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Subject: Valve Stem Separation Flats
From: Jobst Brandt
Date: June 16, 2003
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.
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Last Updated: by John Allen