The Stovebolt Site Forums The Shop Area Engine Shop Rear main seal

Hi Bolties! I have a question about rear main seal preference. My 49 Chevy truck w/ a 1957 235 has been leaking oil from the hole in the flywheel cover, so I figure it must be the rear main seal gone bad. The engine was freshly overhauled about 600 miles ago (about 1 year). Tonight I pulled the pan, and the rear bearing cap to discover that instead of the ol'e wick seal material, there are real nice rubber seals in the block and cap. They look to have wire in the center. I guess I haven't been very observant, because I got into a new gasket set I had sitting on the shelf and found a new set of rear main / rubber seals.
I don't know why the rubber seals would have already gone bad, and started leaking, but something is causing the ol'e girl to leak. Do any of you think the wick seals are better or are the rubber seals better at sealing?
Thanks!
KT.
1949 3/4 ton Chevy..1950 1/2 ton Chevy.

I never realy liked replacing rope seals. They can be a real pain in the a$$. But I have been sucessfull with every one I put in. The 2 piece rubber seals work as well just make sure you use a little dab of sealant between the 2 halves. I have done several of these as well and only had 1 fail to seal.
Fail rate 0 for rope, 1 for rubber. Close call.

Hi Toyvo. You know now that you mention it, I don't belive I seen any sign of sealant between the block, and the cap. That may be why it was leaking. I can't believe the guy that did the work failed to do this. He has forgot more about engine (especially 235's) than most people will ever know, but I didn't see any sealer.

I guess even the good mechanic/machinist's can make a mistake!!!
Thanks!
KT.
1949 3/4 ton Chevy..1950 1/2 ton Chevy.

kthomas,

You have already attacked the problem with enthusiasm, so this might be a moot point.

If it was a quality rebuild by a experienced mechanic, maybe is isn't the rear main leaking.

Oil can flow down a surface and appear to be leaking from somewhere it isn't. The flywheel cover is the lowest point on the back of the engine...a natural for a drip!

Only 600 miles on a new main seal says "sumpin's wrong here".

Stuart

I just replaced my rear main seal last week. The Neoprene seal(rubber) installs only one way. The long lip towards the front of the vehicle. If installed backwards.....the possibility of a leak is increased.

Hey guy's! Thanks to all who replied. I really believe that there wasn't a "Dab" of sealant between the bearing cap and block which would allow oil to get between them, and run it's way back and down to the flywheel cover. I am going to put it back together tomorrow night, give a night to for the silicone to dry, and I'll put the oil back in it, and give it a drive to see if I got it cured.

Thanks!!
KT.
1949 3/4 ton Chevy..1950 1/2 ton Chevy

Well bolters, I got the engine back together, allowed the silicone on the pan to dry for 24 hrs., and even put a "Dab" of silicone between the rear main cap, and engine block around the area where the 2 seal halves meet. I put in new rubber seals in place of the rope seals. I made sure the lip was toward the front of the engine, and not put in backwards. Tonight I took it for about a 9 mile test run, and to my disappointment, there is just as much oil leaking from the flywheel cover as before.

Well, I guess it's back to the drawing board. I think I will replace the rubber seals with the rope seals. This working under the truck is getting old!

KT
1949 3/4 ton Chevy..1950 1/2 ton Chevy

kthomas,

Sorry to hear the news. I will mention it once more then forever hold my tongue...are you 101% sure the oil is coming from the rear main seal?

What does the area INSIDE the bell housing look like...is it coated with oil in places where it would have to have been thrown.

Stuart

A neat trick to when installing the rubber seal is to put the ends offset a bit. One end of the seal will be recessed in the cap and the opposite end will be exposed. The same thing when you put the upper half in, offset it the same amount.
Might be hard to understand by my description but if you look at the seal installed in the cap under normal installation, one end is at 9 o'clock and the other end is at 3 o'clock. Both ends line up with the flat (mating) surface of the cap.
If you install the seal so one end is at 10 o'clock the other end will be at 4 o'clock, this method will help prevent oil leaking along the bearing cap mating surfaces because when the oil gets on the surface it doesn't meet a seam or butt joint in the seal.
Hard to explain or understand I am sure. Another way to describe it is if your mating surface is horizontal you should put your seal in above horizontal.

