Which engine do you have? A GMC or a Chevy? I have replaced a few 216-235-261 rope seals using the procedure described in the Tech Tips that Waldo53 recommended (follow the "Engine" sub-link), using both a home-made or a real "Sneaky Pete. I've also replaced rubber rear main seals in SBC engines with the same method.
If you have a Chevy, the 1929-57 Master Parts Manual shows only the rope rear main seal for all 40-57 sixes. However,
Best Gasket shows the rubber seal for the 1956-62 235 and 261. For further confusion, a friend of mine bought a new 1954 261 short block from Chevrolet in the early seventies and it came with the rubber seal. I think (but do not know for sure) that the groove for the rubber seal is smaller than for the rope seal.
Chevs of the Forties shows the rope seal for 235 and 261s through 1958 and the rubber seal for 1956 and newer.
"CHEVROLET SEAL -REAR MAIN, RUBBER 2-PIECE
Seal-rear Main Rubber (2 pcs) designed for 56 and newer blocks, some earlier blocks may not seal correctly. 3835774 is original style rope seal."Perhaps amore experienced Stovebolter or a tech person at Best Gasket could tell us the real details about the differences in the seal grooves. An Inliner with a GMC had success with the Best Gasket replacement rope seal.
Hoyt
