The Stovebolt Site Forums The Shop Area Driveline Bellhousing parallelism

I have a 1941 1/2 ton, model AK with a 235. Is there a parallelism spec for the standard transmission bellhousing? I’m getting: 12 o’clock = 0 , 3 o’clock = .003”, 6 o’clock = .008” and at 9 o’clock = .005”. These measurements were taken from the bellhousings transmission mounting surface. Any help would be appreciated.

The late model overdrive transmission require bell housings to be dead true to the flywheel and the crank, the input shafts can't take much out of alignment issues. I doubt your old trans will care about a few thousands here or there, but if you wish to align it, I purchased shim washers from McMaster Carr when I dialed in a Pontiac bell housing for a Tremec 5 speed. I placed them between the bell housing and the block. I also used offset alignment pins to dial in the bell housing opening so the input shaft would be centered to the crankshaft. I started out using brass shims stock then purchased hard shim washers once I knew what sizes I needed.

I had the mag base/dial indicator on the block and checked the run out of the flywheel first, just to make sure I wouldn't be chasing some odd problem around for no reason. It was flat and true to the block face.
Next the mag base was on the flywheel with dial indicator on the inside of bell housing opening while bolted to the block. By rotating the offset pins, you can move the bell housing around to center it to the crank.
With the mag base still on the flywheel, I moved the dial indicator to the face of the bell housing ( what you did ). In this position, you can align the mounting face with the flywheel, " make them parallel " .

It wasn't to hard to dial in, and sure won't hurt even if it's not needed. On the Pontiac, I had thick shims at the top, half amount mid point and none at the bottom, mine was even side to side, but off top to bottom.
The alignment pin reading showed the opening was a little to the left and down from center.

Always be sure to check both measurements after making adjustments with shims or pins.

Joe

Don't ignore the thrust bearing on the crankshaft. You'll need to have it preloaded one way or the other, or none of your measurements will be worth a concrete water barrier. The whole pursuit sounds pretty OCD to me.
Jerry

Yes, I made sure the crank was pulled back for each measurement.

So you made sure no burrs or high spots on bolt together surfaces?

The bell housings were not machined 100% accurate, so being a few thousands out is not unheard of. Thrust bearings are important, but being inline and parallel will make it all work better. One cannot get it perfect, but if you have the time, why not. The owner of the Pontiac took pictures of the dial indicator readings for his warranty, they required the alignment as part of the warranty.

Why would anyone want to pay an outrageous price for a transmission so flimsy that it can't stand a few thousandths of runout? I wonder what that transmission will be experiencing in the way of wear 75 years from now? The old SM 420 will still be going strong then.

Yes, I stoned each surface and made sure there were no nicks. So, the measurements that I came up with will be all right for a stock transmission? The bore runout is: 12 o’clock = 0, 3 o’clock = .0005”, 6 o’clock = -.001”, 9 o’clock = -.001”. Thanks for the information.

Those measurements are almost perfect. Anything under .005" or so would be acceptable, and a transmission with a little bit of wear on the shafts and bearings would probably be OK with quite a bit more. The adapter ring for a Hydra-Matic transmission was the only thing that was critical on alignment, and that was lateral runout that needed to be dialed in with offset dowel pins.
Jerry

I remember reading that some bell housings were as much as 0.040 out from the factory and in one factory manual I recall a method for correcting it with offset dowels. That seems preferable than shims to me. That said, I've never even checked mine and that bell housing has been on two different engines and I've had no clutch-bearing or pilot bearing problems in 50 years. And that's just my present truck not to even remember all the other ones I've worked on. I've never had a bell housing misalignment cause a problem. If you're within 0.010, which you are, you're well within the manufacturing tolerances of the day.