Rear Main Bearing Cap- Modification ?

Ron in Quincy

Ron in Quincy

Well-known member
A Question for Engine Builders !!!!

Back in the early 60's when A engines were easy to find and cheap; I took several apart looking for a crankshaft that would clean up .010 -.010. On two of the engines I found the same modification on the rear main bearng cap: in front of the rear main thrust, at the end of the existing oil groove, another oil groove had been cut vertical across the cap, at the bottom of this groove was a 3/16" hole which had been drilled into the oil return passage. It apperaed the excess oil flowing toward the slinger would start to exit through this hole and any oil that got by would be picked up by the slinger.

About 10 years later, in the 70's, a friend of mine had pulled the engine out of his 30 coupe to replace it with a B engine. The A engine he removed had always had an excessive oil leak from the rear main; he was going to install the A engine in a Speedster he had just completed. I suggested to him we pull the pan and check the rod and main bearing clearances before we installed it in the Speedster. When we got the pan off and checked all the bearings most had .0015 and the rear main, as I recall had .0040, we corrected the clearance to .0015.

I mentioned to him about the modification I had found on a couple of rear main caps ; he said lets do that to this engne and I did.

That A engine is still in that Speedster, has gone thru two different owners. I recently was talking to the present owner and asked him if there had been any problem with oil leaks ? he said " not a drip"

Would some of you engine builders please comment on this modification to the rear main cap, good or bad ?

Thanks, Ron
 
Ron - I am trying to picture the modification you described, but I just can't. I too would be interested in what the engine experts have to say, plus a picture or a diagram would really help my brain.
 
I'm no expert on leaks but I have seen that done, with the hole about 3/4" in front of the rear thrust. The hole was about 3/16" in diameter, and went down into the return passageway. Did it work? I have no idea but it does make a certain amount of sense.
 
Mike V. Florida said:
I wonder if the mod fixed the leak or did the clearance change? Although I would like to hear from the experts.
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You are right Mike, Clearance change, .004 will pump oil real bad, and or to much end play. I have seen those caps, just more work to fix before rebabbitting. If you had to have that hole, all Model A's should leak with out it.Thanks Herm.
 
I have seen it and have done it and other things to help a customer make a couple of seasons before having to re-babbitt. I have found rear mains with a channel (groove) cut from the cross groove to the front of the bearing, I have seen channels/grooves cut from one well to the other and a hole drilled in the cap into the tunnel under the cap, I have seen oil tubes with holes drilled in them to let the oil out.

This is done so the valve chamber will empty through this relief and bypass the rear channel so the channel won't be overwhelmed and leak out the rear. The valve chamber is so full of oil that the main won't starve when the engine is running but when stopped it will drain before it can over whelm the rear channel.
 
James Rogers,

This was my thinking, most A's leak after shutting down do to all the oil in the valve chamber, and the ability of the slinger end to handle the volume. I realize excess clearance on the rear main will cause leaking, but believe the modification assists with this problem.

As we all know, the oil return pipe was lowered on the valve cover end of later engines to get the excess oil out of the valve chamber. The A oil pump moves a lot of oil to the valve chamber, a great deal of it dumps into the timing gear case and on to the dip tray to keep it full so rods don't starve for oil.

Thanks for the reply, and keep the comments coming, both pro and con, as leaking rear mains seems to continue to plagg the A'ers.

Ron
 
A properly installed rear main will not leak more than a few drops. I have seen these caps with drains to the return channel and they would seem to me to just remove oil from the bearing. Better to fix the problem than apply this type of band aid.
 
There was an article in one of the national club magazines within the last two years that showed this modification. I think there was also a thread on this forum regarding the same. Anyone remember the thread? If so, there may be some photos available from the thread.
Good Day!
Dave in MN
 
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