Thanks to everyone who responded.
All the posts have been remarkably helpful!
I am homing in on these two posts because they have made me wonder whether my impressions have been accurate.
Originally Posted by
TimothyH
Overtightening the jam nut can pull the tube up through the valve hole causing it to fail. Many just throw the nut away. It isn't really needed. -Tim
Yes. Good point, Tim. I realized a few (several?) years ago that tightening the lock nut might be making trouble. So, I started just loosely tightening it. But you may have a good point. The failures that I mentioned may indeed include a time when I was over-tightening it. And, yeah, I noticed that Michelin tubes have smooth valve barrels--no lock nut at all. So, I may just start leaving the lock nut totally loose. (The lock nut is good to have at hand, though, only when I need to press hard against the valve, on the road, when I have to use a CO2 cartridge to fill the tire.)
Originally Posted by
Andrew R Stewart
[...] After you fix a few thousand flats you begin to see that just about every rim has a sharp valve hole edge yet the number of flats due to it are quite rare. I've deburred many rim valve holes and seen flats close to the valve, on the tube, no longer being able to be claimed from a sharp hole edge. [...] I should add that if one is wagging the valve back and forth (as in when using a frame pump) or are tugging on the valve (as in when removing a stuck air chuck) the tube can tear at its base. But again the rubber is being stressed in a way it isn't designed to be.
Andy, yes, that's the problem I'm trying to address. I have a Blackburn floor pump. The chuck is pretty hard to remove, requiring a fair amount of force. I fear that it might be placing excessive stress on the tube, and this is happening 4 or 5 times a week. On the other hand, you say you have rarely encountered a problem at the valve base. Have you really fixed "a few thousand flats"? Are you a professional mechanic?
--David