There are a couple of major differences. First, the included angle of the
thread is 60 degrees on American threads and 55 1/2 degrees on the British
Standard Whitworth (BSW) and British Standard Fine (BSF) threads. Second,
there are indeed differences in pitch. For example, a 1/2 in UNC bolt has
13 tpi, whereas a 1/2 in BSW bolt has 12 tpi.. A 5/8 bolt has the same tpi
in both standards, but the difference in thread form would make a nut and
bolt of the different standards bind.
The other difference is in the size convention for wrenches, or spanners if
you're on the UK side of the pond. A half inch unified wrench fits a bolt
with a head measuring 1/2" across flats (or AF, which is what these wrenches
used to be called in England). A half inch spanner, on the other hand, fits
a half inch diameter bolt, which has a hex that is between 0.812" and 0.820"
across flats.
So, there you have it.
Michael Marr
1960 TR3A
Naperville, IL, USA
Ex: Tilbury, Essex, England.
----- Original Message -----
From: "Paul Dorsey" <dorpaul@negia.net>
To: "triumph list" <triumphs@autox.team.net>
Sent: Tuesday, April 12, 2005 9:55 PM
Subject: British or SAEt
>I had someone tell me that the British used a different 'pitch' on the
>thread
> of a bolt. IOW, a SAE 7/16 x 24 (threads/inch) but, perhaps the closest
> British sized bolt is 7/16 x 25 tpi. Is this true or is that guy making
> this
> something that "the British did this on purpose".HWNM (his words not
> mine).
> Thanks,
> Paul Dorsey
=
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