Originally Posted by surreal

The Schwinns of that era, excepting some paramounts, have an American BB hell which will take a standard-size OPC BB. However, the Schwinn OPC's were 28tpi, and took a 9-ball retainer on each side, and many folks feel that the Schwinn cups work best with the Schwinn bearings. For comparison's sake, "standard" OPCs are 24tpi and take a 10-ball retainer on each side. Short story: you can run a Schwinn spec OPC OR a standard OPC in a Schwinn frame, but you'll need to make sure that the BB itself matches the crank. The sprockets are interchangeable. (You may find some variation on the width of American BB shells, side-to-side, but mmost OPCs have enough threads to compensate for this...) 1/2" pedfals for all OPCs, btw.

Headsets: These are 1" threaded, but they are the old-school american bike size, which means anywhere from 32.5mm to 33mm cups. I have found Schwinns from the 50s to be bigger (@32.7) with more recent (1970s) Chicago Schinns to be closer to the "bmx" size of 32.5mm. All take 24tpi forks. Basically, a nice starting point for a current-production headsets would be a tioga beartrap 2, but you may have to shim it a tiny bit. The safest bet is the Wald 4080, which you can make fit, but it will take some MacGuyver moves.

Other than that, basically every Chi-town Schwinn takes a 13/16" seatpost, and the stem will be 22.2mm (7/8") on very old Schwinns, and 21.1mm on the majority of them.

hth
-Rob