Rear Main Seal

[John S.]

As part of a major refresh to a 1969 Corvette my mechanic replaced the rear main seal 3 times and the seal still leaks. The leak results in a silver dollar sized spot on the garage floor. The rear main is being replaced with the engine in the car, removing the steering linkage and pan to gain access. He cleans the seal area and seals with RTV. He put fluorescent dye into the crankcase to accurately see the leak origin. The engine is original with 89K miles and has had no major work.

- First time: Fel Pro flouroelastomer seal and also replaced pan gasket with a Fel Pro one piece rubber/steel core pan gasket

- Second time: New Fel Pro flouoelastomer seal and reused the pan gasket

- Third time: Fel Pro offset rear main seal and new multi-piece cork pan gasket.

After the 3rd try the seal did not drip for a couple of days <150 miles and then started leaking.

The mechanic works at the local Chevy dealer and has much experience with hot rods and restorations. He isn't a Corvette expert. He's done much of the work on the car including suspension, brakes, clutch, diff with mostly good results. $7,500 invested in parts alone. The car runs well and most of the work has been satisfactory. The diff cover flange broke and I'll always wonder if this was due to installation error. One rear strut rod bushing failed. The part from Zip looked lower quality than a new set purchased locally. The failure could be due to the part or installation error. Other systems work well, so I have reasonable confidence in the mechanic's ability and would expect him to know how to work on a small block Chevy.

The car received a chapter Top Flight last month. I plan to have it judged at a regional next year. Not sure if I'll attempt a PV. I frequently drive the car and plan to put on 5-6K miles per year.

Do you have suggestions?

Should I just live with the drip?

1969 Corvette
350CU/300HP
4 SP
AC, PS, PB
Chapter Top Flight 96.7%

Thanks

[Clem Z.]

Re: Rear Main Seal

As part of a major refresh to a 1969 Corvette my mechanic replaced the rear main seal 3 times and the seal still leaks. The leak results in a silver dollar sized spot on the garage floor. The rear main is being replaced with the engine in the car, removing the steering linkage and pan to gain access. He cleans the seal area and seals with RTV. He put fluorescent dye into the crankcase to accurately see the leak origin. The engine is original with 89K miles and has had no major work.

- First time: Fel Pro flouroelastomer seal and also replaced pan gasket with a Fel Pro one piece rubber/steel core pan gasket

- Second time: New Fel Pro flouoelastomer seal and reused the pan gasket

- Third time: Fel Pro offset rear main seal and new multi-piece cork pan gasket.

After the 3rd try the seal did not drip for a couple of days <150 miles and then started leaking.

The mechanic works at the local Chevy dealer and has much experience with hot rods and restorations. He isn't a Corvette expert. He's done much of the work on the car including suspension, brakes, clutch, diff with mostly good results. $7,500 invested in parts alone. The car runs well and most of the work has been satisfactory. The diff cover flange broke and I'll always wonder if this was due to installation error. One rear strut rod bushing failed. The part from Zip looked lower quality than a new set purchased locally. The failure could be due to the part or installation error. Other systems work well, so I have reasonable confidence in the mechanic's ability and would expect him to know how to work on a small block Chevy.

The car received a chapter Top Flight last month. I plan to have it judged at a regional next year. Not sure if I'll attempt a PV. I frequently drive the car and plan to put on 5-6K miles per year.

Do you have suggestions?

Should I just live with the drip?

1969 Corvette
350CU/300HP
4 SP
AC, PS, PB
Chapter Top Flight 96.7%

Thanks

does he seal the area marked with hash marks with non hardening permatex ?? go to https://www.forums.ncrs.org/attachme...24117&uid=6619

[John H.]

Re: Rear Main Seal

The diff cover flange broke and I'll always wonder if this was due to installation error.

John -

If you mean one of the rearmost "ears" on the cover that the rear spring retaining plate bolts thread into, that failure is almost always the result of fully-torquing the spring plate bolts with the car on a 2-post lift. Those bolts can't be torqued unless the car is on wheels, with the suspension at normal ride height, so the spring is nearly flat. That note didn't appear in the Shop Manual until 1971, although it has always been in the Assembly Manual.

[Michael B.]

