There is no question that the screw used to extend the finger
is not an ACME thread. It isn't a worn ACME either.
When this part is threaded into the finger, there isn't any play (if it were a worn
ACME, there would be lots of play). Here is a close-up.
What I see is, this thread form is a 60° but has a shallower depth. I haven't measured
the depth yet. From the formula in the HTRAL handbook, full depth 10 pitch thread
should be .064952" or .065" (rounding to the nearest thousandth). This would give
a minor diameter of .245".
The pilot hole in the finger is about 19/64". This is .297" (rounded). This means the
tap doesn't have to be made to full thread depth and that should make it stronger.
Some have suggested substituting a 3/8-16 LH tap. Some have done this when
fabricating parts for these telescoping steadies but if one uses a finer pitch, one
does not get the full range of finger movement and loose some capacity (either
loosing the ability to come all the way to center or not able to open all the way to
maximum for large diameter parts).
The depth of the thread isn't completely known at this time. The fingers were modified
to accept a replaceable tip and they were drilled all the way into the bottom of
the 3/8-10 LH threads. Even though this has happened, I can estimate the original
depth to be about 1-7/8" to 2". I have purchased some worn fingers that haven't
had any mods so I will be able to get a more accurate measurement once they have
arrived. Once I know the correct depth, I can pass that on to the makers of the tap
so it can be made accordingly.
Anyway, I do appreciate the suggestions and help I've received so far.