"LH thread on rachet wheel screw" - list?

[caseback]

Well, there has to be a first time for everything. I've managed to work on watches for three or four years without ever breaking a single screw....until today.

Several other screws on this movement were on very (!) tight and had the markings to match, so I used a bit more force than usual. No markings on the head as a warning, so "snap" it went. Furtunately, last year I've bought a shoebox filled with leftovers from a watchmakers estate, and it held a little box marked "AS1012" with 5 new LH threaded arbors and matching screws. 

Has anyone ever compiled a list of movements with a LH threaded arbor? We could start a thread on the subject?

Edited September 8, 2024 by caseback
Correct typo

[mbwatch]

The HMT 0231 have left threaded ratchet wheel screws. I have never seen one marked. And the screws are very soft metal, extremely easy to shear off.

I have not worked on its progenitor, the Citizen 0201, to confirm whether it was also left threaded. But I would approach one expecting it to be.

[caseback]

22 minutes ago, mbwatch said:

I have not worked on its progenitor, the Citizen 0201, to confirm whether it was also left threaded. But I would approach one expecting it to be.

I have, and that one is left threaded as well.

[mikepilk]

I hadn't come across a LH threaded ratchet wheel for years, until I worked on an AS 984.  I'm always wary when working on AS movements that it may be LH. 

[Jon]

Yep, it was a tiny AS movement that I broke a ratchet wheel screw on! I can't remember which one exactly.

There is no feasible reason to put a LH thread on a ratchet screw. When it spins it would undo itself if it were loose which makes no sense as it turns clockwise, the same way the screw will loosen

Edited September 8, 2024 by Jon

[nevenbekriev]

The reason is simple. The screw is identical to the crown wheel screw, so one mashine less for making screws in manufacture

[nickelsilver]

Aside from AS, I only really recall seeing left hand threads on ratchet wheels on 8 day movements. The reason being the ratchet winds in reverse.

 

There is a rather special form to ratchet wheel screws aside from the thread; they (the good ones) are reinforced where the thread meets the head with a fillet.

 

 

[mbwatch]

1 hour ago, nevenbekriev said:

The screw is identical to the crown wheel screw, so one mashine less for making screws in manufacture

That seems a logical reason I would expect to find on some movements. On the HMT 0231 the screws are not the same though. The crown wheel screw has a larger head and shorter shaft.

[nevenbekriev]

3 hours ago, nevenbekriev said:

 The screw is identical to the crown wheel screw...

That's what I have met in many AS pocket watches