Join Date: Apr 2012
Location: Denver, Colorado USA
Bikes: '86 Moots Mountaineer, '94 Salsa Ala Carte, '94 S-Works FSR, 1983 Trek 600 & 620
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The clamp diameter should be stamped on the inside of the derailleur clamp, and is determined by the diameter of the Seat Tube, not the seat post.
There are three sizes of clamps; 28.6, 31.8, and 34.9 mm. Shims to make the larger clamp sizes work on smaller seat tubes are available, but only
to a degree.
Find out the clamp size, and someone will advise you forthwith.