I have looked through the Model T Ford manual and the booklet on Transmission and don't find the maximum clearance for the 4th main.
Lang's catelog shows .004 - .006 for a newly poured 4th main.
What is the maximum clearance allowable for use on an existing 4th main?
I have two cars I am working on. One has .014 and the babbit looks like new. The other has .20 and is worn out. I know I need to replace the second one, but what about the .014? is it useable?
assuming that the transmission really spins on the crank's axis with no wobble (a big assumption), then the .014 clearance is excessive. Even with a small amount of wobble, the flywheel action of the transmission assembly will try to straighten wobble out and it will appreciate the support given by a new 4th main.
I sent my 4th main and drive plate to George King III for rebabbiting and custom fitting to the shaft. If no one here can answer your question, perhaps you can contact George and see if he can answer your question. www.enginerestoration.com. He uses the original Model T babbit formula used by Ford and did a superb job on mine. Jim Patrick
A: I shoot for Lang's measurement B: It's the 4th main and supports the transmission in line with the crankshaft C: The bigger the gap the more oil you will loose out the rear into the U-joint and down the drive shaft and onto the ground.
Don't know the max but I am real sure someone will chime in on that.
You've kind of answered your own question. If .004/.006 is the recommended new part clearance, and you've got .014, I'd say you need a new 4th main. Since it's apart that far now, it's an easy fix.
Simply buying a new babbited 4th main is not as good as having yours rebabbited with your drive plate shaft by George or your machinist of choice, because a new 4th main from one of the vendors has not been custom fit to your drive plate shaft, which may be worn and certainly not able to attain the closest fit necessary to minimize the leaks that this area is known for. My 4th main does not leak from the rear of the engine like it did before having George rebabbit it and machine fit it to my drive plate shaft. Jim Patrick
Mark is right, Ford made his tail shafts O.D. at 1.560-00, he cut the Ball Cap hole standard to 1.9/16's, or 1.562-50.
That gives a clearance at .002-50.
If it is larger, it should not go over .003-00 thousandths at assembly.
If you cut to .002-00 thousandths, it has a 50-50 chance of smearing, as when the ball cap flange is bolted tight, the bolting surface is not true, and can pull the ball cap so it will twist a little, which in turn binds the babbitt on the shaft, and cuts the clearance down in some spots to let the babbitt smear.
So, like Mark says, the bigger the gap, the more oil down the drive shaft tube.
Norm -- If the babbit's fit on the crankshaft is good, you don't need to re-do the whole rear main cap, just the thrust face. The front face is the one which wears, so just build it up with some babbit, then turn it to length on a lathe. Accurately measure the clearance as it is, then figure out how much longer it will need to be to end up at 2-3 thou. If you're not equipped to do that, have someone do it for you. That saves all the work of refitting the "new" main to the crankshaft.
Mike, Norm is talking about the 4th main,or ball cap, the "other" rear main<g>. Dave
After Reading the above posts, and measuring the shaft and the bearing, I have come to the conclusion that the shaft is 1.555 which is .003 undersize, but someone in the past bought a bearing which was made for a standard shaft. The babbit looks almost like brand new, but too much clearance. I will buy new bearings for both cars.
Thank all of you who posted.
Norm, can you specify the exact ID size of the babbited opening, or do you get it undersized and machine it to precisely fit your drive plate shaft?! Jim Patrick
My shaft also measures 1.555. I ordered a rebabbitted one from Langs. I could have ordered 1.561 or 1.559 as both might work. I decided to order the tighter one as I can shave a little material off but, I can't add any on.
You will be surprised at how much run out of centre there is on a drive shaft, unless you can re-machine all mating surfaces like Herm has shown, it will never be true, just rotating the drive plate on each bolt hole on the brake drum and checking will vary several thou, so if you are lucky enough to get it down to 3 to 4 thou run-out, you don't want a tight ball cap, it will smear real fast, 3 to 4 thou gives it a fighting chance.
Opinions may vary.