The CTA Rear Crankshaft Seal Installer Tool - 6.6L Duramax Diesel, No. 3878, is designed to fit the rear seal of the engine block and crankshaft for proper fitment of inner and outer seal body. It services GM/ISUZU 6.6L Duramax 2001+ GMT800, 2500HD, and 3500. To install, simply set new seal on inner body of tool and attach outside ring of tool to the end of the crankshaft assembly. Then, turn forcing screw to press in new seal. The tool compares to GM's J-44642 tool, the company says. It also works with CTA's Front Crankshaft Seal Installer Tool - 6.6L Duramax Diesel, No. 3879.