The rear crankshaft seal is part of the rear cover, and is installed to the rear face of the block as an assembly on HF V6 engines. For Gen II HF V6, the pilot on the rear flange of the crankshaft has an increased diameter, and the flywheel mounting bolts have changed from M10x1.0 to M10x1.5 thread pitch, so the current GM-47839 does not fit. GM-51766 fits the HF V6 Gen II crankshaft and positions the rear seal housing without allowing the seal lip to roll over the crankshaft lip. The OD edge of GM-51766 fits over the lead-in chamfer on the crank, preventing the seal lip from catching. If GM-51766 is not used, the seal lip may roll on the edge of the crankshaft, causing a leak.