Gene from web site provided this great info on figuring out your old trusty Schwinn; Schwinn Bike Dating After 1965, Schwinn serial number sequences, while appearing to beconfusing on first sight, follow several general rules.If thefirst letter is a "A" through "M" it refers to production month("A" = January, "B" = February, etc.).Many older bikes are still quite useable and still satisfy the casual recreational user.
(For additional strength Schwinn also brazed steel sleeves into the interior of its frame tubes at the joints.) Fillet-brazed bicycle frames are strong and have a neat and clean appearance, but they are uncommon because of the additional craftsmanship required.
Lugged bicycle frames, for example, are now manufactured by automated machines.
The rest have been acquired used, primarily at garage sales.
Bike technology, of course, changes, but nowhere near as fast as computer technology.
Above are links categorized by decade which document visually the images of Schwinn bicycles as far back as 1895.
We use vintage catalogs, books, and any other publications we can find to document Schwinn’s history.
The letter "G" followed by a two-digit month code (01 = January etc.) and two more digits for the production year (81 = 1981 etc.).
An example would be 'G0181' and equates to "Giant, January, 1981".
(See illustration below) In this method of bicycle frame construction, "mitering" or cutting the tube ends so that they fit together precisely is critical so that capillary action will draw the molten filler into gaps for a strong joint.