Re: Rear Main Seal

The sealant has to be used sparingly and only placed in the right place. RTV is not the correct sealant for this area. Be sure to rotate the seal about 3/8" so that the ends don't lineup with the parting line of the block and main cap.

Here is a good page with good info. http://www.corvettehacks.com/article5.html

[Edward J.]

Re: Rear Main Seal

John, theres one thing that would bother me doing it again, tightning and re-tightning, the tin oil pan so many times the tin tends to bend an get weak and I am sure the he 's straightening after each replacement.

If it were me I would try the one piece rear seal, but really should have the engine out, and the main caps removed and crankshaft lifted slightly to install correctly.and you could have a closer inspection of the crankshaft where the seal rides, they have been know to wear a groove where the seal rides. and there is a repair sleeve for that.

Mikes article on the rear seal replacement is excellent, but one caution is the use of to much sealant on the rear of the main cap can be harmful to the main bearing if to much is used.

[Joe L.]

Re: Rear Main Seal

John, theres one thing that would bother me doing it again, tightning and re-tightning, the tin oil pan so many times the tin tends to bend an get weak and I am sure the he 's straightening after each replacement.

If it were me I would try the one piece rear seal, but really should have the engine out, and the main caps removed and crankshaft lifted slightly to install correctly.and you could have a closer inspection of the crankshaft where the seal rides, they have been know to wear a groove where the seal rides. and there is a repair sleeve for that.

Mikes article on the rear seal replacement is excellent, but one caution is the use of to much sealant on the rear of the main cap can be harmful to the main bearing if to much is used.

Edward-----


I'm not aware of any one piece rear main seal that can be used with a crank and block designed for a 2 piece seal.

[Tom P.]

Re: Rear Main Seal

If it were me I would try the one piece rear seal, but really should have the engine out, and the main caps removed and crankshaft lifted slightly to install correctly.and you could have a closer inspection of the crankshaft where the seal rides, they have been know to wear a groove where the seal rides. and there is a repair sleeve for that.

HUH?!?!?!?!?!?!?!?!?!?!?!?!?!?!?!?

[Edward J.]

Re: Rear Main Seal

Joe, you are correct I was thinking Fel Pro made the one piece seal.

[John M.]

Re: Rear Main Seal

John,
Just had a similar problem that turned out to be an oil gallery plug leak. The engine was bulit by previous owner and when stared for first time leaked. We replaced oil pan gasket rear, main seal, used proper technique, leaked again. Took off starter motor and inspection plate and looked up in back and the oil was coming from the plug. The plug was scary loose. Took out transmission, clutch, flywheel, etc. and replaced the plug ,which solved the problem.
I would have your guy at least check the area with inspection plate removed.

[Clem Z.]

Re: Rear Main Seal

i would also check the cam bore expansion plug for leakage

[Steven G.]

Re: Rear Main Seal

My 72' small block had a rear leak and it ended being the rear cam freeze plug that failed from factory assembly, not usually a problem but it can happen. Steve

[Michael B.]

Re: Rear Main Seal

Is the crank surface smooth? The Fel-Pro BS118291 is a special double-lip offset seal for use on a journal with groove wear. The standard blue seal (silicone rubber) is Fel-Pro #2900; the best one (flouroelastomer) is Fel-Pro #2912.

One other area to consider but rare is the PCV system. If you have a plugged or non-operating system crankcase pressure can build and cause seals to leak.

[Timothy B.]

Re: Rear Main Seal

Installed a Fel Pro 2900 rear seal on my 63 and it leaks. I purchased the GM flouroelastomer seal but have not installed it. The Permatex ultra black RTV is what I used to seal the main cap, is this the correct sealer to use for best results or is the non hardening # 2 sealer better.

[Clem Z.]

Re: Rear Main Seal

Installed a Fel Pro 2900 rear seal on my 63 and it leaks. I purchased the GM flouroelastomer seal but have not installed it. The Permatex ultra black RTV is what I used to seal the main cap, is this the correct sealer to use for best results or is the non hardening # 2 sealer better.

i always used permatex non hardening as i am not sure how good RTV is squeezed out very thin

[Michael B.]

Re: Rear Main Seal

I agree with the non-hardening sealer when sealing a highly machined part using a high clamping force. We don't use RTV on head gasket surfaces and I wouldn't use it here either. Just my opinion though.