From 1955 through 1985 the small-block Chevrolet used a two piece rear main seal. See Fig. 54.1. The first seals from 1955 through 1958 were Graphite rope. In 1959 the seal was changed to a Neoprene lip seal. In 1986 the rear main seal was changed to a one piece seal Fig. 54.2 so 1986 and newer crankshafts will not fit into 1985 and older blocks. There is an adapter from GM PN 10051118 which can be used to install the pre-1986 crankshafts into the 1986 and newer blocks.
Fig. 54.1
Fig. 54.2