker arm and can fall out of the rocker arm while servicing the engine. A variety of types of service to the head of the engine such as removal of the head, service to the valve train, or service or replacement of the fuel injectors or injector seals would allow the power piston to fall out of the exhaust rocker arm. This would result in lost time to reassemble the power piston and may result in lost or damaged parts. 
     The factory service procedures for these engines instruct the service technician to place a rubber band around the power piston to keep it from falling out of the exhaust rocker arm. The rubber band, however, is difficult for the technician to use, does not hold up well to the oil and other chemicals present in the engine, and may damage the engine if the technician forgets to remove the rubber band after servicing the engine. If used, the rubber band may weaken and break due to the oils and solvents used in service and operation of the engine. This may cause the power piston and associated parts to fall out of the rocker arm and be lost or damaged, in addition to increasing the time to service the engine. If the technician forgets to remove the rubber band after performing the desired engine service, the rubber band may interfere with the operation of the engine and may clog oil passages or other parts of the engine, causing damage to the engine. This may occur as the rubber band collects oil and becomes more difficult to see while service is being performed. 
     There is a need for a tool which is suitable for retaining the power piston in large diesel engines while servicing these engines. There is a need for a tool which is easy to use and reliably prevents the power piston from falling out of the exhaust valve. There is also a need for a tool which is readily visible and which is not easily left in the engine after completing the engine service. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved retaining tool for the exhaust brake power piston of large diesel engines. 
     According to one aspect of the invention, a retaining tool is provided which is easily secured to the exhaust valve of an engine to secure the engine brake power valve during engine maintenance. According to another aspect of the invention, the retaining tool may be highly visible so as to reduce the likelihood that a technician accidentally leaves the tool in the engine. 
     These and other aspects of the present invention are realized in a retaining tool as shown and described in the following figures and related description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the present invention are shown and described in reference to the numbered drawings wherein: 
         FIG. 1  shows a perspective view of a portion of the valve train of a diesel engine; 
         FIG. 2  shows a perspective view of the exhaust rocker arm of  FIG. 1 ; 
         FIG. 3  shows another perspective view of the exhaust rocker arm of  FIG. 1 ; 
         FIG. 4  shows a perspective view of a retaining clip of the present invention; and 
         FIG. 5  shows a side view of the retaining clip and exhaust rocker arm. 
     
    
    
     It will be appreciated that the drawings are illustrative and not limiting of the scope of the invention which is defined by the appended claims. The embodiments shown accomplish various aspects and objects of the invention. It is appreciated that it is not possible to clearly show each element and aspect of the invention in a single figure, and as such, multiple figures are presented to separately illustrate the various details of the invention in greater clarity. Similarly, not every embodiment need accomplish all advantages of the present invention. 
     DETAILED DESCRIPTION 
     The invention and accompanying drawings will now be discussed in reference to the numerals provided therein so as to enable one skilled in the art to practice the present invention. The drawings and descriptions are exemplary of various aspects of the invention and are not intended to narrow the scope of the appended claims. 
     Turning now to  FIG. 1 , a perspective view of a valve train of a diesel engine is shown. The valve train includes an exhaust rocker arm  10 , engine brake rocker arm  14 , fuel injector rocker arm  18 , and intake rocker arm  22 . The exhaust rocker arm  10  moves the exhaust valves (not clearly visible but located inside of the exhaust valve springs  26 ) via an exhaust valve bridge  30 . The exhaust valve bridge  30  allows a single exhaust rocker arm  10  to move two exhaust valves. The exhaust rocker arm  10  applies force to the exhaust valve bridge  30  via a power piston  34  ( FIG. 2 ) and a poppet  38 . Raising the oil pressure to the power piston  34  and pump piston  42  ( FIG. 2 ) causes the exhaust brake rocker arm  14  to press against the pump piston to thereby inflate the power piston  34  and cause engine braking. 
       FIGS. 2 and 3  show partially transparent views of the exhaust rocker arm  10 , illustrating the internal components such as the power piston  34  and pump piston  42 . The power piston  34  and pump piston  42  are hydraulically connected via a passage, and pressing on the pump piston will inflate the power piston when they are filled with oil. The engine brake rocker arm  14  will press on the pump piston  42  as discussed. 
     The power piston  34  and poppet  38  are not captively held in the exhaust rocker arm  10 , and will fall out of the rocker arm if not held in place. Removal of the heads and other engine service will remove the pressure on the power piston  34  and poppet  38  and allow them to fall out of the exhaust rocker arm as discussed. Servicing the head of the diesel engine is thus more difficult as special care and attention must be paid to the power piston  34  and poppet  38 . If these parts fall out, they may be lost or damaged, or dirt or other contaminants may get into the power piston  34 . If the power piston  34  and poppet  38  fall out and are not noticed, the valve train will not function properly and further damage to the engine may occur. 
