What's "new"?

[Steve Jelf]

All bushing tolerances for the Model T Ford transmission are .002 for a new job and .005 for a repair job.

Wasn't assembly at the factory the only new job? Aren't all transmission overhauls now repair jobs?

[Scott_Conger]

The real question is: 100 years later what should we be using for dimensions for a repair?

According to FordOwner and other sources tells us that using a bushing that has .005" or less "play" on it's worn pin is "repair" and used...play over .005" is a replacement item and done with new pins.

Now for practical use in a Ford mechanic's shop in 1918 and how we should be applying it today:

in repairing something like this back when T's were being serviced by a dealer, the new pin would have been .6770/.6775 from the factory. New bushings installed into triple gears would be expected to be .6785/.6790.

The big caveat that some folks ignore in their repairs is they are not done yet!

The last repair method documented by FordOwner January 1918 advised dealers to size finished/installed bushings with 3 successively larger polished mandrels, the finish mandrel diameter being .6820. In Bronze, this would leave a hole somewhat smaller, somewhere in the .6815 range for a total bearing clearance of .0045/.0050 coming out of a dealer repair shop with the blessing of the factory

With respect to .002" from the factory, according to the drawing it was a "running fit" (which MEANS something and cannot simply be ignored) and within SAE guidelines for that heavy service, the allowable tolerances stacked up create roughly a .004 total clearance for that dimensional call out. I'll reiterate for total clarity: the .002" factory callout results in a .004" total clearance, NEW. KRW reamers for this job give (as I recall, a .0035/.0040 clearance). Lots of electrons and tortured logic came up over the years with ".002 on each side" silliness and so forth. The reality is that the logic and home-made terms was faulty but the resulting conclusions were accurate, as borne out by factory drawings and then applying SAE standards. Much ire has been directed back and forth between the folks who rebuild to .002 total clearance and the .004" total clearance crowd. The difference is that the ".002" crowd must walk up to their soapbox and the ".004" crowd can still drive there.

I'm sure there were others, but I give credit to Glen Chaffin for standing up to withering assaults from experts regarding his position on .004" total clearance, followed by Herm Kohnke who repeatedly said that KRW reamers created .004" clearance which matched Glen's assertions. Myself, I read and reread several Engineering/Tool Makers books published in the 'teens and 20's plus period SAE literature to independently come up with the "running fit" dimensions, which too, mirrored Glen's assertions, and Herm's empirical results. I'm satisfied with that.

Hard to go wrong with .004" total clearance and I believe that there has never been a seized bearing failure legitimately attributed on the forum to this beginning clearance.