It goes without saying that casebacks that have the engraving “Fit Washer in Back” require an internal gasket as shown in the ref 2757 photo below. However I noticed casebacks that aren’t engraved “Fit Washer in Back” may or may not have a gasket fitted. What’s appropriate for such casebacks or does it depend on the reference?

 

I always thought that phrase referred to fitting the washer to the case back, rather than main case, before assembly. Watches would have the washer fitted to the main case, & then the back screwed in.

 

These screw back cases typically used a flat plastic gasket or an o-ring which fit into a groove in the case.

 

Does the red arrow indicate the o-ring in this ref 2627?

 

No, its further out, this one actually still has an old lead gasket in place.

 

Does the arrow point to a gasket on this 2576 case or is it my imagination ?

Edit: My imagination. Arrow points to where an o-ring should exist.

 

The first case back in the top post is a snap on case back, and the gasket goes inside the case back. It's a BUNA O-ring, and it does not affect how well the case back snaps in place, so it is often left out. Example shown here:



This is different from a modern snap on case back that uses a hard plastic gasket to seal the case - without that gasket the case back would not stay in place. With those the gasket goes on the outside of the case back, and there is a leading chamfer that has to be positioned correctly:



These are one time use only, because when they are pressed in they deform and will never hold properly after the case back is removed.

The standard screw in case back will typically have a groove in the case where the O-ring seats, and the case back just screws in place after the O-ring is installed and lubricated, like so:



Of course there are other materials and variations, but with Omega these are the three most common types found.

Cheers, Al

 

Thanks Al. I assume the arrows point to the proper locations of where an O-ring should sit in ref 2627 and 2576 caseback photos below? Also should an O-ring be fitted on the caseback when a lead case gasket exists as @X350 XJR post above demonstrates?

 

Please look at the last photo I posted above...

You don't use both...

 

@Archer please look at the last photo I posted above...
Would you use a flattened or rounded case o-ring for my type of screw on caseback?

 

I would use what Omega says to use...

 

These should use a flat washer. Looking through an old Bestfit catalog there is a listing for a lead washer for case 2577. Don't think lead is the preferred material now these have been replaced by plastic.

AFAIK if there isn't a channel to fit an o-ring, then use a flat washer.

 

It's a flat BUNA seal, not plastic as you are showing...

 

Sorry, now I know the correct term.

Al, is the rest of what I said generally correct?

 

Omega doesn't use those flat plastic gaskets - they are flat gaskets but made of the same material as the O-rings are, so black nitrile rubber.

In terms of making blanket statements on seal types by looking at the case groove, I've not taken the time to look into this in a big way, so I can't say one way or another.

Cheers, Al

 

Lol... thanks

 

Hello Archer how do I find the correct oring for a 2975-1 SC?

 

Omega part no. 088NS0607.

 

Thank you very much.. Can you tell me where do I find this kind of information?