In article <00b301c0be0f$8ec16600$66b4403e@zebu>, Graham Stretch
<technical@iwnet.screaming.net> writes
>
>Hi Richard, Mike
>The 1/2" AF bolt should by rights have a 5/16" thread on it.
>On the whole Triumph stuck to the system of into steel use UNF (fine) thread
>form, into aluminium use UNC (coarse) thread form.
>Common sizes on Triumphs
>
>5/16 AF = 3/16 UNF or alternatively 2BA
>3/8 AF = 3/16 UNF or alternatively 2BA
>7/16 AF = 1/4 UNF or UNC
>1/2 AF = 5/16 UNF or UNC
>9/16 AF = 1/2 UNF or UNC
>5/8 AF = 9/16 UNF (bolt often found with a 3/4 AF nut on)
>3/4 AF = 5/8 UNF
Many thanks for the info. I am bemused as to why Triumph should have
selected the Unified series of threads in the first place. My grasp of
engineering history is shaky at best, but I would have expected BA or
BSF/BSW threads on a piece of 1950s/60s/70s British engineering. My
only experience with Unified threads in the past has been on 1950s
Canadian engineering (Westinghouse turbines), and I was under the
impression that the Unified series was a (failed) American attempt to
standardise threads throughout the English-speaking world, and to
present a united front to "those damned foreign johnnies" and their
metric system!
Two little thread anecdotes:
At least one British engineering company (Belliss and Morcom) devised
its own thread/head ratios - they took the egregious Whitworth ratios,
and staggered them, so the Belliss and Morcom head on a 3/8" BSW bolt
was the same distance A/F as on a standard 1/2" BSW bolt. No wonder
British goods cost more than any other!
When W. H. Allen designed a new medium-speed diesel generator (VS37G) in
the 1980s, they took the decision to use metric threads throughout.
Unfortunately, it turned out that the longitudinal stress on the
cylinder head studs was such that the metric (M36 from memory) threads
would strip themselves when the engine was on load. They had to
replace them with a BSF equivalent...
ATB
Mike
--
Michael Hargreave Mawson, author of "Eyewitness in the Crimea,"
published by Greenhill Books on 28th March, 2001:
http://www.greenhillbooks.com/booksheets/eyewitness_in_the_crimea.html
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