Question for the watchmakers out there...

[timoss]

I have an offer to buy a period-correct zenith crown I need, but it has damaged threads. The way it was described: "It's as if you had a nut and you rubbed it against something and the threads got flattened." So I assume the outermost theads are flattened and the stem can't be screwed in.

So the question: Is this easily repairable? Since the outermost threads are flattened it doens't seem as simple as just running a tap through it, for instance. But maybe it is?

I know it's hard knowing without seeing it, but that's all I got (no picture)!

 

[Geezer]

It all depends on how badly damaged it is, I guess. You're right, you can try to take a shortened tap and go through the thread again. But if it is bad, it might be difficult to find the start of the thread and force your way through.

Depending on how much solid material the crown has, it might be possible to drill it out and insert a new tube. But you will need a pretty enthusiastic watchmaker to attempt that...

There are always ways to repair things, some better than others. Unless it is really hard to find a functional part for a good price, I would avoid this crown and wait for a good one.

 

[Archer]

You need to buy this prepared to write it off. A bottoming tap could chase the threads, provided you can find one of the right diameter and pitch.

 

[timoss]

Thx guys. Good advice. It’s a small fraction of the price of a “good” one but of course hardly worth it if it doesn’t pan out.

 

[ExpiredWatchdog]

If you can get the stem in the crown, you might try Loctite 660 retaining compound.

I had a cheap diaphragm compressor where the motor bearing at the crank end began to slip on the shaft and wore a groove in the shaft. Between a metal shim and this stuff, I got several more years from the compressor before replacing it.

They also make PC 3967 thread repair compound which touts the ability to replace worn threads with new "metal" and still have a functional threaded joint. Can't vouch for it.

The sales blurb for 660 says "LOCTITE 660 is a high strength retaining compound with good gap filling properties, ideal for repairing worn-out seats, keys, splines, bearings or tapers without remachining." That's pretty confident, but in line with my experience.

 

[timoss]

Thanks. I’ll have to check that stuff out either way.

 

[timoss]

This probably won't change anyone's advice, but the seller attempted to take a pic of the condition:

 

[Archer]

Threads look to be in decent shape...

 

[timoss]

Yes indeed. Just the very first “layer” appears flattened. I wonder if you could run a slightly larger tap through first to push the threads back close enough to original position to run a tap of the correct size through to straighten it all out.

 

[Archer]

I would try just carefully running the stem in and out, and see how it goes. If you have a spare stem of the right thread size and pitch, you can file or grind some flats on it, leaving a sharp edge on some of the threads, and that will act as a tap for softer metals. That might work here as well.

 

[timoss]

Nice idea thx.