I'm about to install a bolt-on valve stem on my rear wheel. How tight do these have to be, when do I know when it is tight enough?
This sounds good. I already have the tire mounted and the wheel installed and there is a good chance I tightened it a tad too much but I'll keep an eye on the cracking you referred to. I'll install the next one a little different. Thanks for the advice.Rule of thumb: when compressing a rubber seal like the one under a valve stem, compress the seal until it's sides bulge to a gental curvature. If it is curvature begins to bulge up past the flatness of the surface compressing it (usually a washer) that is way to tight. Usually, a seal like that is way over tightend. As it ages, that will so with lots of cracking around the edges.
Here is another way to do it. Think of it like this. When airing the valve stem, it should never be able to rock allowing anything under the seal. Tighten the nut to that point only ... most likly a slight movement will still be there and that is normal.
Here is another way to look at it. If you over tighten it to the point of a double nut not needed, then you don't need to double nut it. So don't tighten it that tight. The double nut is there for a reason.
Also, just like all chemical, never use them unless they are called for. Just because you think they are needed is usually not a good reason. So unless lock tight is required by the manufacture, don't use it. If installed correctly, none is needed. Just make sure the lock nut is tight.
I don't think I tightened the nut to much to cause the washer to bent from hitting the wheel but this is what i'll look for when I do the front wheel.Like Greg says, be careful not to over-tighten the nuts, the metal cup on the outside should not touch the rim. Too much torque and the outer washer will hit the rim and bend the stem. I use a torque screwdriver and apply 20 in/lbs. I also use a drop of red loctight on the second nut because it is tough to get a wrench to hold the first nut while tightening the pinch nut so I add a drop of red threadloc to be sure.
I've installed a few hundred of them over the last few years and not one complaint or leak.