During that era Reynolds offered their 531 seat tubes with the following wall thickness dimensions:
BSW Gauge (the British Standard for tubing also SWG Standard Wire Gage)
22/24, 21/24, 20/23 and 19/22
the rounded off metric equivalents were:
5/7, 5/8, 6/9 and 7/10 meaning 0.5mm wall thickness in the thin section at the top of the tube and 0.7mm at the bottom and so on.
These were also listed as the actual "nominal" dimensions, .7/1.0mm and so on.
These tubes were made in both metric 28mm and Imperial (inch) 28.6mm outside diameters.
The thinner 5/7 size tubes came in 531SL sets (and 753) Reynolds 531P sets and others used the 5/8 tubes.
Reynolds would supply any combination of tubes as long as you placed an order for at least 100 sets so you will find lots of variations.
7/10 and 6/9 were the most commonly used wall thicknesses found in production bikes.
Theoretically a 7/10 wall thickness inch size seat tube would use a 27mm seat post and a 6/9mm tube, a 27.2mm post.
That's assuming that the seat tube is round to the proper size to begin with.
The high heat involved in brazing on the seat stays and creating a fillet where needed frequently warps the seat lug and can cause the tube to bulge inside the lug. Also not all seat lugs were round to begin with!
I had a set of homemade steel mandrels that I used to use to round out seat tube IDs. By gently hammering in the smallest size that almost fit the ID and then working up from there until the seat tube was close to round.
Once near round the seat tube could be reamed to the closest standard seatpost size.
These are bikes not Swiss watch movements. It's very difficult to economically produce accurately dimensioned tubing. Most "precision" tubes are resized after the drawing process by running them through angled rollers or centerless grinding the tubes to the specified OD.
If you take side to side and front to back or top and bottom measurements of bike tubes about 4"-5" (100mm - 125mm) from the lugs you will frequently find that the tubes are not round!
BTW, add 0.1 to 0.2 to the diameters for paint thickness.
Another thing, seatposts are not always round or true to the marked size. I have a NIB 26.6 Campagnolo NR seat posts that measures 26.5mm to 26.7mm depending on the location on the post. Also most used seatposts have been sanded and or polished at some time in their lives. This leaves them slightly undersize.
2nd BTW, Columbus SL had 6/9 wall thickness and SP was 7/10.
The Prugnat or Bocama lugs with the factory stamped cutouts didn't appear until the mid 70s or later. This would place the frame at 1975 or 76 at the earliest. These lugs were more common in the late 70s and 1980s.
The Cro-moly sticker looks similar to ones used on some Japanese bikes in the 1970s.
Chas. Colerich Oakland, CA USA
Giles O'Bryen wrote:
> Marten, do I take from your reply that Reynolds did make a 27.0 seat tube
> that was slightly thicker than the 27.2?
>
> Barrie, I am sure the frame was not built for a professional rider -- it has
> a decal saying Professional across the top tube, that's all!
>
> Forgive my ignorance, but what is an FKW?
>
> Giles O'Bryen
> London UK