Worldly Scions,
As it's the weekend and the bandwidth is down, I'm exceeding my
self imposed posting limit. I thought the following might be of
interest. I got it from the "Brit-Iron" British motorcycle list.
Although written for a British motorcyle audience, some of it may apply
to the earlier cars represented on this list. It is reproduced here
with permission from the author Charles Falco.
Enjoy,
Bob
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Subject: A lecture on the meaning of "Whitworth"
First, a tiny bit of history. In the 19th Century every British
factory which needed to bolt something to something else devised
their own fasteners to do it. Clearly, this caused all sorts of
compatibility problems. So, along came Mr. Whitworth (I forget his
first name right now) who invented a standardized system of coarse
threads (with 55 degree thread angle and rounded roots and crests).
This standardization was a Good Thing. Along with his threads came
heads for the bolts that were based on the length *along* the side
of one flat, rather than across the flats. Hence, there is no
simple fractional number for the length across the flats, which is
why your American wrenches don't fit. The fractional number on
your English wrenches refers to the diameter of the bolt (which is
1/4", 3/8" etc. just like in the U.S.); not to the distance across
the flats (which ends up being various weird dimensions). Some
years later the Brits decided they needed a finer pitch for some
applications, so another thread series was introduced (same 55
degrees). They also decided that the heads were too big for the
bolts, so for most applications they switched to using the next
size smaller heads. Because of this, and to add one more bit of
confusion to life, one manufacturer will mark a particular wrench
(spanner) "3/8BS," while a different manufacturer will mark the
same sized wrench "7/16W." They fit the same diameter bolt.
The first thing any fledgling Brit biker learns is that his (or
her) motorcycle has "Whitworth bolts." They think this is
interesting, buy a set of "Whitworth wrenches," discover these
wrenches fit their bolts, and believe they now know everything they
need to know about British fasteners. Unfortunately, at this point
they know only enough to make themselves dangerous. Instead, what
they *should* have said to themselves is "Ohmygod, what other weird
and incomprehensible things have the Brits done to the fasteners on
my machine?" The answer to this question is:
British Standard Whitworth (BSW)
These are the original, 19th Century, coarse-threaded industrial
bolts designed to hold locomotives together. Because of their
coarse pitch, they are more prone to vibrating loose, so are little
used on motorcycles. _Except_ for threading into Aluminum (e.g.
crankcase studs), where a coarse thread is less prone to stripping
than a fine one. It turns out that, except for 1/2" (where the
Brits use 12 tpi, and the Americans 13 tpi) the thread pitches are
the same as for American Unified Coarse (UNC). However, the thread
*form* is different; Whitworth = 55 degrees; UNC = 60 degrees. In
spite of this, mismatched nuts and bolts mate nicely, so you're
likely to find UNC bolts or studs where BSW should have been.
British Standard Fine (BSF)
A finer pitch series, analogous to the American Unified Fine (UNF),
although--unlike the case of BSW/UNC--with none of the pitches in
common with UNF. Many motorcycle manufacturers commonly used a lot
of BSF threads.
CEI (Cycle Engineers' Institute) or BSC (British Standard
Cycle)--these are different names commonly used for the same
threads.
60 degree thread angle, rather than the 55 degree of BSW and BSF.
For sizes from 1/4" through 1/2" by far the most common are 26 tpi,
although 24 tpi appear as well. *Most*, but by no means all,
fasteners on post-War BSA's (through the late '60's, when it got
more complicated) were CEI. Although the thread form and pitch is
different, the head sizes on CEI-threaded fasteners use the same
wrenches as BSW/BSF.
British Association (BA)
47-1/2 degree thread angle. This is a metric thread system devised
by the British for small screws used in components like speedos.
Not metric like you might expect, but with diameters determined by
a factor proportional to a power of the logarithm to the base 10 of
the thread pitch in millimeters. I couldn't possibly be making
this up. Ah, the English. You'll find lots of BA threads on any
British bike, but only for fasteners smaller than 1/4". BA
fasteners have their own set of wrench sizes. Typically, a set of
"Whitworth" sockets will include a 0BA (and maybe a 2BA--bigger
number = smaller size) socket.
British Standard Pipe (BSP)
A tapered, self-sealing thread system used to seal fluids
(interestingly, the US *and* the metric world standardized on the
BSP system for threading all their pipes).
UNF and UNC
In the late 1960's, when even the U.S. was thinking of going
metric, the giant BSA corporation decided it was finally time to
scrap that old 19th Century Whitworth-based system, and switch
to....yes, you guessed it, American. Since they had lots of money
invested in tooling, the switch wasn't made suddenly (or
completely), so bikes from the late '60's and later had a mix of
all sorts of thread forms. Typically, engine internals (e.g. the
thread on the end of a camshaft) stayed with whatever form it used
to have, while simple fasteners (e.g. holding the fenders on)
switched to UNF.
"None of the Above"
While the above systems account for well over 95% of all threads
you'll ever run across on a British bike, some manufacturers--again
BSA springs to mind, but others were guilty as well--couldn't
restrain themselves from inventing a few oddball pitches of their
own. This is why, when dealing with British bikes, you should
assume nothing. You must have a pitch gauge and calipers.
So, let's get back to the question someone asked a few days ago
about buying a set of "Whitworth" taps and dies to last him the
rest of his Brit biking life. The first thing I'd say is that if
you wants it to last, be sure you buy a HSS set (rather than carbon
steel), even though it will be at least 2x the price. Then, in
order of overall usefulness, I believe you'll find in first place
it will be a CEI set, then BSF, then BA. Unless/until you really
get deeply involved, make do with UNC to "simulate" BSW (although
you'll want to buy an individual 1/2"-12 at some point to
supplement the UNC set). Thus, if you follow my recommendation,
the very *last* thing you'll want to buy for working on your
British motorcycle is a "Whitworth" set (I have one, by the way,
but only got it after I'd had CEI, BSF etc. for a long time).
Charles Falco
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