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As evidenced by the spotting on the garage floor, the trickle of dirty oil out of the bell housing inspection hole, and a clutch disc shudder during high torque acceleration (oil is not conducive to good friction), I have a failure of the one piece rear main that I had added to my 404 stroker Cleveland. That took some machining, but uses the same seal as Ford used on the Windsor small blocks.
Engine was assembled by my first builder, but I found several issues and had the engine totally disassembled, checked, and reassembled by another builder. They did not install a fresh seal, as the engine had no use, but I suspect that is the genesis of why a 6000 mile seal is no longer performing.
I have a Viton replacement sourced from SCE already on hand, 23605.
I’ve done the usual Google and YouTube searches, but would love additional feedback from any of you that have experience with this task.

Sent from me using a magic, handheld electronic gizmo.
Original Post

One piece rear mains on a 351-C are not worth the expense for many owners since it requires the engine block to be set up on a big mill and carefully bored to take the Windsor main seal. Careful builders then add J-B-Weld to the large unsupported area thus exposed in the block, but it seems to work OK without a JB seal support at least on street engines. Such machined blocks then will NOT ever accept a 2 piece seal again without a LOT of fussy work.

The problem shows up when the rear crank journal is undersized by almost any amount. With a 2-piece seal you trim the ends sightly to compensate for an 010"-030"-under crank. This is not an option with one piece seals and there are no known undersizes. On the good side, if you have a std sized crank, one-piece seals are available from Rock AutoParts in teflon which reduces drag a tiny bit. FWIW, I and thousands of other builders don't get leaks with stock 2 piece seals.

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