Here is a Seamaster screw down crown - brand new right out the package. I've removed the O-ring to get a better view, deeper inside the crown.
The red arrow points to the first thread, and you can see maybe one more below that - the angle makes it difficult to see much more that that.
Case tube wear - while tubes do wear, the ones I replace most often are from vintage watches where the tube was originally made of brass. These wear over time pretty regularly. The new tubes are made of stainless steel, and last much longer obviously.
For screw down case tubes, the only reason I replace those is from damage to the threads or the tube say from an impact- not wear. While I'm sure given a long enough time, with a dirty and dried up seal, it could wear the case tube, that will take a very long time indeed.
