The Stovebolt Site Forums The Shop Area Engine Shop 1955 Chevy 235 Rear Main Seal Leak

I have a 235 6 cylinder engine in a 55 chevy . The rear main seal started leaking so I first replaced the lower half, the leak got worse. I then replaced both the upper and lower half with a new seal,no improvement. I am using the neoprene type seal. I installed the seal per the instructions and I did not use any sealers and I am keeping the ends of the seal dry before assembly. I am facing the seal the proper way. I am lubing the seal with motor oil before assembly. The local auto parts store only lists 2 types of seals, the one I am using and the old rope type. Is there something I am missing? Is there another seal available?

I called Fel Pro and the suggestions they had was to stagger the seal slighty and check the bearing clearances. I checked the bearing inserts while I had it apart and they look fine, I used plastigauge when I built it and the bearings were at factory specs. The block was line bored and the mains on the crank were turned .030 during the rebuild. Is .030 enough to cause problems with the seal? The seal surface on the crank is fine. It has about 20,000 miles on it since a complete rebuild. Should I be using some type of sealer in the main cap grooves? Does anyone know of a way to fix the leak? The current leak will cover about 1/2 of the surface of a flat drip pan in one week. The engine runs fine but I want to end this leak. It leaks when it is running and after it stops it will drip about 1 drop per 30 seconds out of the bearing cap for about 3 minutes.

I have heard that 55 was the first year that the neoprene type seal was available and that the caps didn't necessarily fit the new seal that well. If that is the case, will silicone fix the fit?

don't know of anyway to fake it w/ a main seal - if the seal don't work the seal ain't right - especially if Fel-Pro didn't suggest anything other than changing install technique a bit

I used the rope seal in my 57 and it worked fine - was advised by an oooold mech that the neoprene didn't work well where rope was original, tho I seem to recall others here saying they'd used it

Bill

I am rebuilding my 55 and was advised to use the rope seal because the way the 55's were machined would not allow a complete seal with the neoprene type.

Welcome to The Stovebolt Site, Venture !!

Always good to have another Texan on the site!

I'm going to make a suggestion that could be out of left field, but you might want to check to be sure, because I had this same problem and it WASN'T the rear main seal. It turned out to be missing alignment pins on the back crankshaft flange where the flywheel bolts on. There are three alignment pins in the flange of the crankshaft and if one, or more, is not installed, the oil pumps right through the hole, and dumps it into the front of the flywheel cover. It usually comes out right at the rear seal area. Unless you are sure those pins are installed, you might want to check it out.

There is one other spot where it can come from and fool you, but you probably checked that spot - the rear of the valvecover. If it is leaking it can run right down the back of the engine and make you think it's coming from the rear main seal area.

I'm like you - I hate oil leaks. People will say to me, "It's an old truck, it's supposed to leak oil and stuff!" Well, not my truck! I hate oil leaks - any kind of leaks!!

Good luck and I hope maybe this helps you out!

Joe

Venture,
Would it possible that it is the rear pan gasket seal that is leaking and not the main seal?


Stuart.

It is not uncommon for a crank grinder to turn down the rear seal area. If it has been turned down more than a few thousands a neoprene seal will not work.

Dave/Venture,
I am not sure on this, but I remember the rear main seal surface on these engines being separated from the main bearing journal area by a slinger that was part of the crank. The seal surface was very narrow...too narrow for a crank grinder! When a crank was ground, this seal area wasn't touched. I don't understand how a neoprene seal (top half) could be installed with the crank in the engine. I have done several rope seal installs with the crank in the engine ( loosen all mains and drop as much as possible) using the chinese finger trap. This requires lots of slick-um, rolling of the crank and later, precise trimming of the ends. Seals will seal when properly insalled, so keep at it.

Stuart.

I had the exact same problem, tried the exact same things to fix it. Then I forgot about what everyone else said and did it my way, it does not leak at all now! My sealing surface was perfect and original size.

I have a 55 235 in my 52 truck. I rebuilt it a few years ago, had a friend help me with it who had done it many times before. We put the two piece rubber seal in correctly. It leaked about a quart every week of local driving.

So I had another friend, a old mechanic and school teacher, help with the replacement of the rear main seal again, as per the instructions. No difference in the leak.

The third time I got a NOS two piece rubber seal and did it again as per the instructions. No difference.

Then I promptly pulled the engine out of the truck so I could get to it really easy and also check for a leaking cam plug, as it's impossible to tell with the bell housing on. I put a rope seal in this time, but without removing the crank, so it was not done exactly right. It leaked about two quarts in 25 miles, that sucks!

Then finally I put in another rubber seal, this time I ground the ends of the wires in the seal so it could close up better. Made some brass shim strips about 0.003" or 0.004" thick, I forget to go between the block and the seal to make the seal fit tighter on the crank. Then put it all together with black silicone behind the seal and also on the ends, just like they say not to do.

I have driven it 8,000 miles in a month, all the way around the country, with only 2 quarts added. How is that for results?

The shims may be overkill, but it did work for me. At a minimum I would use silicone behind the seal.

Someone asked how it is possible to put the upper half of a neoprene seal in, with the crankshaft in place. Well I have done this before and it wasn't all that hard. The trick is to compress the seal against the crank, away from the block, at the inserting point. It you don't do that, the sharp edge of the block will carve a strip off the seal.

Thanks for the help guys. I recently replaced the valve cover gasket and of course the oil pan gaskets and they are sealing fine. The next time I have a couple of days off in a row, I will check the crank pin and cam plug areas.

In answer to the question on putting the seal in while the crank is still installed, it is pretty straightforward. Just lube the lip of the seal up, keep it against the crank and it rolls right into the groove around the crank.

In case it can help anyone else, I am also relating an oil leak that I did solve on this same engine that was pretty strange. The engine used to leak twice as much and from the rear crank seal area. I finally got sick of it and jacked it up ( on jack stands of course), had my wife run the car at 55 MPH on the speedometer for about 5 minutes and I finally spotted this leak at the rear of the head in the head gasket area at the very back of the head. It was running down the passenger side of the bell housing, down the inside of the dust sheld and then out of the hole on the bottom of the dust shield. It only leaked when it was hot and under load? I replaced the old steel shim head gasket with one of the new composition type and no more leak!

Venture, I wrote this Tech Tip Rear Main Seal Replacement some years ago. I prefer the wick (rope) type seal because rust and abrasion can make it so that the neoprene seal doesn't seat completely. The wick seal will expand to fill the minute gap that the neporene misses. I would suggest trying the wick seal with the asbestos in it (that's the old rope seals).