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)

[Dennis P. DiBari]

Hi Pat,

     This is Dennis DiBari.  Google TA Motor AB and the site (from Sweden) should come up.  It has an English translation on top and it references Matti's exact set of bearings.  Good luck and let me know.

Dennis DiBari, NY

[Christian Doerk]

got mine from there, too. Since this modification it's not 100% seal but WAY better than before. Worth doing it. Just about one drop still hanging around ;)

[Patrick Westphal]

OK Dennis I found the website but the site tells me to measure the crankshaft diameter to determine which bearings to get.  So do I have to take the engine apart before I can even order the bearings?  Any suggestions would be appreciated.  

[Dennis P. DiBari]

Hi Pat,

     I did not realize they wanted the crankshaft diameter, as it is the 429 CI engine - thought it would be a standard set of bearings for our engine.  I went to the website again, BUT could not find the double slotted bearing Matti ordered.  They have bearings sets with options for four different diameters, as you said.  I don't have Matti's phone, but I would call him first - he has the right information!!  This is a lot of extra work to remove the entire engine as you well know.  Maybe Jason Edge has his phone.  I'll continue to research it to help you.  Let me know please!

Dennis DiBari

[Dennis P. DiBari]

Hi Pat,

     On the website, it is the Frame Bearing Kit that has the double slotted bearing.  However, as you said, there appears FIVE different sets to order.  It seems that without the right crank diameter, you would not be able to order.  Maybe the engine would have to be pulled to do it correctly???  Doing the best I can to help.  Matti is overseas (I did not realize), so maybe you can FRIEND him and send a message!!!!

Dennis DiBari 

[Matti Roth]

Hi

It is necessary to check if the crank is worn so much that the gap between crank and bearing is too big. You can measure that with Plastigauge but yes, that requires opening the motor. If engine is worn, you need to use non-std size main bearings. Luckily in my case the crank was still in std diameter.

[Dennis P. DiBari]

Hi Matti,

     Thanks for responding and I hope Pat Westphal sees your posting about the bearing!  All the best to you!

Dennis DiBari

[Patrick Westphal]

Matt and Dennis. Thanks for all the help and information on the rear seals. I think I have enough information to take it from here. 

[Dennis P. DiBari]

Pat,

     Good luck with the bearings and seal.  I hope you get resolve with this and let me know if you can!

Dennis DiBari