Vintage Longines 17.79ABC screw thread type - to replace missing

[JGrainger]

Hello,

I got this watch a while ago and replacing a hairspring stud screw seemed like an obviously easy fix. It's been in a draw for some time while I've been trying to work on watches I'd be more interested in selling (this is one I was intending to keep, so a low priority).. not to mention competing for time spent doing other activities such as Swing dancing and Tango or a music degree as a mature student (finished last year).

There are a few similar movements on Ebay and it's conceivable I could easily replace the screw with one from another movement. It seems a bit extreme and wasteful to buy another (lower grade) movement of the same size and type for only one screw when they're not overly common.

I wondered if anyone can advise what type of thread pitch the screw for the hairspring is likely to be - what system were Longines using back then? - this way I could more easily find, modify, or make such a screw.

Using the matchstick method and microscope graticule I've some up with the following information:

It seems to be approximately a 0.119mm pitch with an ID of 0.416mm and an OD of 0.516mm (guessing a bit here as the wood doesn't produce a full thread pitch).

[nickelsilver]

It will be metric, to the NHS Swiss norm for watchmaking. An 0.50mm thread has a pitch of 0.125, so that sounds like what you've got (0.40mm is 0.10 pitch, and 0.60mm is 0.15). Back then one would encounter sometimes "half sizes", like 0.55mm but I doubt you have that. The image is a screenshot from the Asco Schuch catalog- they are the go to guys for small watchmaking taps and dies and they provide a lot of useful info.

www.schurch-asco.com

[JGrainger]

Thank you,

I'm never really sure what type of thread will be used on most vintage watches.

[JGrainger]

Figured I'd just post a followup.. one of my weekend projects.. it now runs but I found a couple of cracked jewels during servicing - will do that once I've got some which are suitable.