MTFCA Forum

A friend gave me 2 pistons today, a .031 and a .033 , factory ford markings.
NOS

Hey there Mack! Ford back in the day had several "standard" size replacement pistons in cast iron in what we would think of as "odd" sizes.
I have I think two full sets of .031 pistons, and have seen a few individual .033 pistons over the years.
It is interesting to note that they also had (I have seen myself?) .0015 oversize whish isn't much at all, and if I recall correctly, a couple .005 which isn't much more. I suspect that those slight oversizes were so that a missed or slightly damaged bore could be honed slightly and better fitted with pistons closer to standard.
Just a guess on my part, but the .033 may be the same idea in that the .031 was Fords "standard" oversize near the modern usual .030 range. And the .033 was available to clean up a rough rebore at slightly over the "standard" .031 size.
Rings for cast iron pistons have been available for many decades in the .030 size, which works fine for the .031 or .033 pistons. I do not know if they are still available or not.

I believe you are correct. The "slight oversize" rings could be used in a cylinder that was good enough to hand-hone in the field to take out light scoring or other minor defects, provided taper and out of round were within reason. I'd think that .030 (for instance) rings would work just fine in a bore that was .031 to .033 as long as the taper was not excessive and they had enough gap in the lower bore. The closer to proper spec everything was, the longer the job would last, but a good mechanic could get good results using good judgement.

1928 parts books, USA piston sizes from Ford, .0025, 005, 010, 025, 03125 and 033
Canada Ford sizes .001, 002, 003, 004, 005, 010, 015, 020, 025, 030, 033, 035, 040 and 045.