Steph :
UNF - Unified National Fine : name for a range of fine threads, based on
inch measurements. Still used in the US, but AFAIK (as far as I know)
obsolete in the rest of the world. _Very_ common on Triumphs.
Sometimes called SAE threads or SAE fine threads. Also sometimes called
simply NF.
SAE - Society of Automotive Engineers : Group formed in the US in 1905
to set standards for cars. Original creators of what became UNF thread
standard, along with many other standards. Now expanded to 97 countries
and all forms of transportation. See http://www.sae.org/ for more info.
BSF - British Standard Fine : name for a bolt and nut standard,
including thread form, (fine) pitch and head size, based on the bolt
definitions created by Sir Joseph Whitworth back in the 19th century.
AFAIK obsolete everywhere in the world, but found in odd places on
British cars and motorcycles. See
http://triumph.utah.edu/sol/tech/whitworth-hist.html for more info.
BA - British Associated : similar to BSF, but for screws smaller than
1/4 inch diameter.
UNC - Unified National Coarse : common, coarse equivalent to UNF.
'Standard' threads in the US, although most new cars are ISO metric.
Sometimes called USS or American threads.
BSW - British Standard Whitworth : Original 'Whitworth' threads. Coarse
equivalent to BSF, just as obsolete. BSW threads in sizes 1/4" through
7/16" will actually interchange to some extent with UNC threads (with
some loss of strength), but use different wrench sizes.
TPI - Threads Per Inch : measure of the pitch, or distance from one
thread to the next. Higher TPI means finer pitch.
NPT - National Pipe Tapered : US standard for tapered thread used on
water pipes, etc.
BSP - British Standard Pipe : Whitworth equivalent to NPT, commonly used
everywhere but the US.
NPS - National Pipe Straight : Like NPT, but without the taper. Used in
some odd places like lamp bases (where it is standard).
BTW (by the way), the 5/8" measurement refers to the diameter of the
threaded part, the 1 1/8" refers to the size wrench used to turn the
bolt. Whitworth wrenches are marked with bolt diameter, all others are
sized by the opening (distance across the flats of the bolt head) which
is normally much larger. A 'normal' 5/8" bolt has a 15/16" head, so
perhaps the crank bolt on a TR6 is a 'special'.
Hope this clarifies a little.
Randall
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