Repairing stem threads

[ManSkirtBrew]

I finally have a replacement crown for the Longines I'm working on and I can't get the stem to thread cleanly into the crown. It goes in a fraction of a turn and stops.

Not about to force it, I tried a different stem and it threads into the new crown just fine.

The first thread looks a little smushed to me, but it could also be an optical illusion from having been filed at a weird angle.

I obviously can't shorten it to get to fresh threads, so what would you do here?

 

[HectorLooi]

If you have a die of the correct thread, you could use that to freshen up the threads on the stem.

[Neverenoughwatches]

Same as Hector the correct size of die will feed on easier. Although you cant apply it in this case ( shutting the gate after the horse has bolted sorry about that and the pun )  but a popular trick that is used to straighten out the end threads of a bolt after it has been cut is to have a nut threaded on before the cut, the nut is then spun off. For future you can use this method by trimming off the blind end of a crown and have that threaded on before cutting the stem.

Cutting it back to the root portion of the thread will help it to enter better. How thick is the crown ? If there is plenty of thread in it you might get away with a little trimming on the stem, add something in to the crown hole afterwards to stop the stem bottoming out past the required length you need. Make sure whatever you pop in the crown is small enough to come out in case adjustment is needed.

Bit difficult to do at this size, but  another idea is to clip on a halved nut and back it off the stem

Get yourself a screw thread polisher for shaving the ends

[oldhippy]

You could use a stem extender or adapter. When fitting a new crown to make sure it screws in correctly file the end so it has a bit of a point. 

[mikepilk]

13 hours ago, ManSkirtBrew said:

It goes in a fraction of a turn and stops.

That usually means the wrong thread. Was the 'different stem' the same as this stem?

The threads look fine to me. 

(What's the movement?)

[nickelsilver]

I agree with Mike, those don't look great (I've seen way worse that still worked fine), but that shouldn't keep the crown from going on. I'm guessing the crown has a smaller thread in it.

 

A really handy tool is one of these plates, we always called them "Borel plate/gage", but I see Bergeon has one too. They aren't cheap, but very useful. You have male and female threads to check stems and crowns, tubes for checking crowns (the fit of the gasket) and holes for checking tubes and crown ODs, plus gages for straps.

 

 

[ManSkirtBrew]

1 hour ago, nickelsilver said:

I'm guessing the crown has a smaller thread in it.

I think you guys are onto something. The movement is a Longines 343, case #2579-343. The crown is a Longines 13WP.

I grabbed another Gruen stem I had and it does thread in a few threads, but still stops as if it were the wrong thread pitch.

 

I compared it (top) with the original stem and a brand new replacement 510 stem, and all seem the same. The other 510 stem doesn't thread in at all, just like the factory one.

 

I compared the factory crown on the left to the new one, and they sure seem the same to me. I used my microscope's measurement feature and they seem very, very close.

[Neverenoughwatches]

Seem like you will need to tap the crown to the correct size thread. Unless there are stem adapters available.