This is quite common for the era, the reason being that many of the glass crystals of that time shattered quite easily if they struck something, whereas plastic could take hard knocks although it scratches fairly easily. Also, back then they didn't have the technology easily to make curved, domed or rectangular domed crystals, such as you see on many of these old watches, out of glass. It sounds like yours may fall in that category from your description.

I inherited my Dad's gold Jaeger-LeCoultre from that era and it also has a domed plastic crystal. He also had an old rectangular curvex Girard Perregaux (his high school graduation gift from his parents in 1940) that originally had a fancy domed curvex plastic crystal on it, but when he had it restored and gave it to me when I was in college, the restorer put a simpler sapphire glass crystal on it (which frankly doesn't look as nice although it doesn't scratch).

His GP looks similar to this (this is being sold on Ebay) although my Dad's is gold with a silvered face, the sub-seconds dial is a little different, and the original crystal stood up higher from the watch face (so it actually kind of magnified the dial):

I understand with these plastic crystals you would take the watch to a jeweler from time to time and he could buff out the scratches at the same time he was cleaning and oiling the movement. Also, if necessary, replacing the crystal with a new one was easy and fairly inexpensive.

 

How often do these mechanical watches need to be cleaned and oiled?

Do the good ones ever wear out?

 

How often do these mechanical watches need to be cleaned and oiled?

Do the good ones ever wear out?

Advice varies (depending on the age/make of the watch and how aggressive the advice-giver is in promoting his repair business), but 3-7 years between servicings seems to be usual. The watch will likely keep running past that time, but you run an increasing risk of parts wearing out the longer your let it go.

I think that's the answer to your second question, too: inevitably the crown will break at some point, the mainspring will eventually wear out, and non-jeweled points of friction might occasionally wear away, but so long as parts are available your watch should indeed run forever given proper attention.

 

He also had an old rectangular curvex Girard Perregaux (his high school graduation gift from his parents in 1940) that originally had a fancy domed curvex plastic crystal on it, but when he had it restored and gave it to me when I was in college, the restorer put a simpler sapphire glass crystal on it (which frankly doesn't look as nice although it doesn't scratch).

His GP looks similar to this (this is being sold on Ebay) although my Dad's is gold with a silvered face, the sub-seconds dial is a little different, and the original crystal stood up higher from the watch face (so it actually kind of magnified the dial):

That's a beautiful watch.

 

In addition to what others have said, changing to a sapphire crystal would be a very expensive proposition and it's not the type of thing that you can easily find a local shop to do. One of the crown jewels of my watchbox is an IWC Jubilee Portuguese. Although it was made in 1993, it has a plexiglass crystal as it would have in the 1940s. Any scratches buff out easily with mild abrasives like silver or brass polishing wool or Cape Cod cloths.

 

I thought that synthetic sapphire crystals became available for watches in the 1970s, and that everything before then was either plastic or tempered glass.

 

As a follow up that bugged me though; as they definitely had glass then, and I have had several glass watches that have never shattered, merely scratched;
so why didn't they put glass on these older watches? Unless, for whatever reason in chemistry or processing, the glass was more brittle then than they have managed to make it now.

I understood that sapphire was around <$40? But that might have been for a flat crystal.



-Bernard

 

I don't know why watchmakers used clear plastic on dress watches.

It made sense for sport watches. You're more likely to scratch the crystal on your watch when you're driving a race car or playing cricket. It was critical for diving watches--the tempered glass couldn't withstand the pressure, but the clear plastic can. A glass watch crystal thick enough to hold up to pressure at 1000 feet would have to be absurdly thick.