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When I first started working on watches I tried to buy the correct size O ring seals on an individual basis after carefully taking measurements. I soon realised the futility of this, it took 3 or 4 goes to get the right one. I bought a box of O rings for around £30 from Cousins, this a few of every size you're ever likely to need. After cleaning the old seal remains out I measure roughly then try one from the box. Invariably it'll be wrong, but after a couple of tries I end up with the right one. How do you know it's right? Fit it in the groove then check with a magnifier that it's sitting slightly proud to allow some compression. Prior to fitting, I fit the caseback and screw it home to the point where the edge is contacting the case with no gap. I mark the caseback with marker pen, try tightening up with the seal in. You can feel the seal compressing and ideally the caseback should tighten to the point where there's metal to metal contact at the edge and the mark you've made is aligned. To get the case fully screwed on should require light nipping up using a tool if all's well. Proof of the pudding is a pressure test, but usually if it looks and feels right it is right.
If the selected seal's too thick the caseback won't screw down fully. If the caseback also serves to secure a movement ring ( a common arrangement on cheaper watches) this will be a problem.
All this is a lot simpler than it sounds. It also pays to clean up the threads on the caseback if necessary to ensure the back screws on nicely. Don't forget to lubricate the O ring with a trace of silicone grease, and a tiny spot of watch oil on the caseback threads can help if the threads are a bit rough.
If an existing seal is still in reasonable condition it can be carefully measured with a digital caliper and a magnifier to get an idea of the thickness required, but it still comes down to trial fitting and finding what works.
Paul
Originally Posted by walkerwek1958
