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Old 01-10-2019, 09:49 AM   #1
stewgee
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Default Main bearing oil leak

Just got finished reinstalling my original 4 in my 30 A after replacing all seals and gaskets short of the head gasket. The old car was running great with no smoke but was leaking oil profusely from the rear main. Now it doesn't leak "profusely" from the rear main but still leaks. I cleaned all removed parts and surfaces and used a quality gasket sealer when I put it back together. Cleaned about 2 inches of sludge out of the oil pan and installed a new oil pump screen. Anyone have any ideas or suggestions as to what I may have done wrong (or could have done differently)? It did seem a bit difficult installing the rear seal. Sure don't look forward to pulling the engine again.
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Old 01-10-2019, 10:05 AM   #2
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Default Re: Main bearing oil leak

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Old 01-10-2019, 10:29 AM   #3
BRENT in 10-uh-C
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Default Re: Main bearing oil leak

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Originally Posted by stewgee View Post
Just got finished reinstalling my original 4 in my 30 A after replacing all seals and gaskets short of the head gasket. The old car was running great with no smoke but was leaking oil profusely from the rear main. Now it doesn't leak "profusely" from the rear main but still leaks. I cleaned all removed parts and surfaces and used a quality gasket sealer when I put it back together. Cleaned about 2 inches of sludge out of the oil pan and installed a new oil pump screen. Anyone have any ideas or suggestions as to what I may have done wrong (or could have done differently)? It did seem a bit difficult installing the rear seal. Sure don't look forward to pulling the engine again.


It appears you shave too much clearance between the rear main bearing surface and the crankshaft journal. This likely needs to be adjusted. Th engine will not need to be removed for this procedure, but you probably will wish you had done this when it was removed from the chassis. There are many publications that outline how to properly do this bearing adjustment procedure.
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Old 01-10-2019, 12:12 PM   #4
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Default Re: Main bearing oil leak

Is the "rear seal" the stock slinger type, or an aftermarket rubber seal? Reason I ask is that stock seal should not be difficult to install.
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Old 01-10-2019, 03:48 PM   #5
Kurt in NJ
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Default Re: Main bearing oil leak

Where is the oil level--- if it is touching the "F" lower the level till it's a ways below the full mark
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Old 01-10-2019, 03:56 PM   #6
Charlie Stephens
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Default Re: Main bearing oil leak

Did you put a gasket between the engine and the flywheel housing? It seals the back of the camshaft.

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Old 01-10-2019, 09:02 PM   #7
James Rogers
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Default Re: Main bearing oil leak

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Originally Posted by PC/SR View Post
Is the "rear seal" the stock slinger type, or an aftermarket rubber seal? Reason I ask is that stock seal should not be difficult to install.
There is no "seal" for the rear main.
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Old 01-10-2019, 09:02 PM   #8
James Rogers
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Default Re: Main bearing oil leak

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Originally Posted by stewgee View Post
Just got finished reinstalling my original 4 in my 30 A after replacing all seals and gaskets short of the head gasket. The old car was running great with no smoke but was leaking oil profusely from the rear main. Now it doesn't leak "profusely" from the rear main but still leaks. I cleaned all removed parts and surfaces and used a quality gasket sealer when I put it back together. Cleaned about 2 inches of sludge out of the oil pan and installed a new oil pump screen. Anyone have any ideas or suggestions as to what I may have done wrong (or could have done differently)? It did seem a bit difficult installing the rear seal. Sure don't look forward to pulling the engine again.
Oil is controlled by bearing clearance only. If the bearing needs shims removed that will cause a leak. If the bearing clearances out between .002 and .0015 it is probably the thrust. You can check this by trying to move the crank forward and backwards. This should not move more than .012. If it does, you will need the bearings repoured and align bored or just deal with the leak.
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Old 01-10-2019, 09:26 PM   #9
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Default Re: Main bearing oil leak

There a rear seal on the oil pan to block at the 3rd main as well as the front rope seal. The rear pan seal needs to be trimmed down some as they are too long.
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Old 01-10-2019, 10:14 PM   #10
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Default Re: Main bearing oil leak

