Erik Levin via OldTools wrote:
> Then you can check the pitch using a thread pitch gauge. If you don't have one
> (and can't borrow one), you can use a caliper and magnifier. Measure as many
> threads as possible and divide into the number of threads measured. If you
measure
> five thread pitches (four untouched thread marks between the caliper points)
and the
> measure is 0.180", then 5/0.180=27.8, giving 28 threads/inch as the most
likely, with 27 a nut
> unheard of option, 26 and 30 being unlikely, but possible given the precision
of
> the technique. The more threads in the measurement, the better.

It is possible to perform this kind of measurement rather accurately by taking a
macro
photograph of the item with a ruler (or other accurate scale) in shot.

You ideally need a zoom macro lens, so that you can be far enough
away to minimise parallax issues.

Having done this, count the pixels on the largest possible convenient distance
on the scale; this can be used to set the resolution of the image.

 From hereon, most image editing tools will allow you to measure
directly in the units of your choice.

If not, simple pixel distances can be converted using your reference measurement
pro rata. This method allows half-threads and thread "tips" to be referenced
very accurately, since the photograph (and image editor) allow in effect
high magnification and easy manipulation.

  BugBear