Production Chevrolet small-block V8 and V6/90-degree engines were redesigned in 1986 with a leak-resistant one-piece rear seal. This change required a revised oil pan design to fit a new rear seal retainer which is bolted to the back of the engine block. A new one-piece pan gasket replaced the four-piece gasket set used on pre-1986 engines. As a result of these changes, oil pans and gaskets are not interchangeable between 1955-85 and 1986-up engines. If you have installed crankshaft seal adapter on a block which is machined for a one-piece rear seal, you must use an oil pan and gasket designed for 1986 and newer engines.