[MikeK]

Originally Posted by Benson

Mike,
As we know the differential bolts and case are a tighter thread than common taps and dies. Are those 3A and 3B or are they a different, even tighter tolerance? Transmission cases also? Thanks

Tough question since Ford, and the industry in general at the time, did not use today's grading standards for either strength or fit. You would have to find the prints/specs (Benson Ford Research) and both old '20's and modern copies of Machinery's Handbook to compare the tolerances.

That said, I have run class 3 cutters over differential, trans, and engine threads and fasteners without removing anything more than the deformed metal. A class 2 cutter would leave the walls of the thread cut or scraped down to new metal- bad, bad, bad.

In any case that type of clean up is a last resort after steel bore brushes and 60 degree die sinkers files fail to give the desired outcome.

Another option is thread forming rather than cutting taps, also available in various tolerance specs. I used those a lot on AlMag castings when I had a foundry back in the 70's & 80's.

As far as dies go, the adjustable versions permit dialing in the clearance you want. Not something for the casual user.