This is probably a stupid question but I just don't know the answer. I tried to Google it up but I ended up with excellent advice on how to take care of dental crowns.

Judging by the number of vintage watches with incorrect crowns this seems to be rather common. Why is it? Is it a crown/stem replacement thing or just the crown?

 

keep the watch water tight for one, and because they wear out from constant use and become loose and slip.

Think about it, what is the most commonly used part of a watch?

 

Crowns fall off, break off, and can wear out. There were almost no crown guards on vintage watches back then. One snag on a sweater and boing!

 

crown/stem/tube are all commonly replaced during servicing.

 

Crowns need to be replaced for several reasons. One is that they have gone missing as Dennis noted, another is that soft metals like gold do wear out. More commonly though on a watch with a screw down crown is that the threads are stripped, and on all types of crowns that have a seal, the seal inside the crown goes bad so the whole crown is usually replaced. On most crowns there is typically a washer that is swaged in place after the seal is put into the crown during manufacturing, that will not allow the seal to come out. The whole crown is replaced because of the bad seal.

Often the stem can be used again if it's not corroded or damaged, and is of the right length. The case tube is something I don't always change - it depends on the situation. If I'm changing a screw down crown, then yes crown and tube are changed as a set. But for the non-screw down crown, I look at the condition of the case tube and decide if it needs to be done.

Cheers, Al