Steve admits confusion about the "dual size" markings on the wrenches, which
went something like:
1/4" W 5/16"BSF x 5/16"W 3/8"BSF. Then he compounded the confusion into SAE
and metric sizes. Bear in mind that the W stands for Whitworth and the BSF is
short for British Standard Fine.
It took me a long, long time to sort it out but there is a beautiful logic to
both systems.
Hear me out on this - most of us Yanks grew up with the SAE system. From
experience, we know that a 1/4" bolt has a 7/16" hex head so you use a 7/16"
wrench (or socket) to make it turn. The measurements used in this example are
the 1/4" diameter of the rod stock used to make the bolt and the 7/16" is the
distance across the flats (AF) of the hex head. Are you with me so far? Does
it make sense that you use a 7/16" wrench on a 1/4" bolt?
If you are dealing with a 1/4" Whitworth bolt, you use a 1/4" W wrench which
just happens to be .525" AF (that means across the flats, if you weren't
paying attention before). If it's a 1/4" BSF bolt, you would use a 1/4" BSF
wrench which is .445" AF. Those Brits were all screwed up, weren't they?
Actually, it signifies technological progress. Back in the older olden days,
blacksmiths formed the bolts, each to his own design. There was little if any
interchangeability. Then this dude Whitworth came along and set up some
standards about thread pitch and angles and diameters and hex heads and all
that stuff that no one understood, but they didn't want to appear stupid so
they went along with it and called it British Standard Whitworth. Remember,
this was in the early days of the Industrial Revolution, the threads were very
coarse, so they used BIG heads to hold everything in place. And, if it was a
1/4" bolt, you used a 1/4" wrench. Utter simplicity.
Over time, they learned how to machine tighter tolerances and were able to
rely more on the surface friction of the threaded sections to hold everything
in place and keep the damn parts from working loose. In stepped Mr. Fine with
a new set of standards that he called British Standard Fine using more turns
per inch at a different angle and because of this was able to save on
materials by reducing the size of the head. Alfie and the lads refused to buy
a new set of tools so Lord Fine adjusted to the tools in use. But - tradition
required that a 1/4" wrench be used on a 1/4" bolt. Easily done - we'll just
call a 3/16" Whitworth wrench a 1/4" BSF - get out your stamps, boys.
So, in Britspeak, if you are trying to intall a 1/4" bolt, you will use a 1/4"
wrench. It's that simple. Well, as long as you know whether the bolt in
question is Whitworth or BSF.
Damn - I started out to give a complete answer in three or four sentences and
still left a lot of territory unexplored. Will someone else get into the
esoterica of BA (British Association) and BSC (British Standard Cycle)
fasteners?
Anyhow, some of the Haynes workshop manuals have a table showing by decimal
inch the various spanner (wrench) sizes in AF (SAE), metric, Whitworth, and
BSF.
For example: .433" = 11mm, .438" = 7/16" AF, .445 = 3/16"W and 1/4" BSF.
"Ole" Olson
Steve wrote (in part):
<< I am somewhat
confused, however, by the "dual size" terminlogy. I have
seen wrenches marked in this manner. Having had my MG-TD
and Jaguar XK-120M both with the Whitworth system, I know
that SAE wrenches do not fit Whitworth nuts/bolts, and
vice-versa (unless you just like to round corners. In the
late 50's the British started converting over to UNF threads
and SAE wrench sizes.
What is the meaning of dual-marked wrenches? They can't
possibly fit both bolts. I know there were cars built with
both systems in place. Even in the US, we started
converting engine threas and nuts to meteric, while leaving
suspension and body boltss SAE.
Do you have the rationale? Which Rootes (or other English)
car did use those dual-marked wrenches?
>>
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