Rear Main Seal Oil Leak Fix with Bearing Cap Modification

[Kurt]

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.

Here is the February 1965 Serviceman bulletin:
Feb 1965 Serviceman Bulletin - Rear Main Bearing & Seal

Rear Main Seals Available - which is better?
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 3812S rear seal at places like EGGE (Click Here) and other sources.

VERDICT: Unclear if Main Bearing Cap Alone Works or of the Rebuilders Choice Seal is needed.
At least two members have made both the rear bearing cap mode and installed the Rebuilder's Choice rear main seal.  It is unclear if the rear main seal brand really matters. If someone has made the main cap mod and installed the Best brand (or other brand) seal please provide feedback. For now, the assumption is the best approach to make the mods and use the Rebuilders Choice rear seal. 

UPDATE Nov 20, 2024:  It has been stated that someone had made the main bearing cap and installed a rear main seal sourced from Kanter (part #  RMS13329FEL) and this fixed the rear  main seal leak.  A chapter member then talked to a Kanter rep and said their RMS is a Best brand seal, rebadged in their box. So I think that is at least one example where it is the bearing mod that is making the difference, not the brand of rear seal.  I think we need more examples of the rear main cap mod being made with different brand rear main seals.

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)

[Jason Edge]

I just cleaned up the header section  up top on the rear main bearing cap mod and rear main seal to use information. If others have experience making the rear main cap mod and any of the available seals please reply.

[Russ Austin]

I just got word that Kanter has a good rear main seal. The mod and install went perfect and the engine is leak free after some miles were put on. 

Engine Rear Main Seal - RMS13329FEL

[Jason Edge]

I tried to add one to the cart but would not let me.  It said to contact for pricing?  I wonder who they are sourcing their rear seal from?   Here was the link: Click Here.  
So here is the question put another way:
If:
A= Rear Cap Mod
B= Rebuilders Choice Rear Seal
C= Best 3812S seal
D= Kanter Seal

and we know:
A+B = no leak
and A+D = no leak
but do not know results of A+C unless Kanter is actually selling a 3812S or Rebuilder's Choice seal.

Put another way the common piece to the puzzle that results in no leaks is the rear main cap mod. I have a the Best brand and Rebuilders Choice rear seals and may try the Best 3182S if no one chimes in before I get around to it. People trashed the Best rear seal saying it leaked but that was without doing the rear cap mod... at least to the best of my memory. I just want to make sure we state this as correctly as possible.

[Russ Austin]

Caddy Daddy was carrying Best brand and another brand. It sounds like they ran out of the brand. 

[Jason Edge]

A chapter member just emailed me this morning and said he had talked to a Kanter rep and their RMS is a Best brand seal, rebadged in their box. So I think that is at least one example where it is the bearing mod that is making the difference, not the brand of rear seal. 

I will copy the above into the original post summary up top.

[Christian Doerk]

I did the same modification end of May this year. For the first 300 mls it was perfect. After that it leaks a little bit. I would guess one or two drops after shutting down with hot oil for 1 hour. So totally acceptable. But it’s still very good in comparison with the original rear main bearing. I bought the seal at Caddy Daddy.

[Jason Edge]

Christian, Thanks for the update. Do you know if the rear main seal received from Caddy Daddy was the Best brand or Rebuilder's Choice?  I am being told the rear main seal from Caddy Daddy, at least as of this past week, is the Best brand.

[Dave Fehrle]

My question is for those that have done the Mod, do you have to buy the whole set of bearings? And if so, do you just replace the rear?