OldTools Archive

Recent Bios FAQ

264205 Erik Levin 2017‑12‑12 Re: Thumb screw wanted
Don Schwartz wrote:


> I tried all of my loose odds and found nothing that fit. The local
> Bolt Supply House agreed that an 8-32 almost fits, but jams up quite
> quickly, and suggested it may be an 8-28 which is not available here
> due to lack of demand. ;-(
 I would suspect that imperial measure is a reasonable assumption given the
origin, but the actual thread size, pitch, and form have a lot of options. A
LOT. What I would do is run a soft, formable material into the hole and back out
to get as much information as possible. Thread pitch is easy this way. George
has suggested wood, and it has worked for me, but I have also used a variety of
plastics. You need a material that will form, not break, and hold dimension.

 For the pitch, use a round slightly larger than the ID of the thread. 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.
If you can't get a good measure directly, I have run a chunk of material into
the hole, marked the end for alignment, measured how far it sticks out, and then
backed out 10 turns and remeasured. Quite accurate, though awkward and it needs
a reference surface.

In some cases, you can actually get a good enough cast of the threads to get a
true major diameter, and even see the thread form under a microscope. Don't
count on it, though.
Just for fun, also consider that even though sizes were standardized in the
1860's, many manufacturers didn't adopt the standard sizes (I mean you, Stanley,
and Starrett, and Brown & Sharpe, and, well, most of them) across the board.
0.150" (just under #7) isn't uncommon (Again, I mean you Starrett), and, if in
the 1860's to 1910 range, the numbered size could be old standard
(0.01316*size+0.0578), which makes a #8 0.163" rther than 0.164".  Not enough
difference to matter for most purposes (#8 through #12 are nice this way, number
14 being a dead match), but it can be an issue with screws in old size holes
especially if the thread forms are not close.

That said, if it is a #8, 8-36 used to be fairly common as a fine thread. 8-28
would be a tad course and isn't in any of the tables I have, going back a long
way. 8 40 is not unheard of, but I have only seen it a few times, typically on
adjusters.


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Recent Bios FAQ