Oil leak - rear of SB engine

[Ed S.]

I have a 64, original L76, rebuilt (stock) about 13 years ago. I developed a slow oil leak at the rear of the engine - it started about a year ago. Following a short drive it will lose about a tea spoon of oil, but it will continue to drip slowly for up to a week after the engine has been turned off. Oil drips from the bottom of the bell housing where it meets the inspection plate.

Thinking it was the rear main seal, I pulled the starter, oil filter and inspection plate. Have not yet removed the oil pan.

The flywheel is very clean and completely dry - it even had a slight coat of rust on the front (engine) side in some areas. Bell housing is mostly dry, some moisture at the bottom towards the front but not much. According to other threads I've read if the rear main is leaking the fly wheel should be wet. The back side of the inspection plate is pretty dry except towards the bottom. The block looks dry in the vicinity of the rear seal - at the very back of the oil pan.

I don't think I have a rear main seal problem. I checked the valve covers - dry all around, the block and top side of the bell housing are dry too. I did put a wrench on the back two bolts that hold the oil pan on, one side took about a quarter turn to make it snug, the other side took a little more than a quarter turn - they were both loose. Some wetness around both bolts.

Could those two rear pan bolts be the problem? If not, any thoughts or suggestions on where the oil is leaking from?

[Mark E.]

Re: Oil leak - rear of SB engine

If it leaks for a week after shutdown, it's unlikely the problem is the RMS or oil pan gasket.

These engines are tilted back, so leaks can travel back and look like they originate from the rear.

Easy to fix yet common culprits include: oil sending unit, oil filter, drain plug (fan can blow oil from this to bell housing), valve covers, fuel pump.

Check for oil traveling back from the front seal too.

Usually these only leak while or soon after the engine is running, but it takes awhile for the oil to travel to a low spot where it drops.

To pinpoint the sources, you may need to clean the entire engine really well, then look for newly emerging oil with a black light.

[Edward J.]

Re: Oil leak - rear of SB engine

Ed it surely sounds like a rear seal on the oil pan, The small block oil pans can be a nightmare to get 100% sealed, it takes a really nice oil pan without bends and a super flat surface to start, and then a good prep to remove the oil gasket and super dry surface, and some good luck sealing the corners of the rubbers, and then the proper torque at least three trips around all the bolts starting with lower torque values, and then after a few road trips recheck again. I have seen other say they like the one piece oil pan gasket from Fel Pro, I have had no luck with them, 50 year old tin oil pans are you to distorted, cork is more forgiving.

[Mark E.]

Re: Oil leak - rear of SB engine

Also want to mention the rear intake seal to check...

[Ed S.]

Re: Oil leak - rear of SB engine

Update - my engine was pretty clean to start with - I have since wiped the bottom half & oil pan down with denatured alcohol - front to back. The paper towels I used were not stained with wet oil. I notices some "wetness" along the mating line between the trans (4 speed) and the bell housing. Washed it with brake cleaner & dried it - then wiped it clean with a Q tip.

Thirty minutes later there was some wetness along the point where the two components meet. I think I found the culprit - front transmission seal. The oil that drips on the epoxy floor is not very "fluid" - it doesn't run or spread out - might be 90 weight trans fluid. Will monitor closely - right now I am not taking the oil pan off. Thanks to all that provided advice.

[Frank D.]

Re: Oil leak - rear of SB engine

If its a Muncie it could well be the countershaft hole....a known issue...

[Ed S.]

Re: Oil leak - rear of SB engine

Frank - it's an M21 Muncie, it's late, I will check tomorrow. After I thoroughly cleaned all areas I observed some wetness right at the seam where the trans case mates to the bell housing. It would be about an inch above the counter-shaft hole. Just above the arrow head in your picture.

[Edward J.]

Re: Oil leak - rear of SB engine

Ed gear oil has a weird smell that does not wash of easily when you get it on your skin. so it should be easy to know what your dealing with.if it is gear oil from the counter shaft, I have had some luck smearing some sealant over the shaft, just be sure to have the area clean. and as you see in frank picture a couple of longer bolts will let you slide the trans back, put the shifter in 1or3rd gear and remove the hanger and mount bolts.

[Duke W.]

Re: Oil leak - rear of SB engine

If the front face of the flywheel is dry, then I doubt if it's the rear main seal. One culprit could be the oil gallery plugs at the rear face of the block. Engine shops usually drill and tap for pipe plugs rather than replacing the OE Welsh plugs, and properly installed pipe plugs rarely leak.

