Many owners have reported oil leaks at the rear main seal even after replacing the seal, myself included. Recently Tony Lentini found a Serviceman bulletin published in February 1965 which details a new Rear Main Bearing design and a chamfer modification to the bearing cap which is applicable to early 1965 engines as well as 1963 and 1964 engines. The double-slotted bearing and chamfer improve oil drainage from the seal area to reduce the possibility of leakage. This information is new to us so currently we don’t have any testimony about the effectiveness of this fix. We can assume Cadillac did their homework and the fix seems plausible. Member Matti Roth received this information while rebuilding his engine and has applied this fix. We all look forward to hearing positive results when the car is back on the road. I plan to apply this fix to my engine the next time I have the oil pan off.

Here is the February 1965 Serviceman bulletin

Feb 1965 Serviceman Bulletin - Rear Main Bearing & Seal

I am aware of two Rear Main Seals for the 1963 to 1967 Cadillacs: one by Rebuilders Choice (no catalog #), and a Best 3812S.

UPDATE Jason Edge

The two 1963 to 1964 Cadillac rear main seals available are: Rebuilders Choice (no catalog #), and a Best 3812S.  (Perhaps there are others, these are the two I am aware of). 

It  has been stated that the Caddy Daddy supplied 1963 to 1967 Rear Main Seal is the best one out there. It is a “Rebuilder’s Choice” brand rear main seal.

You can see it on the Caddy Daddy site by Clicking Here.

You can get the Best 3812 rear seal at places like EGGE (Click Here) and other sources.


Below are photos posted by Matti of the bearing cap modification and the double-slotted bearing. (Thanks to Matti)

I created this gif (from Matti’s photo) of the oil flow in the slot and chamfer.

When purchasing main bearing sets. please make sure the set contains the double-slotted rear main bearing. Here is a photo posted by Matti of the bearing set. (Thanks again Matti)

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