Learning about fitting an unusual size gasket

I need to replace the Gasket on an Aquastar Benthos 500 case back. It's 70 duro hardness but the size is gargantuan for the watch world.

In my search I did learn something which I thought was pretty cool, but basic to most, I know.

I learnt that the formula for a gasket's cross section is CS = OD - ID /2.

and in this case the gasket/O-ring needs to be:

ID=33mm
OD=36mm
CS=1.5mm

and that's spot on to the Channel width at 1.5mm.

An engineering trick I learnt from Rileynp on another forum is that a certain amount of compression should yield an optimal seal, so that the o-shape can turn into a square-shape as the case-back comes into contact with the case frame.

So with that in mind the search was on and eventually found this supplier so I could get some sizes ranging from 1.1 to 1.5, to most likely end up throwing out the largest and smallest options.

https://www.theoringstore.com/store/index.php?main_page=page_2

Plumbing suppliers also yielded some close sizes in a pinch.

https://www.theoringstore.com/store/index.php?main_page=page_2

I will let post here when they arrive.

This is the watch and the empty channel to be filled...

Yes, the cross section of the seal should be equal to the width of the seat in the case. The seals should be slightly proud of the groove when seated, so it can be compressed.

Most seals top out at 1 mm cross section, even for watches rated to extreme depths. I do see on vintage Doxa's for example some very thick seals like what you need, so if this one is vintage and related at all to the Synchron group, it may be of the same design team. I have hundreds of different O-rings in stock, and looked in my stash for one for you, but this is the closest I have...



Note that watch material suppliers do stock these odd sizes sometimes, so not sure what they are like where you are, or if they deal with hobbyists, but it might be something to consider if you can't find them elsewhere.