It seems there was a time when some wrench manufacturers tried to market the same wrench as fitting both Whitworth and SAE fasteners. Below, a Dreadnaught brand wrench (made by Gray Tools) marked 3/4" and 3/8" BSW on one side, and 5/8" as well as 5/16" BSW on the other:
They're not really identical. 3/4" is, of course, .0750", but 3/8 BSW is 0.710" across the flats, a difference of 0.040". Similarly, while 5/8" is 0.625", 5/16 BSW measures 0.600", a difference of 0.025". So, the wrench is going to be a poor fit on either the SAE or Whitworth fastener.
Here's the size comparisons in table form:
The openings are clearly made to best fit the Whitworth sizes. In particular, the supposed 3/4" SAE opening is only 0.715", and will definitely not fit a bolt head or nut of this size. The end marked 5/8" SAE is actually 20 thou smaller than this decimal size, and probably wouldn't fit a modern fastener of this size.
Below, another wrench of unknown American manufacture, with 19/32 forged on one end but stamped 5/16 W on the opposite side:
Here's how it measures:
So, even though it looks like it was made as a 19/32" SAE wrench (since that designation was forged in), it would actually best fit a 5/16 W fasteners.
Other manufacturers tried the same trick with SAE and metric wrenches. Below, a wrench marked 9/16" and 13 mm on one end, and 1/2" as well as 12mm on t'other:
Here's how it actually plays out:
Most standard wrench openings are between 5 and 10 thou larger than the fastener size. So, aside from the 1/2" size, this particular wrench would be a pretty sloppy fit on any other fastener it was supposed to fit.
Below, a somewhat more accurate attempt. 1/2" and 12 mm on one side, 9/16" and 14 mm on the other.
Best fit here is the 9/16 and 14 mm side. Don't use it on a 12 mm fastener!
Below, two wrenches. The larger one is marked 25/32" and 20 mm one one end, and 7/8 and 22 mm on the other. The smaller wrench is marked 5/16" and 8 mm on one end, and 13/32" and 10 mm on the other end.
Best fit would be 25/32" or 20 mm, 5/16" or 8 mm, and 13/32" (but not 10 mm!) Forget the 7/8" and 22 mm size altogether.
Finally, one American manufacturer made a wrench with all three measurement systems on it!
On the left, the forged measurements are 5/8" (0.625"), 16 mm (0.629") as well as 3/8 Whit (0.710") stamped on the same opening. Quite the dimensional spread. On the right, the forgings indicate 3/4" (0.750"), 19 mm (0.748") and the stamped measurement 7/16 Whit (0.820'). I can't see how the wrench could possibly have fit the larger Whitworth fastener.
My conclusion: a bad experiment in trying to make one size fit all. Arguably better than a vice grip, but generally worse than an adjustable wrench.
Interesting. I've never seen a Dreadnaught wrench marked like that...
Did you measure the openings? What are the actual sizes?
I've added the information you requested above. I had omitted it initially because I just couldn't turn that wrench up in my shop! I couldn't remember if I had put it in the drawer with the Whitworth wrenches I use, or in the tool box with my mostly British-made tools, or hung it on my weird tool wall, or put it in with my extra Whitworth stuff. Anyway, I searched more thoroughly and finally found it!
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