I recently installed a rear main seal in a 312 I had on the stand. It was installed without removing the crankshaft similar to doing it in the car. The engine was installed in the car and it leaks really bad; either a bad job, bad seal surface, or bad seal, and leaks so much it has to be replaced again. The seal came from my favorite T-Bird supplier and was black. I read that  the Best Gasket 312 rear main seal is red or orange. I plan to do the engine in car (57 T-Bird) seal replacement. I came across the following article in Gil's Garage of the CTCI website which references another article written by Ted:

"Ted Eaton has written a great article regarding installing the neoprene rear seal. (found in Gil’s Garage). His article also covers other oil leaks associated with the 292/312 Y block engine. Ted finds that as a general rule if his installation instructions are followed the early made for 312 engine neoprene seal featured in his article holds well if the crankshaft seal surface is polished. If not it will hold for a while then usually leaks before 3500 miles. It may leak sooner if the crank shaft seal surface is not perfect.

He believes as I and others believe that the early seal material may be too soft allowing the seal to wear. It has been found that a USA made modified Chrysler seal researched, developed and sold by Fordcraft, works equally well in the 312 engine and it is not as sensitive to crankshaft seal surface irregularities. I have used this seal for several years with very good results. It can be ordered from Fordcraft 972-423-4636 and many other Thunderbird parts dealers."

My question is, since I didn't thoroughly check the condition of the seal surface (although I remember it as OK) before, would I be better off getting the Fordcraft seal over the Best gasket?

The Red/Orange seal was the problem seal mentioned and Best Gasket has replaced it with the Black one for many years now. It works well if Ted's recommendations/instructions are followed.
You will need to remove the seal and look at it for wear but I would be surprised if that happened. It could be the side seals or the retainer porosity.

Does it leak at idle? Or only at higher rpms?

Rob


1936 Ford 3W Coupe : 56 T-Bird 312, 47 Packard 3 speed, 40 juice brakes.
1968 Merc Cyclone FB GT 390, Getting a better front clip!
1977 Ford F-250 Supercab RWD Explorer Long box.
1976 Chev Camaro RS LT
Future rebuild : 1949 Ford F-1

Rob:

Thanks for the reply!
It leaks at idle for sure, I will have to get help to look at it at high rpm's but I expect it does then also .

The part number for the Best Gasket neoprene rear main for the 312 engines is 3A52.  The early seals by Best Gasket for the 312’s was orange and had the same part number.  The revised seals are now black and have been for several years.  I suspect some of the suppliers may still have some of those older orange seals on their shelves so they must be watched for and not used.  The seal for the 292 (also black) will not work on the 312 engines.

When you remove the rear seal retainer, ensure that both seal halves have the lip facing the inside of the engine. Offsetting the seal in the groove so the ends do not align with the main cap parting line will also help to align both seals halves to each other.  Another thing to watch for is a seal groove that’s wider than it should be.  If the rear main seal is sideways loose in the groove, then some RTV on the outer edge of the seal will help in that instance.

Here's the link to the seal article that Gil Baumgartner references.
https://www.eatonbalancing.com/2008/09/10/neoprene-rear-seal-installation-for-the-y-and-others/

Lorena, Texas (South of Waco)

I installed a trimmed 318 MoPar seal in my 312, eliminated the side seals and used Ultra Black RTV instead...No leaks 6 years and counting!

"FREE SAMPLE"

Should the crankshaft be fully torqued down before or after the rear main seal is installed? Thanks.

Ted:

Thanks for including the link. That article has more info than the CTCI version. 

Before installing the seal retainer torque the main cap bolts. The seal retainer partially covers the #5 main cap.


1936 Ford 3W Coupe : 56 T-Bird 312, 47 Packard 3 speed, 40 juice brakes.
1968 Merc Cyclone FB GT 390, Getting a better front clip!
1977 Ford F-250 Supercab RWD Explorer Long box.
1976 Chev Camaro RS LT
Future rebuild : 1949 Ford F-1

http://bigdclub.org/assets/files/bdlb_tt_wn_rearmainseal.pdf

"FREE SAMPLE"

Thanks Lord Gaga:
6 years and counting sounds very good!