If you have a '13 or '14, you may remember the article I wrote back in the '70s about the windshield clamp screws and nuts being brass plated. Back then, everyone was painting them black. Remember the aluminum crank handles? Well, when I was powder blasting the nuts for the bolts that hold the windshield support rods to the side of the body, I discoverd that they too were brass plated. This is for my new '13 roadster. They are the same nut that is used for the windshield clamps on the firewall. And, by the way, they are 19/32" across the flats.
Hmmm, that's only .003" larger than 15 mm; do you suppose Henry was getting a jump on the future?
Doing any job on a danged Ford takes just about every tool in your toolbox, plus one. And then you have to drop the rearend to take off the radiator cap...
some model A original rod nuts take a 19/32" socket. I can't find mine so I use a 15mm.
So many of Ford's dimensions are so close to metric sizes (British, French, International and/or Whitworth at the time), I think it was intentional as part of his business plan.
The U.S. Standard for fasteners (NOT the SAE standard) called for the distance between the parallel sides of heads and nuts to be equal to 1-1/2 times the diameter of the bolt, plus 1/8 inch. Thus for 5/16 diameter fastener the standard size head would be 1-1/2 x 5/16 = 15/32 then 15/32 + 1/8= 19/32. That is why you have a 19/32 socket in your old set of tools. It was the U.S. standard size for a 5/16 threaded fastener. The thickness of the head of the bolt for that same fastener would be 1/2 of the dimensional distance between the parallel sides so the head thickness for a 5/16 bolt would be 19/64. The NUT thickness using that same standard was for the NUT to be equal in thickness to the diameter of the fastener so the nut would be 5/16 thick. Ford seemed to use a lot of different standards for various fasteners so I suspect he bought what was in common use at the time for any given fastener. Another interesting detail is that almost ALL fasteners of this era were "domed" on the ends rather than being simply cutoff square. Modern bolts are usually rolled so the ends are in fact concave a bit. Just more silly research.
A GREAT way to find out about old standards and sizes of bolts and things is old high school mechanical drawing and drafting class text books from the T era to about 1940 or so. They can be found in old book stores and such. Students had to learn the various standards and how to draw them.
I haven't bought a set of Craftsman sockets or wrenches for a long time, but they always used to have a couple of those odd sizes in their sets.
I remember when working on bicycles as a kid, the rear hubs of US made bikes used an axle shaft nut that used a 19/32" wrench, We all rounded off a bunch of them with 5/8" wrenches until we realized that there was another size in there, between 9/16" and 5/8". (before the metric onslaught) if you picked through dad's or grandpa's toolbox the right one could be found.
There was also a GM bolt that looked like 1/4' and the head looked like 7/16, but it wasn't.
'41 Buick tail lights used 'em.
How about the Bendix cover on a T?
There were also 3/16" bolts, now they are #12.