Anyone who likes old wrenches and sockets will have examples marked in 32nds of an inch. Apparently, before increments of 1/16" became the standard, 1/32" were produced particularly since bolt head and nut sizes could vary depending on the quality of the manufacturer. So, you might need a tool size a little larger or smaller than the fastener would otherwise accommodate. This was the case for SAE (American) wrenches and sockets, and I have even found one example of a 32nd inch increment in Whitworth on a Triumph wrench:
Along the same lines, I found the following curious entry in "What Makes It Tick." Elementary Information for Automotive Technicians prepared in the 1940's by General Motors Products of Canada, Oshawa, Ontario in cooperation with the Jam Handy Organization of Detroit (which is best known for its training films).
So, at one point there was a 7/16" drive "preferred for all-around work"? I wondered why I had never encountered such a beast.
Back in 2010 I emailed the good folks at Alloy Artifacts about this. Here's what they said:
"If you check our site index under "Socket", there are several 7/16-drive sockets listed. These sockets and tools were made by Blackhawk from the late 1930s through 1950s, and you can read about it in the Blackhawk article.
The 7/16 drive size wasn't made by other manufacturers so you don't run across the sockets and tools that often. In addition, people sometimes mistake them for 3/8 or 1/2, the way 9/32 drive is mistaken for 1/4."
So, it turns out that there were actually 6 socket drive sizes at one time: 1/4", 9/32", 3/8", 7/16", 1/2" and 3/4"!
And you can add 5/8"-drive to that list too!
Snap-on (at least) offered such tools from the early 1920s to the 1970s sometime...
Of course, there are also 1"-drive, 1-1/2"-drive and 2-1/2"-drive that still exist in heavy industry too...
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