The best rear main seal, and the only one you should use is made by best gasket. Here is a link to what you need. When you install the seal, make sure to put a light coating of silicone sealer where the two gasket ends meet.

Pardon Our Interruption...

 

Sorry that information isn't correct the OP states it's a 350 block not a original 265 or 283 with a rope seal which that information is great for any early block that used rope seals.Have you looked to see if there is any wear on the rear crank journal where the seal rides. It can be that the rear main journal in the block may be oversized NAPA years ago made a shim to go between the underside of the seal & the block to tighten up the seal. Have you made sure the leak might not be from above the rear cam plug or the threaded plugs in the oil galley.You might clean everything really good & add dye to make sure it's actually the rear main & not the pan,or one of the plugs I mentioned or even from the rear of the intake manifold. Fel Pro makes or at least did make a main seal listed for racing that is red in color supposed to be better. Fel Pro #2912 I believe

 

When installing do you have it flush with block and main cap, or is it 3/8 down on one side and out the slot 3/8 on the other…..

 

I offset the seals, so they are NOT flush with the block and cap. I also add RTV to the ends where they join and on the outside edges of the cap and block. the last seal I installed was Fel-Pro, P/N BS40013. I do appreciate the feedback. I can't see up in the back of the block but will try running some dye after I get a stronger blacklight. Thanks

 

It may not be the main seal. Are you certain of that??

I built a 283 for my 57 and it was a leaker right off the bat. Had pan off 7 times ! One of those times I changed the rear main with no effect. Also, know there is a thin pan gasket (at the timing cover ) and a thicker gasket, if you’re unaware.

I think I finally have mine fixed. I used a (thick) one piece Fel Pro gasket(see above) and more Right
Stuff gasket maker than I really wanted to, but I firmly believe it was the RTV that done it for me.

Also, all kinds of places above the crank can leak and run down between the bell housing and block and drip off the pan, like corner of the intake, “China wall”, oil pressure sending unit area, distributor (not likely) cam plug, lifter gallery plugs.
Hope you don’t have to R&R the motor and you find an easy solution

 

It may not be the main seal. Are you certain of that??

I built a 283 for my 57 and it was a leaker right off the bat. Had pan off 7 times ! One of those times I changed the rear main with no effect. Also, know there is a thin pan gasket (at the timing cover ) and a thicker gasket, if you’re unaware.

I think I finally have mine fixed. I used a (thick) one piece Fel Pro gasket(see above) and more Right
Stuff gasket maker than I really wanted to, but I firmly believe it was the RTV that done it for me.

Also, all kinds of places above the crank can leak and run down between the bell housing and block and drip off the pan, like corner of the intake, “China wall”, oil pressure sending unit area, distributor (not likely) cam plug, lifter gallery plugs.
Hope you don’t have to R&R the motor and you find an easy solution

napa has a very good rear main sealer worked for me at one time
Larry

 

I seem to recall there being two types of rear main seal. One a silicone base and the other a urethane base. I may be wrong on the materials. But one was a “premium” and the other a standard replacement. Again if memory is correct premium one was a reddish brown color and the other blue or black.

 

As I listed above the Fel Pro #2912 is listed as the high performance seal & is red while the standard seal is blue in color #2900

 

I called Fel-Pro and talked to a tech guy about my leaking problem. Seems they make a seal that is .030 offset in case you have a worn groove on the crank. P/N BS11829-1. Not sure when I will jack the car up again and swap them out, but I'll let you guys know if it fixed the leak. Thanks for the help.

 

That's good to know I seen quite a few cranks that have a slight amount of wear where the old seal was riding that seal should help sealing.

 

I had changed my rear main seal and still had a leak out of the back of my motor. After a lot of cursing I took a deep breath and discovered that was fixed, but my "new" valve cover gaskets were leaking and I had an oil sender switch mounted on the back of the manifold also leaking. I was able to use a selfie stick with my phone to take pictures at the back of the motor to see the problem.

 

Sometimes it can be a real B---- to locate the actual source of a leak especially at the rear of the engine where it's hard to see because of the tightness to the firewall.

 

Raider, please advise after you change out your rear main seal as to whether the offset seal works or it doesn’t
Thanks

 

I also used an endoscope camera to help locate leaks. It is not great since it has a very narrow view through the camera and additional lighting is often helpful. I find it is often less helpful than I would like but useful in some cases.

 

Hello,
did you looked at you crankcase ventilation, pcv and breather related?
I had similar problems after i changed my valve covers with other pcv and breather than before.
Too much pressure in the crankcase.
After i taked care for better ventilation no seeping rear main seal anymore.

Markus

 

Before you tear it apart, you might try some Barr’s rear main leak stop. It’s not the old radiator crap they used to steel, but a clear stuff that supposedly swells the seal up. I had to use it in my 261, it was dripping like a sieve. Worked pretty good. Not 100 percent but way way better and I did not have to pull the motor apart.