Hi Stuart, and olblu49! You guys have always been great to help me and I appreciate that! Stuart, as best as I can recall is that there was some light coating of oil on the front of the flywheel but not too much. I didn't see oil any further back, such as on the pressure plate or I would have thought I would have seen it dripping from it. I have noticed that not only is there oil leaking from the flywheel cover, but it's also on the right side of the "Hogs head" near the right rear motor mount. Olblu49, I think I understand what you mean and I even considered doing what you suggested but I guess I thought the silicone would seal off any gap. Your saying to rotate the top and bottom half of the seal so that one side of the joint is in the engine block, and the other is in the rear main cap. Am I right?

KT
1949 3/4 ton Chevy..1950 1/2 ton Chevy

kthomas that is correct. It just adds an extra bit of security.
Going back to your original oil leak, how much are you losing? The hole in the flywheel cover is to let oil drip out.
I had read somewhere that GM had special cardboard mats which they placed under the new trucks and cars in the show rooms to catch the drips.
BTW I rely on Stuart too!

kthomas,

The oil on only the front of the flywheel sure points a finger at the rear main.

I am not sure what the 'hogs head' is, but it should be below the level of the main seal area for a leak to affect it.

Have you checked all the gasketed surfaces above the bellhousing...rear of the valve cover, side cover etc. Anything that could run down or be blown back to the area where it drips.

The pan is sealed in this same area with the strip over the rear main bearing cap.

It very well could be the rear main seal that is truly leaking, but it would be great if it was just some simple old 10 cent gasket.

Stuart

Hey Stuart, I know this may sound far fetched, but I wonder if it's possible the oil that is leaking now, could be coming from up in the bell housing (hogshead)?

I did not clean any oil from inside the bell housing before I put the pan back on. The reason I say this is because the oil that leaked out lastnight is used/brown.

I just put fresh oil back in the engine. I thought maybe when the engine warmed up from the 9-10 mile drive, maybe it started the old oil running out. Knowing how my luck goes, that would be too good to be true!

KT
1949 3/4 ton Chevy..1950 1/2 ton Chevy

kthomas,

It would be great if that were the case. My luck wouldn't run that way, but maybe you were born under the proverbial lucky star.

I think I was born under a old leaky Chevrolet truck.

Stuart

I just noticed your comment about oil on the rt motor mount. Check the side cover and valve cover closely for seepage. Oil running down the right rear of the engine could be making its way to this area from these.

Also, I know that the early full pressure 235s were not machined to take the neoprene seals. But, I don't know where the change was made. If the gaskets are ok, I think it would be wise to go back with a rope seal. Pull the trans far enough back to clear the pilot bearing, loosen the main caps and the crank will drop far enough to easily roll a new upper in place.

I don't know if this has been mentioned yet, but have you felt behind the back of the valve cover on the head. If this is leaking, it will end up in the same place. The bellhousing does not seal anything. It is just a big supporting member of the engine/transmission combination.

Hmmmm.

I've been replacing these seals in 235's since I was just a kid (dang has it been that long- well, no-never-mind) and I'm seeing that others are putting you in the right directions.

Check the rear of the valve cover on both sides and rear with a clean white cloth. Any oil? If not, then it is in the rear main seal.

Back to the rear main- For some reason your mechanic (old school- I presume) felt that the rope seal, although a 57 235 comes with the neoprene, was what was called for (He should have known this). Unless, he saw that the surface where the seal mates had some sort of imperfections on it. These old engines can sit for long periods and the mating surface for the main seal can pit or build up gums and tars, often when the crank is turned this surface doesn't get the attention it needs.

If there are imperfections in the rear main mating area these can, at times, be overcome by using the 'rope seal' because it will expand when the oil gets to it, there-by letting it 'fill in' the entire area.

I know this is being fastidious of me, but, check for tar a gum buildup (or heaven forbid, pitting) on the rear main seal mating area.(this is usually where the problem arises) This can be dressed out with some fine emery cloth (being sure to clean the area properly afterwards)

Then follow this Tech Tip don't, I repeat- DON'T put sealant under the cap- it won't torque nor seat properly.

If you still are having problems with this, feel free to contact me or just keep posting here.

Hey guys! Thanks so much for all the advice!! I have had a 216 valve cover on this engine, and I have noticed some oil seeping out under the right side of it, but not real significant.

Today I replaced it with a 235 cover and new gasket. I will double check the back of the engine, but I have looked at the top of the bell housing and so far, haven't seen any signs of oil.

I will take a look at the pushrod cover and make sure there is no oil coming from it.

I purchased a new pan gasket set, and rope seals. I plan to replace the neoprene with rope seals.

