Many owners have reported oil leaks at the rear main seal even after replacing the seal, myself included. Recently Tony Lentini (aka Tony and Ginny 429) 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 (Thanks to Tony and Jason)
Feb 1965 Serviceman Bulletin - Rear Main Bearing & Seal
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)
SUGGESTED REAR MAIN SEAL
It has been noted that the Caddy Daddy supplied rear seal by Rebuilder's Choice is better than the Best Gasket 3812 in preventing leaks. In the future, we hope to have a separate Help Topic article posted with pictures and details of the differences between the gaskets and perhaps other sources for this Rebuilder's Choice rear seal. This rear main seal can be ordered thru Caddy Daddy by Clicking Here.
Thanks Tony & Kurt. This has been added to our Help Page.
I have added an addendum to the end of this article discussing the Serviceman Bulletin regarding the use of a new double groove rear main bearing and modifications to rear main bearing cap, to include a link to the recommended Rebuilder's Choice rear main seal and noted we will hopefully have a separate help topic posted regarding the rear main seals in the future. When a concise article with pictures is written regarding the rear main seal options, I will simply link to it from this article.
Since this is another location and topic dealing with oil pan installation and not the rear main seal leak, I do not want to add that to the rear main seal and cap modification article. There are no pictures and just a newer Cadillac Part # (363 2764) referenced and believe the design would be incorporated on any newer oil pan gasket set going forward. If someone wants to write up an article I can scan and send that short section on the oil pan real seal.
As of a few months ago ( is now 3/23 ) we have accumulated enough mileage on the first rear main seal repair done by the above rear main seal crankshaft info - ( 6-8000 miles ) to be Sure —- it is the very best known rms repair available for the 63—67 390 and 429 cu in engines —to date —-and the Only seal repair — to not start leaking immediately after a new engine start up for many years ( that’s right years )
The only correct main seal is available from—- Restores choice ( the supplier ) or Caddy Daddy ( a distributor ) for the same price
New engine start ups— are a lot more enjoyable and less frustrating —-without oil leaking issues after start up
A Serious long term repair problem solved—- due to a new replacement seal available for our wonderful very old Cadillacs and some very minor machining of the rear bearing found in a 65 Cadillac Service Bulletin somewhat by complete accident
It has been a highlight for me to have discovered the info and to have share it with those rebuilding there engines in our community to improve that process
Thanks to Kurt and Jason for there help in the process
Need to do a shout out —to Kim Kaiser —for being the very first to do this rms repair correctly with the info offered and sharing his experience with the community for others to benefit
Repair details :
The seal is offset —-by about one inch from the crankshafts end cap seam joint
I recall the seal is one inch lower ( deeper ) on the left side and one inch higher on the right side standing in front of the engine ( engine in the car )
Use black Permatex 90 minute dry time as the sealant at the seals joint and on the flat metal surfaces of the end caps seam
Happy oil leak repair ——-solved at last !