One can also search for parts for sale on various forums or discussion lists (see the Buy/Sell page for a list of some of these).One also can post a wanted notice on the forum or list describing what you want.Mountain or hybrid bikes (with straight or or raised handlebars) take a 25.4mm handlebar. When replacing the headset, remember that stack height (the vertical space taken up by the headset) is important. If the new height is too tall, you won't be able to screw on the lock nut (big problem).If too short, you will need to add a spacer(s) that you can buy at your LBS (local bike shop).Chat about other than vintage lightweights is done elsewhere.The i BOB discussion list is another good source of information.
the French-made Belleri bars, used on some mid-80s models, have a clamp diameter of 26.0mm (our thanks to Richard Kaufman). Vintage Treks take threaded headsets in contrast to the more modern threadless headsets.
Exceptions are Models TX200 and TX300, which have a diameter of 26.8mm, and the Model 170 at 27.4mm.
Additionally, a 1985 Model 2000 (aluminum bonded frame) with a 27.4mm seatpost has been reported.
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Dale Brown runs a tight ship, making the C/R list wonderfully informative.
This page includes some tips concerning the maintenance, refurbishing and upgrading of vintage bikes.