Transmission main drive gear
- Thread starter miken7132
- Start date
pat in Santa Cruz
Well-known member
you are at the limit of wear. The bearing I.D. is .625, slip fit to the pilot (.001) with a tolerance of .002. Too loose will contribute to slipping out of high gear when coasting downhill. If your gears fit the main shaft snugly, then I bet it will work OK for a while, then start slipping out of gear some time further down the road. If your gears are loose on the main shaft, it might start slipping out immediately. I would replace it since I do not like pulling the transmission a second time if it were to slip out of gear and I want my machines to last reliably for some time. Some guys would take the risk and go with it anyway. I suppose it depends on how much driving you expect to do. Its your call.
Actually, the pilot bearing inside diameter is 0.669" (17mm) and the shaft outside diameter is 0.668". Most of the repair manuals say that the minimum diameter to use is 0.666" or it should be replaced. I have done a number of transmission rebuilds for members of our "A" club. I take the worn shafts to a local machine shop (owned by my brother) and they turn them down about 0.030 on the diameter and then build them up with weld and then put them in a lathe between centers and turn them to a diameter of 0.668". When I install the transmission into the flywheel pilot bearing I use LocTite Thread Lock on the shaft so it will not spin in the bearing. I refrain from turning the transmission for 24 hours to insure that the LocTite has set up.
Refering to the quote of Pat from Santa Cruz, I understand that there is a bearing available with a 0.625" inside diameter. I would not use that because it would make it a nonstandard shaft in the event that the bearing would need to be replaced in the future. This is obviously a personal preference. I believe there was a thread recently referring to that special 0.625" id bearing, but I looked thru the first 20 pages of postings and could not find it. Hope this information is of value. Don/WI
Building up with weld and turning back to standard can get pretty expensive around my area. The 0.625 ID bearing from Graingers is my preference. I think they list it as a 40mm OD / 15.875mm ID. The stock bearing is 40mm OD and 17mm ID, one of those rare metric anomalies that were somehow incorporated into the Model A.
Tom Endy
Well-known member
Contact Jim Nichols at Jim's Automotive Services in Costa Mesa, CA 714-546-2969. He can repair the shaft for you. He turns the pilot end down on a lathe then interference press fits a sleeve on that will be a nice slide fit into the pilot bearing. His price is very reasonable. He has done countless numbers of them.
Tom Endy
Tom Endy
pat in Santa Cruz
Well-known member
Don
thanks for straightening that out. I was taking numbers off the top of my head,, since my book is out in the shop. I did turn a shaft to .625 once as a solution, which must be where that number came from. It works, but the problems is, as you describe, remembering to fit the correct bearing every time its needed.
I also tried the loctite trick you suggest once on the recommendation of a machinist. It has problems as well if you ever want to pull the transmission. The pilot sticks pretty well into the bearing, making separation of the clutch housing from the flywheel housing a real pain.
The shop I go to charges $30 plus postage for repairing the shaft to original size. The LocTite I use is not the permanent type, it is called Screw-loc. We had one occasion where we had to remove an engine two times after the original installation and did not have a problem with the Loc-Tite on the shaft. If you take a new shaft or a rebuilt one, place the new pilot bearing on it and turn the outer race, you will see the inner race turn on the shaft. That is what causes the shafts to wear in the first place. I have never taken a transmission apart for rebuild that did not have a worn pilot bearing shaft. The reason it turns is that there is less friction between the outer shaft diameter and the inner bearing race than there is between the inner and outer race of the bearing because of the grease in the new bearing, thus the need to use LocTite. Just my experience. Obviously, you should do what you feel is right based on your experience. Don/WI
Bill Goddard
Well-known member
There is a short article in no.4 pg 48 of “Technically Speaking” that describes an alternate fix. There is a replacement pilot bearing made by NTN (6203ZZ/15.875/2A) with a 40mm o.d. and a 5/8’dia bore to fit the drive shaft if it is turned down. It is available in Baltimore from Applied Industrial Technology. I also think I have found it at Tractor Supply. It is double sealed, 40mm o.d. and 5/8” dia bore for $4.50 .
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Bruce
Senior Member
I am in the throes of this very job. My machine shop estimated the cost to build up the shaft with weld and then turn it down at "about $100." Given that the cost of a new shaft is not a lot more than that, I placed the order. Now I am waiting for the UPS truck. I have to admit that I really wanted to just put it back together ... afterall, what could that itsy bitsy wobble really hurt? I suspect that we are a lot more exacting than those guys back in the day when all Model A's were daily drivers.