This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <email@example.com>
Date: Fri, 07 Nov 1997 16:46:59 PST
Jam nuts on Presta valve stems and pumping.
1. The jam nut holds the stem when pumping so that it does not recede
into the rim when pressing the pump head against the tire. This is
especially useful when the tire is flat (after installing the
tube). It also keeps the stem from wiggling around while pumping.
Removing the nut should present no difficulty unless the threads
have been damaged or the hands are cold. The cold may present a
problem, but then just opening the valve nut on a Presta valve
under such conditions is difficult.
2. Breaking off stems with a frame pump comes from incorrect pumping.
The number of new tubes with broken stems lying along the road
proves that this occurs far too often. To avoid breaking the stem,
the pump head should be be held in the fist so that the pumping
force goes from one hand into the other, not from the pump into the
valve stem. To practice the correct action, hold the pump head in
one hand with the thumb over the outlet, and pump vigorously
letting out no air. All the force goes from one hand into the
other. This is essentially what should take place when inflating a
It does no good to "get even" with the stupid tube by discarding it
on the road for all to see. Most riders understand how to pump a
tire and see this only as evidence of incompetence rather than a
faulty tube. Besides, this ostentatious behavior constitutes
littering for which the the fine in California is $1000. Bike
shops should instruct new bike owners about the use of the frame
pump. Along with this there should be some tire patch hints like
don't try to ride a freshly patched tube, carry a spare tube and
always use the spare after patching the punctured tube. Of course
this is a whole subject in itself that is also treated in the FAQ.