     As mentioned, technicians often place a rubber band around the exhaust rocker arm  10 , power piston  34 , and poppet  38  to keep the power piston and poppet in place. There are several disadvantages to this. The rubber band is difficult to place around the power piston  34  and exhaust rocker arm  10 . Furthermore, the rubber band may slip off of the rocker arm  10  and release the power piston  34 , allowing the power piston to fall out of the rocker arm. Rubber bands are adversely affected by engine oils and other chemicals, and may break or leave debris in the engine. Additionally, rubber bands may be easily missed when reassembling the engine and left in the engine where they may damage the engine. 
     Turning now to  FIG. 4 , a perspective view of a retaining clip  50  of the present invention is shown. The clip  50  is formed with a generally C-shaped body  54  which is configured to extend around the exhaust rocker arm  10 . The lower portion  58  of the body  54  has an opening or slot  62  formed therein. The slot  62  is shaped to fit between the power piston  34  and poppet  38 . The upper portion  66  of the body  54  extends generally parallel to the lower portion  58 , and fits over the top of the exhaust rocker arm  10 . The upper portion  66  of the body  54  includes a finger grip  70  which allows a technician to more easily insert and remove the clip  50 . The finger grip  70 , as well as other portions of the clip  50  such as the upper portion  66  or even the entire clip may be painted a high visibility color to make it less likely that the clip  50  is left in the diesel engine after servicing the engine. 
     Turning now to  FIG. 5 , a side view of a portion of the exhaust rocker arm  10  with the clip  50  in place is shown. Many parts of the valve train have been omitted for clarity. The clip  50  is easily placed on the exhaust rocker arm  10  by placing the slot  62  around the stem  74  between the power piston  34  and poppet  38  and by placing the upper portion  66  of the clip  50  on top of the rocker arm  10 . The upper portion  66  may rest on a valve lash adjustment bolt  78  and not directly on the rocker arm  10  in many diesel engines. The C-shaped body  54  of the clip extends around the end of the exhaust rocker arm  10  as shown. 
     The clip  50  is made from a resilient material such as spring steel and is sized for a particular engine so that a technician must flex the clip slightly to increase the distance between the slot  62  and the upper portion  66  during installation onto an exhaust rocker arm  10 . This stretching of the clip places some compression on the power valve  34  and keeps the power valve from falling out of the exhaust rocker arm  10  during service. Thus, the finger grip  70  makes it easier for a technician to stretch the clip  50  slightly and install the clip on the rocker arm  10 . The finger grip  70  also makes it easier to grip the clip  50  in order to remove the clip. 
     When servicing the head of a diesel engine, a technician would remove the engine valve cover. At this point, the valve train is visible and accessible to the technician. The technician would then place a clip  50  on each exhaust rocker arm  10 . As there is typically a single exhaust rocker arm  10  per cylinder and six cylinders in a diesel engine, six clips  50  are typically required. Once the clips  50  are in place, the technician may remove or service the head without fear of the power pistons  34  falling out of the head. 
     The clips  50  are advantageous because they are easily and quickly installed on the exhaust rocker arms  10 . The six clips  50  may easily be installed and removed in less than a minute. Using the clips  50  may save twenty minutes or more when working on the head of the diesel engine. Additionally, the clips are more secure than using rubber bands or other means of retaining the power pistons  34 , eliminating the worry about the power pistons coming loose during the service procedure. Painting all or a portion of the clips in a bright color such as yellow or orange makes them easily visible to the mechanic and easily recognized as something which is not a permanent part of the engine. This virtually eliminates the possibility that a clip is left in the engine, and thus virtually eliminates the corresponding risk of damage to the engine which may be caused by a service clip or rubber band being left in the engine. Rubber bands are small and have a color similar to oily metal, presenting a risk that a rubber band is left in the engine. 
     There is thus disclosed an improved retaining tool for exhaust brake power pistons. It will be appreciated that numerous changes may be made to the present invention without departing from the scope of the claims.
A retaining clip fits on the exhaust rocker arms of a large diesel engine to hold the exhaust brake power pistons in place while performing service on the engine. The retaining clips are easily installed and removed from the rocker arm and prevent the loss or damage of the power piston due to the piston accidentally falling out of the rocker arms during service to the engine.