[Shop-talk] Metric Hardware

[David Scheidt]

On 7/6/07, Elton E. (Tony) Clark <eltonclark at gmail.com> wrote:
>
> > David wrote:
> > Should have gone to a brit car shop. Tripod mount screws are
> > whitworth! 1/4 inch for most small cameras, 3/8 for biger ones. Both
> > whit sizes are have the same thread pitch as unc threads(20 and 16,
> > respectively), and nuts interchange.
>
>
> >>>Oops! please 'splain that: If it's 1/4 x 20 & 16 it ought be UNC . . . do
> you mean cameras actually have those beautiful round-contour thread forms of
> Whitworth? If Whitworth, I thought it would be sized by the wrench size . .
> . unconfuse me!

Whitworth bolts are referred to by their major diamter and pitch, just
like American ones.  A number of the diameters have the same pitch as
the American (UNC) bolt does.  Tools for whitworth are named for the
bolt they fit, not their across flats measurement.  So, the tool that
fits a 1/4-20 UNC bolt is 7/16 AF, and is labeled as such.  The tool
that fits a 1/4-20 BSW bolt is about 0.44 inches, but it's labeled
"1/4 BS".

It's actually more complicated than that, though.  The bolts that we
call "Whitworth" on our LBCs aren't really.  They're British Standard
Whitworth (BSW), or British Standard Fine (BSF).  These use the same
55 degree rounded threads as Whitworth, in the same diameters, and for
BSW, the same pitches.  BSF uses, not surprisingly, finer threads.
The difference is that the head sizes for the BS stuff were reduced,
to the next smaller size.  The big heads apparently let people
over-torque fasteners.So a 1/4-20 whitworth bolt has a head that you'd
find on a 5/16 BSW or BSF bolt, which is a tad more than half an inch
across the flats.  It's for this reason that many tools that fit these
bolts list two sizes on them like "1/2BSF | 7/16W".  (Actually, my
favorite Whitworth set are different sized at each end, so they have
*four* sizes on them.


-- 
David Scheidt
dmscheidt at gmail.com