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There is no "seal" for the rear main.
If there is no rear main seal why does Mac's sell one? I'm new to the Mod A world so maybe I'm confused or just don't know. I did install the gasket that Mac's sold me in the gasket set for the engine. Just asking--I'm A learning
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Old 01-10-2019, 10:19 PM   #11
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Default Re: Main bearing oil leak

Charlie Stephens I did put a gasket between the engine and flywheel housing. And Kirt in NJ I have lowered the oil level to about 3/4 full.
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Old 01-11-2019, 12:49 AM   #12
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Default Re: Main bearing oil leak

Installing a seal in the rear main requires modification of the crankshaft to install. I hope you didn't try and fit a seal on a crank that did not have the slinger machined off? If this is a case were you installed the seal without removing the slinger, I think you are going to have to pull the engine and fix.


Re; Oil is controlled by bearing clearance only. The slinger is at the back and it is there to prevent oil flowing out the back with the tube acting as a drain.
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Old 01-11-2019, 09:48 AM   #13
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Default Re: Main bearing oil leak

There is an aluminum half circle part with two grooves that fits into the rear of the engine block . This part , part number 8960 is on page 44 of the 2017 Brattons catalog . Though this part isn't a real seal , it is called the rear oil seal . If this aluminum part is installed too deep in the block , it will cause an oil leak at the rear main , The ends of this part needs to be level with the block or it will leave a gap where oil will continue to leak .
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Old 01-11-2019, 09:08 PM   #14
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Default Re: Main bearing oil leak

I agree with Brent and James, the rear main clearance has always been the issue when this has happened on my Model As. To avoid doing it twice, it needs be set on the tight side closer to .001.-.0015. From my experience, it still leaks if set to .002.

It's a bit of a pain to do while the engine is in the car, but not impossible.
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Old 01-11-2019, 10:08 PM   #15
James Rogers
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Default Re: Main bearing oil leak

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Originally Posted by stewgee View Post
If there is no rear main seal why does Mac's sell one? I'm new to the Mod A world so maybe I'm confused or just don't know. I did install the gasket that Mac's sold me in the gasket set for the engine. Just asking--I'm A learning
Originally there was no seal in the rear main. The seal that the vendors sell are modern "fixes" for leakers that don't work. I have found engines that have had ropes installed in the grooves that never work and leak. Never seen a rope seal or rubber seal or a cork seal that worked for any length of time. Ford knew best.
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Old 01-11-2019, 10:29 PM   #16
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Default Re: Main bearing oil leak

You guys are great. Thanks for all the good info for a newbee A to digest. I know you all have been here just learning the basics of the model A.
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Old 01-12-2019, 11:05 AM   #17
Purdy Swoft
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Default Re: Main bearing oil leak

I agree that bearing clearance is one of the main causes of main bearing leaks . leaks around the main bearing bolts can also be a problem . Snyders offers a new main bearing bolt kit that can also help . the kit contains special sealing washers that help . I feel that it is best to address every possibility while you are at it .
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Old 01-12-2019, 05:11 PM   #18
Kohnke Rebabbitting
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Default Re: Main bearing oil leak

Any end play clearance over .006 will start oil leaking. You have to seal the shim packs, and both ends of the main bolts. The oil pipe has to be utilized, and has to be clear of babbitt, and excess threaded portion of pipe thread. That is a clean 3/8's hole.

Shaft clearance on a Model A should never be .001, you will remove bearing material, and wasting it, and also take a big chance of smearing the first .010, of bearing surface. When this is done, the outer surface starts moving to fill the oil grooving, and creating an irregular bearing surface. Then the babbitt material that went into the oil grooves, break off and run back through the bearing, creating more bearing disfigurement.

The shaft clearance should be from .001-60, to not over .002-25. When the mains are broke in, they will be closer to .003 clearance, when checked cold. The harder you drive the car, the more the crank expands, the more clearance the bearing will have. The crank when normal Temp., will have expanded at least .001-50 thousandths, so at .001, to .001-50, where is the clearance, for oil?

We set Model A cranks to .002-25.

Also, any Model A crank, including all other engines, of course, has to be dead center of the front, and rear main bolt holes. Many of Model A's, " new Pours ", are off by .010 thousandths, that I have seen. That rubs the Aluminum
slinger on one side, and has a big gap on the other side, to gush oil, and there is only one way to fix it.

Herm.
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