The 1st/Reverse Clutch Piston retaining ring can be removed with standard hand tools. However, GM-51920 is a unit repair tool that is required for reassembly to evenly compress the Piston Spring, allowing the retaining ring to be installed in the ring groove of the case. The tool fits into the transmission case and contacts the return spring around its entire perimeter, evenly compressing the spring down past the retaining ring groove so that the ring can be seated in the case. The spring consists of a ring with numerous short coil springs attached. If the Installer is not used, the ring flexes, and it is very difficult to snap the retaining ring completely into the case groove; the spring can easily be bent and damaged during the process. GM-51920 requires the use of two previously essential inline tools: GM-48056 Spring Compressor Bridge and GM-48903 Spring Compressor. GM-48903 is used as a spacer to take up the distance gap between the GM-48056 forcing screw and the GM-51920 Installer. As this three-tool assembly compresses the spring, the technician works the retaining ring into the groove around the perimeter of the transmission case with a screwdriver.