Neoprene rear main seal for small block or big block Oldsmobile engines. Replaces that leaky rope rear main seal. DMR-18692 fits 330, 350 and 403 Oldsmobile engines. DMR-68135 fits 425 and 455 Oldsmobile engines. The rear of crank should be polished for best result but not mandatory. Be sure to lay a thin bead of silicone across the seal ends in the rear main cap and continue across the cap toward the outside to prevent oil from seeping out between the cap and the block or between the seal ends..