Subject: 8b.30   Valve stem separation flats
From:  Jobst Brandt <jobst.brandt@stanfordalumni.org>
Date: Fri, 13 Feb 2004 12:07:59 -0800

A flat caused by valve stem separation, a manufacturing flaw, is less
dangerous because it usually becomes apparent during inflation.  If it
occurs while riding, it causes a slow leak as the vulcanized brass
stem gradually separates from the tube.  When this occurs, the stem
can be pulled out of the tube entirely to leave a small hole into
which a valve stem from a latex tube of a tubular tire will fit.
Stems from tubulars have a mushroom end, a clamp washer, and a
locknut, that fit ideally into the hole left by stem separation.  Such
a used stem should be part of a tire patch kit.  Any good bicycle shop
that handles tubular tires or latex tubes should have used ones if
they weren't thrown away.

In a self accusative manner, riders often place blame for this failure
on errant inflation, the use of the anchor nut on the stem, or some
other feature of the rim that they failed to ameliorate.  On close
inspection, separated stems show that the rubber peeled away leaving
only a slight black trace on the stem where the leak began.  This
isn't caused by any of the usually believed mechanisms.  It is a
manufacturing flaw.