Originally Posted by
exeo
..... I noticed there is a nut at the base of my presta valves, what are they for and are they suppose to be in a certain position?
The nut at the base of the valve is simply a convenience, and isn't critical. It helps by keeping the valve from getting pushed into the rim when you're pressing on the pump head. Some people use them, some don't.
I don't use them on my tubular tires for safety reasons, but I use them on my 1.9" tires on the commuter because otherwise I could push the valve all the way in when pressing on my pump's head.