Presta Valves – About that knurled threaded thingy
If you ride on tubes with presta valves like I do, you may have wondered about that knurled threaded thingy on the valve stem. Do you keep it on your valve stem? Tighten it down?
I ride on Schwalbe Marathon Plus tires, but recently had a flat. Pulling of the tire and dunking the tube revealed no penetration. No puncture. But flat. The problem was the seal around the valve stem.
The tube must be free to move independently of rim and tire. If it can’t, you will stress the joint between the valve stem and the tube proper. Having that nut tightened down will cause the valve stem to move in such a way that it will stress this joint. Sure enough, that was the cause of my flat. The purpose of the knurled nut is to hold the valve stem in place when the tube is completely empty, such as inflating it for the first time. It prevents the valve stem from recessing back into the tire when you put the pump head on.
My trick is to use the nut and the valve cap as a lock facility: I keep the nut on the valve stem up near the cap, and tighten the two together so that they stay in place. That way, should I ever have to re-inflate the tube, I have the nut available to lock down the tube.