As stated 80W-90 GL-5 gear oil that is the proper oil for old manual transmissions has a pungent odor from the EP additive, which should make it easy to ID.

Duke

[Ed S.]

Re: Oil leak - rear of SB engine

Ed, Duke,
Thanks for weighing in on this problem. After close examination of the flywheel and surfaces around the back of the oil pan and rear seal I pretty much eliminated the main seal being the culprit - and, I based that decision on comments by Duke that I have read in other threads - particularly about a clean / dry flywheel.

I will try to confirm that it is gear oil via the recommended sniff test. Judging from the viscosity of the drips I tend to think that is what it is. Hope it is not the pipe plugs. Will report on progress or findings.

[Joe L.]

Re: Oil leak - rear of SB engine

Ed, Duke,
Thanks for weighing in on this problem. After close examination of the flywheel and surfaces around the back of the oil pan and rear seal I pretty much eliminated the main seal being the culprit - and, I based that decision on comments by Duke that I have read in other threads - particularly about a clean / dry flywheel.

I will try to confirm that it is gear oil via the recommended sniff test. Judging from the viscosity of the drips I tend to think that is what it is. Hope it is not the pipe plugs. Will report on progress or findings.

Ed------


The 3 oil gallery plugs on the rear of the block are 1/4" NPT. They rarely leak. In fact, they can be a real bitch to remove. The factory used some kind of hardening sealer on these. The rear cam plug can leak, though. It's just a drive-in welsh-style plug.

If it's the transmission, it could be a leak at the countershaft orifice as others have mentioned. It could also be a leak at the front bearing retainer gasket or a leak from worn threads of the bearing retainer bolts.

One more comment: I've rarely, if ever, seen a small block that did not evidence the "oil drop at the bottom of the bellhousing" syndrome. Of course, there are many possible sources but despite the best efforts to locate and repair, the problem usually persists.

[Ed S.]

Re: Oil leak - rear of SB engine

Thanks Joe - After inspecting the trans / bell housing interface this morning I am pretty confident that it is the trans, either the counter shaft or main. Yesterday I cleaned that area with brake cleaner, dried it, wiped it clean and ran a Q tip over it. It was spotless & dry. This morning there is a slight bit of "wetness" at that interface, not much, but enough to capture on a Q tip. I am pretty sure that is the culprit. Knowing that it is not engine oil - I can live with this leak for a while -until there is a more opportune time to correct it. Before I started this I was concerned that it was the rear main seal - and I have heard that they have a reputation for going from a drip to a gush quickly. I am sleeping better now - again, thanks to all, this is a great forum, and we are all fortunate to have technically proficient friends that we can rely on for assistance.

[Terry M.]

Re: Oil leak - rear of SB engine

Thanks Joe - After inspecting the trans / bell housing interface this morning I am pretty confident that it is the trans, either the counter shaft or main. Yesterday I cleaned that area with brake cleaner, dried it, wiped it clean and ran a Q tip over it. It was spotless & dry. This morning there is a slight bit of "wetness" at that interface, not much, but enough to capture on a Q tip. I am pretty sure that is the culprit. Knowing that it is not engine oil - I can live with this leak for a while -until there is a more opportune time to correct it. Before I started this I was concerned that it was the rear main seal - and I have heard that they have a reputation for going from a drip to a gush quickly. I am sleeping better now - again, thanks to all, this is a great forum, and we are all fortunate to have technically proficient friends that we can rely on for assistance.

It might be prudent to check the transmission fluid level and top off as necessary.

[Frank D.]

Re: Oil leak - rear of SB engine

If it is indeed the countershaft hole then the 'quick and dirty' fix is not that bad.....some RTV and possibly a bit of rubber over the shaft hole - VERY thin rubber. My car has run with this arrangement for about 3 years now with no leaks and no issues.... I really wanted to keep that early Muncie case in the car (its a late May vehicle)....

[Ed S.]

Re: Oil leak - rear of SB engine

I am quite confident that it is trans fluid, which is a huge relief. My engine doesn't leak any oil - a rarity for a SB. Correcting the trans leak is not a high priority - will get to it sometime this winter. Hopefully I will find that it is the counter-shaft hole, if it is I'll attempt the RTV fix - if it is the front main seal..... well... I guess the trans will have to come out for that one. I learned a good bit on this thread - thanks to all those with experience in this issue were kind enough to lend a helpful thought. Greatly appreciated - thank you.