Feel free to quote small parts of the information on this page, but copyright law requires be listed as the source of the information.In this website, we credit the people involved in the understanding of these codes. (Naturally, for information provided by others, such as for the Campagnolo and Williams codes, the original source should be cited.)Ben writes: "Brakes and brake levers often have date codes on them.For Treks, the SR date markings are especially important.
Swaps also can be made as the bike falls out of favor, or is being sold, where the higher quality components are traded for lower quality ones that the owner had onhand.
(Don't all cyclists have boxes and boxes of old components in their garage?
Even those equipped with Suntour Superbe components usually had SR seatposts.
Many components are marked with size descriptors in addition to component manufacturer's date codes.
)Most early Treks (1976 through about 1980) were sold as framesets.
The components were added by the local bike shop or by the buyer. Components also could be swapped from an existing ride to the new frameset.
This clearly is when the component was made and not when the bike was made, but unless the component or bike manufacturer had lots of stock lying around in inventory, the date should be a fairly good indication of the year of the bike.
At least it would be the earliest date that the bike could have been made.
For old Treks, 27.2 is the most common, but for other bikes the diameters can range from 25mm to 33mm.
The rear derailleur can often be dated to a year or two by referring to the book "The Dancing Chain - History and Development of the Derailleur Bicycle", by Frank Berto, 3rd edition 2009.