I set out to make a new battery hold-down for my riding mower. Shouldn't be any big deal -- two J-bolts (actually just straight rods with a small, L-shaped leg on the bottom and with the top threaded), and a cross bar that spans the battery.
The holes in the tractor battery shelf are too small for 1/4" rods, so I decided to use 3/16". Well, for starters, Lowe's didn't have any 3/16" round steel rod, so I had to trek to Home Depot. They had it. Fine.
I've checked multiple web sites, and they all tell me that a #10-24 bolt has a major diameter of .1890". 3/16" rod is .1875" in diameter. I figured that, for a non-critical application like a battery hold-down, the loss of .0015" off the major diameter wouldn't matter. So I clamped the rod in a vise, dug out my tap and die set ... and found that I could NOT get a 10-24 die to even think about starting on the rod. I had to use a 12-24 die. I had bought some 10-24 wing nuts. Naturally, they wouldn't fit.
http://www.fasnetdirect.com/refguide/Machinescrewthreads.pdfSo today I went back to Lowe's in search of 12-24 wing nuts. And ... they don't have them. They don't have ANY nuts, bolts, or washers in the #12 size. They skip from #10 right to 1/4-inch. So I'm going to have to order some 12-24 nuts from Amazon.
But I can't figure out what's wrong here. 3/16" is .1875. The minor diameter for a #12 thread is supposed to be .2078" -- a 3/16" rod should fit right into that and, when I tested it on the thread sample board at Lowe's, it did.
If the major diameter for a #10 thread is .1890" and a 3/16" rod is .1875" -- why couldn't I thread it using a 10-24 die? Why did I have to use a 12-14 die?