Friday, July 18, 2014

One size does not fit all: Marking SAE, metric, and Whitworth sizes on the same wrench

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.


lbgradwell said...

Interesting. I've never seen a Dreadnaught wrench marked like that...
Did you measure the openings? What are the actual sizes?

The Duke said...

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!