Hi Oly. I have seen the mating surfaces and there don't appear to be any pitting. They look clean and smooth.

My mechanic is indeed old school! He's built a lot of 235's, and racing engines. He used the have a full line machine shop business at his home, but sold his equipment and for the last several years has worked for a machine shop in Joplin.

When he put the engine back together, he used the neoprene seals, probably because that was what the gasket set came with, but who knows!

It went for a while without leaking, and then one day I had drove the truck on a 20 minute drive, and noticed an oil leak, and I HATE oil leaks!

Especially on a freshly rebuilt engine. I am going to try the rope seals and see what happens.

Again, a big THANKS! to all who have gave direction and advice on this project!

KT
1949 3/4 ton Chevy..1950 1/2 ton Chevy

Hi Bolters! I thought I would give you an update on this project. I got the rope seal in, and finally got to take the truck out for a drive today. To my disappointment, there is STILL an oil leak!

When I looked under the truck, the oil wasn't coming from the hole in the flywheel cover, but it did seem to be coming out on the right side between the cover and the bottom of the bell housing.

It was running down the right side of the flywheel cover.

The back of the engine is dry, and there is no oil coming from the pushrod cover

It also looks like it was coming from the holes up on the bell housing on both left and right sides.

This thing should'nt be this hard to figure out!Well, I guess it's back to the drawing board.

KT
1949 3/4 ton Chevy..1950 1/2 ton Chevy

Could it be an oil gallery plug or cam plug in the rear of the engine leaking? Either may partially be hid by the bell housing. This would allow oil into the dust cover, but not from the rear main seal. Maybe the cam plug is not installed correctly.

KT are you getting oil on your clutch?

You said oil is coming out the bolts at the top of the bell housing? If the rear seal is leaking oil will fling upwards and give the impression the leak is higher than it actually is.

Remove both bolts and put sealer on them (the holes should not be all the way through) and reinstall.

If you have a bud with a lift this next thing would be tons easier.


Go get some brake cleaner about 3 cans, remove the lower inspection cover and spray the brake clean untill there is no sign of oil. With a helper at the switch and you on the ground start the engine and look up in the bell housing with a flash light. If there are no leaks after 3-4 min. Turn it off and keep watching. Some time seals will leak worse when engine is off.

Remember hot oil is not somthing you want driping on your face. Be carefull!

Hey guys! You know, I feel that I did a good job of getting the rope seal in, and getting the pan sealed. I have to wonder if it could be a gallery or I have even contemplated taking the tranny out (I hate to, it's a heavy 4-speed)!, and looking at the end of the crank just to make sure all the pins are in it.

Hey Toyvo! When I had everything apart last weekend, I looked at the pressure plate, and there didn't seem to be any oil on it, just on the front of the fly wheel.

Also, the clutch isn't slipping, so I figure I'm ok on that at least at this time.

The holes I'm refering to are the timing / pointer hole, and the other similar one on the left side.

I do like your idea about the brake cleaner. The first thing I want to do is to take off the flywheel cover and clean up the area.

I may even want to leave it off after cleaning, and drive it to get the engine warmed up since that is when it really leaks, then I can inspect it to see if I can tell where it is coming from.

And you are certainly right about the bud with a lift! God I hate laying under a vehicle trying to work!

KT
1949 3/4 ton Chevy..1950 1/2 ton Chevy

I have read with interest this thread I have the same problem My old time mechanic replaced the rope seal and he went through all the procedures that have been mentioned I was there for a while and he was very meticulous at what he was doing.

Well I still have an oil leak!!! My tranny has been service all sealed up , the oil pan is tight. It seems like it's hit or miss on these rear seals I'm like kthomas I hate oil leeks so does my wife I'm ok where I park but it dribbles on my drive way when I leave the yard I would hate to have him replace the seal again because he did such agood job. It seem to be leaking more on the passenger side or maybe thats where it wants to flow too. Any suggestions here there is to much oil to be coming from anywhere else.!!!!!

Peter,

If you have your flywheel cover installed, I think a rear main seal leak would travel down the cover and be centered on the truck.

For the drips to be off center, I would look at the side cover, or maybe the fuel pump gasket.

Both those items are on the passenger side. Being "handed" might make it easier to find Mr. Leak!


Stuart

i wondered that but i can't see any oil leaking there (Side cover) i have an electic fuel pump.. There is still oil comming out of the hole on the flywheel cover ....