Fitment and Product Information. Canton 21-850 Rear Main Seal Adapter Converts 1 Piece SBC To 2 Piece Seal NEW. Brand Name: Canton Racing. This SBC rear main seal adapter converts? 86 and newer blocks with one piece rear main seals to accept an older style two piece seal crank shaft. The adapter is essential when installing an early style pre-? 86 & newer blocks. When using this adapter an oil pan for a pre-86 block with a right side dipstick must be used. Made from Billet aluminum, hardware included. WARNING : This product can expose you to chemicals including Styrene, which is known to the State of California to cause cancer.