[Newbie] Equivalent Metric/SAE Threads question

[runswithsizzers]

Short version: Are M22 x 1.5mm - and - 7/8" x 16 tpi - functionally equivalent and interchangeable in non-critical applications?

Details: I just bought a Taiwan-made drill press that is missing a left-hand threaded nut to secure the pulley block to the tapered quill/spindle (no keyway or set screws).

As best as I can measure, the male spindle threads are EITHER: M22 x 1.5mm - OR - 7/8" x 16 tpi.
I believe my 16 tpi thread pitch gauge MAY be fitting a little better than my metric 1.5 gauge, but I am not certain.

I can find left hand M22 x 1.5 nuts at McMaster-Carr, but so far, I can't find any source for left hand nuts in 7/8" x 16 tpi.

If the actual size of the spidle threads is 7/8" x 16 tpi, can I get away with substituting a M22 x 1.5 mm nut for this application?

 

[benmychree]

A definite maybe, if the nut is not too long, as the pitch would be about .003" short on each thread engaged, important to note that the 22 mm thread would be .866" on the major diameter, too small if the thread is 7/8". A pitch diameter measurement would be definitive as to what the thread size really is. If the thread is long enough to span most of thread gage, that should be apparent what the pitch is, sometimes holding a flashlight behind the gage helps in seeing the possible mis fit.
Given the origin of the machine, it would seem likely that it would be metric.

 

[mksj]

If you have access to a lathe you could thread some stock and check, I would also check other fittings on the drill press to see if they are metric or imperial, still on my Taiwanese machines they always seem to be a mix. Other possibility would be to use an M22 x 1.5 and rethread it,
I did find this at Grizzly and inexpensive which could be used as is or grind it down as needed.

Insert - 7/8" x 16 TPI, LH Thread

These threaded inserts adapt the G1194, G1082 chucks and the G1633 face plate to any lathe spindle, whether it's a Grizzly or another brand. Just match up your spindle from your lathe to the corresponding inserts when you order one of our chucks. It's that simple! Each one includes mounting...

www.grizzly.com

 

[Chewy]

Ebay sells the L/H tap for $39. I would go with the nut from Grizzly. Check with Matt at PM and see if any of his stuff uses that nut.

 

[Flyinfool]

If it is a 7/8-16 the major dia of the external thread should be 0.8735 / 0.8641 inch (22.1869 / 21.948mm)
If it is M22 the major dia should be 22.000 / 21.732mm (0.8661 / 0.8556 inch)
While there is a slight overlap in the sizes this may give a clue. In addition to the flashlight trick from above.

So the answer is a definite maybe.

 

[runswithsizzers]

If you have access to a lathe you could thread some stock and check, I would also check other fittings on the drill press to see if they are metric or imperial, still on my Taiwanese machines they always seem to be a mix. Other possibility would be to use an M22 x 1.5 and rethread it,
I did find this at Grizzly and inexpensive which could be used as is or grind it down as needed.
[URL omitted because of forum restrictions on new members]

I happen to have a major Grizzly retail store in my home town, Where I just bought that same item earlier today. That part threads onto my drill press quill just fine, though the fit feels a little loose. I will need to modify it because the outside diameter of that part is too big to fit inside the recessed area of the pulley block.

I also have a couple of thin (11mm) brass nuts on order in the M22-1.5 size, so I can see which feels the best.

 

[runswithsizzers]

Thanks to all who replied and for your helpful suggestions. I am not a metal worker - yet - though I did make sterling silver jewelry for a while, many years ago. While I have just about all the tools needed for hobby boatbuilding and home repairs, I do not have a lathe or any machine shop tools.
EDIT: I did just get a small oxy torch and this old drill press, so there may be more metal in my future.

When I was in the bicycle repair business, I used to be able to read my 50 year old ELDI vernier calipers to 0.02mm, but the calipers - and my eyes - are not as reliable as they used to be. The ELDI has scales in metric and inches, but fractional inches, not decimal. It is plenty good enough for woodworking and home repairs, but for me, there is no third digit to the right of the decimal.

I will try the screw pitch gauges again in better light.