The 400 Rear main seal
The rear main seal is supported by the seal register, which is 2.780" in this photo. The lower rear main bearing shell rests inside the 2.840" diameter. A problem can arise when the main bearing bore is align bored and honed. The seal register often winds up becoming enlarged to the main bearing bore size of 2.840", and doesn't support the rear main seal well enough to keep oil from seeping out. A solution is offered by Fel Pro gaskets, part number 2909. It's an oversize rear main seal made just for align bored 400 small block chevys.
This is my 408's rear main cap, which has never been align bored, so it still has the stock dimensions.
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