Seat post Diamensions for Reynolds tubing ?

Ian Raleigh

rBotM Winner
Can anyone explain why there are so many internal dimensions for Reynolds tubing please ?

I've seen 531c/p frames with 26.8mm - 27.00mm - 27.2mm posts and 753 with 27.00mm & 27.2mm.

Why is this ? Production batch/year/early/late/availability ?

Whats yer' thoughts on this ??


Retro Guru
It's nuts alright! But... isn't it the case that butted 531 is 'mostly' 27.0 and double butted is 'mostly' 27.2? I know there's no hard and fast law, hence the frustration!

On the same subject, can anyone explain why seat post manufacturers aren't obliged to label the widths of their product? :evil:


Retro Wizard
Partly due to metric and imperial measurement the tickness of each tube guage for each tube the fact that larger frames had thicker tubes to prevent the frame twisting.

Lots of combinations there :)



Senior Retro Guru
But as I understand it, the 531, 501, 753 etc originally related to the grade of steel, not the size. It just happens to be the size it is because that is what the builder intended. Later Reynolds numbers don't follow that rule though, because some of them specify different tube grades front and back. One of them has super stiff tubes for the front triangle but the rear stays are more springy 531 to take the harshness out the ride.


Senior Retro Guru

Going back to my time, (50s /60s) the seat tube had an outside diameter of 1.125 inches. Normal butted seat tube was single butted, 20 x 23 guage.
top tube 21 /24G down tube 20 / 23G doble butted. Variations were applied according to specification.
This makes the seat tube inside diameter 27.2mm.
Plain guage tubing was normally 20g.



Retro Guru
Don't rely on this being right but I think I've read that the heat involved means distortion so all seat tubes get reamed to a standard size after assembly. The thin walls of the lightest tubes means tiny steps. My SBDU 753 frame takes the virtually unheard of, but otherwise common, 27.2 post size so nearly anything is possible.

Edited after the following post (sorry) to say that my 753 frame is 1977 so it's 27.2 seatpost size is unusual but not unique.

0.3mm wall thickness says it all: ... ochure.jpg


Retro Wizard
Here's a snippet from Mike Mullett, Raleigh Ilkeston.......

1) 753 is Metric-diameter tubing (28.0 mm O.D. for the seat and down tubes, 26.0 mm O.D. for the top tube), and was introduced in about 1975. Seat posts for 753 frames are either 27.0 (forsmall frames) or 26.8 mm (for large frames).

2) 753R and 753T are Imperial (English/Inch) diameter tubing (28.6 mm O.D. for the seat and down tubes, 25.4 mm O.D. for the top tube), and began production in 1983. Seat posts for 753R and 753T are either 27.2 or 27.4 mm.

Of particular interest is the thinner Imperial tubing which takes the 27.4mm seat pin (pillar). Not a lot of people are aware of this. All these tube sets have a part number starting 80*, but Reynolds seem to have lost these specs.


Senior Retro Guru

Both of my 1990 Reynolds 653 Peugeot frames take a 26.8mm seat post. Kind of ties in with the 753 sizes in the previous post other than they're 54cm frames so not large.