SBC 400 CID Unique Rear Main Seal

After the 400 SBC block was aligned honed, I found out that it needed a special rear main seal. P> - At the machine shop, the caps had 0.003" taken off and the block/cap assembly aligned honed. After, the block was washed and once again needs to be painted :-(

The shop had told me that I would need a "special" rear main seal that cost $33 because the block was aligned honed. I thought that this sounded pretty fishy as only 0.003" had be taken off which is only 0.0015" play per seal face. So I went looking for 2nd opinions about this.

- After 3 days of discussing this with about a dozen machinists, mechanics and partsmen and on the Internet newsgroups and getting mixed answers from "it was a crock of sh**" to "yes it's needed", I finally found out that it was indeed true and was unique with 400 blocks only.

- The 400 block has 2.65" main journals while the 350 has 2.45" - no new info here. But the 400 block uses the smaller 350 oil seal. The rail that the seal fits over is at the 350 size (2.45"). When they go to align hone the block, they have to open up the seal rail to 2.65" dia so that the align hone arbor will fit into the block. The problem that the special seal fixes is not the 0.003" align honing material that is removed but the loss of metal in the seal rail (close to 0.200" in overall diameter) that's needed to hold the seal in place. It is a problem unique to the 400 sbc block hence the high price and need for the special seal. The seal that I used is a Fel-Pro #2909 for bore sizes 2.8406"-2.8415".

- I checked the rear oil seal using the stock seal and something looked definitely wrong like it didn't quite fit properly as it should since the seal rails are now 0.200" larger than stock due to the align honing.

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Copyright May 2011 Eugene Blanchard