Gene from genesbmx.com 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 be
confusing on first sight, follow several general rules. If the
first letter is a 'A' through 'M' it refers to production month
('A' = January, 'B' = February, etc.).
The letters 'I' and 'O' are not used, to avoid mistaking
them for numbers. If there is only one letter in the first two
positions, the first number or two numbers refer to production
year. Date Codes occupy the first two, three or four positions
in a serial number.
Mass produced Schwinn bikes have been followed
by a production month letter and several numbers, the
first two signifying year of production (81 = 1981, etc.).
Example: D72130 = April 1972. LeTour (and Super LeTour)
numbers (1978-80 and 1982 on) include the letter 'S'
with year of production (8 = 1978, 0 = 1980). Example:
SC94653 = March, 1979. Sting 20' racing bikes have a
production month letter 'T' signifying that it is a tri-oval
frame (with three frame tube ends ovalized). Example:
F1T20404 = June, 1981.
Imported Schwinn serial sequences depend upon country
of origin. Bicycles from Japan have a production month
letter in either the first or second position and a production
year number in the other (first or second) position. Examples:
J677123 = September, 1976. Taiwanese imports have a
separate date code number on the right rear fork end that
includes the letter 'G' followed by a two-digit month code
(01 = January etc.) and two more digits for the production
year (80 = 1980 etc.). Example: G04813 = April 1981.
Also Note that if your bike is after year 2001 the
Schwinn/Questor bankruptcy, It has been found
that ( Pacifc Cycle ) bikes do not use this dating.
Also See: The Schwinn BMX Catalogs Time Line Web Page:
( http://genesbmx.bmxair.com/schwinn/catalogs/bmx.html )
CC - Gene`s BMX Schwinn Page
http://www.genesbmx.com/SchwinnPage.html
Frequently Asked Questions About Schwinn BMX Bicycles.
02 - What Year Is My Schwinn BMX Bike?

1980s Schwinn Serial Numbers

The Schwinn serial numbers that date the bike to the fall of 1954 or 1956, but I believe the Serial numbers for 54 and the 56 were located on the Left Rear Dropout - not located on the bottom bracket of the pedal crank housing. Register your Schwinn bicycle with us for up to date information on your model and, if your bike is ever lost or stolen, we can provide you with key information that can help you recover your bicycle quicker and easier. Product registration is quick and easy. Please fill out the form here.Register Product. Schwinn bicycle serial numbers are located on the underside of the bike, on the opposite side of the location of the service sticker. This is the metal space that connects the pedals. Schwinn serial numbers begin with 'IBD' or 'SN,' followed by 11 alphanumeric characters.