Im really tired dating

Some later rims, like late seventies/early eighties weren't stamped. I was going by the front fork being drilled but I just recalled both models had this.

Also, S-7 rims came in several diameters, including 26", 24", 20", and 16". It is also missing the braze-ons for the cable routings that a Deluxe would have. I have had several bikes with numbers stamped into them that were not serial numbers.

The S type Schwinn rims were also called "tubular rims", not because they were made for sew-up tubular tires (they weren't), but because they had a tubular cross section, and were therefore much stronger than simple stamped rims. According to some serial number information I have. In two cases the were a previous owners Social Security number.

Sometimes things don't sink in with us that are new to bicycles.It if was a 65 Ford Mustang I would not have these problems.I just don't like to do things twice and am a purest when it comes to the Mustangs. I really am trying to determine if the rims on the bike are correct. I don't know a S7 rim for a Z7 can you suggest some books for a just beginning senior citzen.The Schwinn Twinn I purchased has 26"x2.125" Tires on it. It says under the picture to specify if for 26"x11/2" or 26"x2.125" Tires. I have a Parts and Accessories catalog that has a 1987 tandem Frame listed and pictured.

How can I tell the difference between the two frames.

Dale , these two tires will not be interchangeable.

If it has 26x2.125 tires on the bike then the rims and probably wheels are not correct for the bike.

As far as riding ability it will make no difference.

The original tires would have been 26 x 1 3/4 for a single speed bike and 26 x 1 3/8 for a 5 speed bike. For an explanation of S2, S4, S5, S6 and S7 got to the Schwinn forum (I know you are a member) and using the search feature try looking for "tire sizes" or "rim sizes". For what its worth the S numbers mean the shape of the rim in cross section and the number size means the approximate diameter of the tire wheel combination.

Roger Schwinn S-7 26" x 1-3/4" rims should be stamped "SCHWINN TUBULAR S-7" near the knurling down the middle of the rim. Cuda you could be correct and its just a standard Twinn.