Nut case.....

[oscarvan]

I think I have a pitch gauge somewhere. DOH, should have thought of that. It's definitely not fine. To the eye they look the same. (Coarse) The 19mm idea is a distinct possibility.

[kingsley_oasis]

Cheap vernier caliper. I use digital because my eyes are getting bad. Magnifying glass for eyes, count threads per inch, measure OD and look up thread spec. Hard to believe it is metric if boat has original Hynautic helm pump. My drawing shows 3/4 * 10 UNC

[oscarvan]

Cheap vernier caliper. I use digital because my eyes are getting bad. Magnifying glass for eyes, count threads per inch, measure OD and look up thread spec. Hard to believe it is metric if boat has original Hynautic helm pump. My drawing shows 3/4 * 10 UNC

That's what I thought.... and ordered. Doesn't fit though.....

[kingsley_oasis]

That's what I thought.... and ordered. Doesn't fit though.....

I feel your pain, I have a boat from the old country which has British, metric, and standard threads. And that is before the piping sizes.
Anyway as your old acorn nut fits shaft we can assume thread is good. So if it is 3/4 UNC then may be nut is just bad. As this thread is common test acorn nut on a 3/4 UNC bolt thread. If OD of thread is indeed 3/4 we can rule out metric as I do not believe there is a 19mm size, just 18mm or 20mm either side of 3/4.
Can you ascertain how many threads per inch? this gives us at target. If you do not have caliper get some plain paper and wrap around thread pressing tightly to imprint. You can then count treads per inch. Paper allows using pencil to mark as you count, and just need good tape or rule to measure. May have to do some match if you do not have a full inch of thread.
You could take original acorn nut to a distributor like Fastenal to confirm size. If you do check in advance because a lot of their locations only open to trade customers nowadays.

[oscarvan]

Brought the caliper with me..... 4 threads in .38-.4 inches would indicate a 10.

But.... putting my face on the spindle, and it being in just the right position i noticed all of the 4 the threads have a flat spot on it. So maybe some more persuasion, combined with some happy juice, is in order and see what happens. (Up to this point only used finger force) Or an investment in a thread chaser this size....

[oscarvan]

Proceeded as described above. Success! It is in fact 3/4"x10 Just needed a little "help". Hubcap is on.

[SEVEN]

Glad you figured it out. That was a lot of posts for a little nut.

[oscarvan]

Glad you figured it out. That was a lot of posts for a little nut.

What are you calling me?

[Bob Bradley]

Ya know, it takes a big man to admit that the biggest problem on his boat is the nut behind the wheel.

[oscarvan]

Ya know, it takes a big man to admit that the biggest problem on his boat is the nut behind the wheel.

You funny too......