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BSF is for
all intents and purposes the same as the SAE's United National Fine
(UNF). That's why you can go down to the local 'Murrican hardware store
and find bolts whose thread size will work on a B.
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Actually, BSF and UNF aren't the same. I had to take a crash course on
this stuff when I got into older MGs. BSF fasteners are supplied in inch
diameters but the thread is cut at 55 degrees and the number of threads per
inch is different than UNF. 55 degree threads with a slight radius at both
the crest and valley of each thread is the whitworth thread form. This same
form is used also on BSW (British Standard Whitworth-coarse thread version
of BSF), BA (British Association) and BSC (British Standard Cycle). The
heads are also made to a different measurement than those used on the UNF
and UNC fasteners we're used to in the US and so need a wrench with a
different opening. The reason you need a set of whitworth wrenches.
UNF are also in inch diameters but the threads are cut at 60 degrees and
have more or less "sharp" crests and valleys. So the two systems aren't
interchangeable. UNF and UNC fasteners are available at most good hardware
stores.
John Morris
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