Literally, the crystal keeps falling off from the watch. Am I missing something here?

 

Tension ring maybe

google Tension ring fitting

 

Tension ring I reckon. If I recall correctly, there are different tension rings for different reference. When you purchase a tension ring, make sure it's the right one for your watch

 

There's a tension ring in the case, it should be removed and it should line the crystal.

 

Wrongly sized crystal. A tension ring won't keep it in, afaik.

 

The cuff of your shirt is not tight enough.

 

I see. Should I buy a part before I talk to my watchman?

 

crystal by itself completely fit to the case. actually I have pull the crystal to remove. Provided that, should I buy new crystal and tension ring?

 

If it is an old crystal, I believe it could be down to shrinkage. Or it could be a veeeeery slightly too small diameter crystal.

EDIT: Is it an Omega crystal? If it is a front-loading case, I would be very careful in finding the correct type and size.

 

No, let your watchmaker get the part - it's his job...

 

Unless they don't have Omega parts access, then they will probably use a generic crystal which will likely fall off again.

 

Of course it should be an Omega crystal no matter who looks for it, but if we are assuming a watch owner can find an Omega crystal, the watchmaker should be able to as well...

 

Excellent work getting the word "shrinkage" into the discussion

 

Doesn't look like a high valued watch, some glue will help save lots of money and headaches.

 

For whom? The current owner or the poor future owner or watchmaker? It's worth doing right.

Tom