Rear Main Leaking on 31 Coupe

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IaFarmBoys

New member
We have a 1931 Coupe that was leaking oil out the rear main. Have read through several posts regarding this issue, but still need a bit of extra advice. We started with basic tests, made sure the oil fill tube was open. Oil was dripping off the bolts on the rear main. Dropped the pan and checked the oil tube on the rear main and it is open. Oil slinger looked good. Replaced shims on all three mains and all are approximately twenty-thousandths. We did not pull the engine, but used the old method of removing the shims from each cap, replacing them with new ones and removing them until the engine couldn’t be cranked. We then added one more shim until the engine could be turned. Torqued all main bolts to 80 lbs. Oil seems to be running out more now than before. Any suggestions?
 
I assume oil is dripping off the cotter key . Usually indicates rear main leak in lieu of pan gasket leaking. Could be coming out the back of the cam which also will drip off cotter key . Oil will also leak around the rear main bolts. Little permetex around bolts will stop this leak. Make sure oil return tube is not screwed in to far . There was a similar thread on here a couple of days ago in which all remedies were addressed.
 
did you plastigage the mains. You need .001 to .0015 clearance. Any thing more than that will cause a leak.
 
If you measure the mains with the engine in the car, put a jack under the crank and gently raise it to fit tightly in the raised position. This will give you a more accurate measurement with your plastigage.
 
You did it correctly by removing shims till it locked the motor then adding a shim till it could be turned. Many engines were done this way for many years and it works as a shade tree fix. Your problem is probably in the thrust. Check to see if the crank moves more than about .012 front to rear. To do this, gently press the crank towards the rear and have someone push the clutch pedal to see how much the crank moves. Anything over .012 will cause the leak to be worse exponentially.
 
You are using the correct shim, aren't you? You mentioned you had 20 thousandths clearance, must have ment .002 not .020.that would be way wide.
 
Just because the drain tube isn't blocked, means nothing. If the drain tube is screwed in the least bit too far it will create a dam that wil hinder the oil from draining completely out of the passage in the rear main bearing cap and out the drain tube. If the cap isn't perfectly flat where it mates to the block, this will create a gap and oil will leak down the side of the cap and drip off the main bolts-nuts. There is an aluminum half circle part that some call a seal that fits up into the block at the rear. If this part is pushed too far up into the block it will leave a gap on one or both sides that will cause a leak. This part needs to be level with the block, if not, remove and try again. If the aluminum seal is level with the block, I don't mess with it at all. This part isn't actually a seal and replacement usually will have no effect. The important thing is that the part is level with the block on both ends. Though possible, its very difficult to do the best work laying on your back with oil dripping when the engine is still in the car.
 
We take a new oil pipe and "V" the pipe thread end with a grinder wheel, but you can also use a file. It has to be across the tube the way the bottom of the pipe points and also in the reverse of the way the pipe points, or on the other side of the pipe.

The "V" normally is about1/8th to a little deeper.

If you are using a early cap with a 5/16 hole, drill it out to 3/8's buy using every drill from 21/64th to 3/8's to open the hole up.

You also have to drill and tap the pipe hole, and it has to be the same angle it is now.

Screw the pipe in until it bottoms out on the threads, and with a wire welder, spot the pipe and cap togather on each side of the pipe, NOT front and back.

Now, take a small rat tail file and clean the rest of the pipe out of the hole to run the hole again with the 3/8's drill bit to clean it wide open.

The "V" was put in so you could get the rat tail file started. If you use the 3/8's drill to start with, you will be pulling the cap or your drill out of the ceiling.

I don't care how flat the cap is or how good the cap fits the block, the oil will go between the cap, block, and shims. You have to put a thin coat of RTV on the shims, and main bolts top and the nut end.

The Aluminum insert in the block is very important, and is where a lot of oil can come from.

If your babbitt is newer, and the crank has a bigger gap on one side then the other, then who ever align bored it got the alignment off center.

Just because the ends of the slinger are even with the block is only a part of the fitting, and it in its self, means nothing, as that has nothing to do with the part of the crank that runs next to the matching slinger.

If you use old Ford slingers, chances are from the crank wearing closet to the block, as they always do the old slinger will be wore deeper from the crank wear, and when new babbitt is poured and the crank brought back out where it should be the gap between the slinger and the crank will be to large.

Also, some rear caps have to be junked for the same reason.

Herm.
 
Thanks everyone for your advice on the leaking rear main. We would like to add just a couple more points to our question and see if we can narrow this problem down further. Back when we were restoring the car, we had the engine mounted on the running gears, we drove the car around quite a bit with no leakage problem at the time. Since fully restoring the car, we drove it to several car shows, parades, and around town last year with no leakage. Even the guy who did the body work for us drove it around a bit and had there was no leaking. If the tube in the rear main tube was screwed in too far, wouldn’t the leak have appeared sooner than now? Again, we have blown through the tube and run a wire down and it seems like the tube itself is clear. The new brass shims we put in were from Snyders, and as I said before, we reshimmed and toruqued all three main bearings. Today we took the inspection plate off the flywheel cover to try and determine exactly where the leak was coming on the rear main cap. It appears to be coming from the left side of the rear main cap, about halfway down. Is it possible, that when we had the engine rebuilt, they didn’t cut the groove in the bearing the right direction? Since the car didn’t start leaking till this year, could they have not aligned the bore correctly and in the time we have driven it, its wearing the bearings out more and letting oil pass by? One final point, when we started the car up today, there was not a drop of oil come out. Parked it on a steep embankment in the yard and let it run, the oil poured out. When we pulled back on the level, oil continued to pour. After letting the car set for a while, started it up and drove it back into the garage and not a drop leaked. Does this help pinpoint where we need to start looking for the problem. Cant take the engine back to the rebuilder, unfortunately he died.
 
IaFarmBoys said:
Thanks everyone for your advice on the leaking rear main. We would like to add just a couple more points to our question and see if we can narrow this problem down further. Back when we were restoring the car, we had the engine mounted on the running gears, we drove the car around quite a bit with no leakage problem at the time. Since fully restoring the car, we drove it to several car shows, parades, and around town last year with no leakage. Even the guy who did the body work for us drove it around a bit and had there was no leaking. If the tube in the rear main tube was screwed in too far, wouldn’t the leak have appeared sooner than now? Again, we have blown through the tube and run a wire down and it seems like the tube itself is clear. The new brass shims we put in were from Snyders, and as I said before, we reshimmed and toruqued all three main bearings. Today we took the inspection plate off the flywheel cover to try and determine exactly where the leak was coming on the rear main cap. It appears to be coming from the left side of the rear main cap, about halfway down. Is it possible, that when we had the engine rebuilt, they didn’t cut the groove in the bearing the right direction? Since the car didn’t start leaking till this year, could they have not aligned the bore correctly and in the time we have driven it, its wearing the bearings out more and letting oil pass by? One final point, when we started the car up today, there was not a drop of oil come out. Parked it on a steep embankment in the yard and let it run, the oil poured out. When we pulled back on the level, oil continued to pour. After letting the car set for a while, started it up and drove it back into the garage and not a drop leaked. Does this help pinpoint where we need to start looking for the problem. Cant take the engine back to the rebuilder, unfortunately he died.
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You cannot park a Model A on a steep incline without it leaking. This is especially, if the oil is at the full mark or slightly more than full. I always suggest for people to keep the oil level at halfway between F and L to minimize this type of leak. Try running the engine and not parking or traversing any steep inclines and see if the leak will minimize itself. This may give an indication of the problem.
 
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