Seatpost diameter for Holdsworth.

[meic]

I have an unidentified Holdsworth and the seat post in it is a 26.4 mm.
I have my doubts that this should be there. The bike is probably between 1978 & 1983 and nearest match is a 531special. On the seat stay the sticker says made from 531 butted (not single or double).
I know that most of their bikes are 27.2 and I think it has been 'squeezed'.
Before I try expanding it does anyone think it was possibly made for a 26.4

[reohn2]

Meic
It could be 26.8mm 531st seat tubes are that diameter,which is a standard size.If it is 26.4mm as you say which I think is none standard a 26.8mm would need little'doctoring' with emerycloth.

[CJ]

Reynolds have supplied 531 seat tubes in many different butting configurations. Mostly, including 531ST for much of its history, they were single butted (which means thickened at one end only, the bottom end in this case) 0.81/0.55 wall thickness and always 28.6mm outside diameter, befitting a 27.2mm seatpost. (Brazing into the seat lug etc. normally causes some distortion, requiring about 0.2mm clearance for a smooth fit, but sometimes a 27.4mm seatpost fits better.)

Reynolds also supplied double-butted 531 seat-tubes, for example in the briefly marketed Club Sport tube-set, that was 0.81/0.55/0.81, in which you'd expect a 26.8mm seatpost to fit. But builders have always been free to select alternative tubes in response to the particular needs of their clients. Strong pedalling may cause premature fatigue cracks in the lower section of a standard seat-tube, so if this frame was built for a particularly heavy and powerful rider, the builder might have used a 1.02/0.71/1.02 thickness tube normally intended as down-tube. In that case 26.4 would be the required seatpost diameter.

The top of the seat-tube however, does not really need to be that thick, and even in the case of a strong rider it would be quite okay to ream it out to take the more common 26.8mm size.

[meic]

Thanks very much for the information. One thing I dont understand is the effect of the lugs. Do the tubes actually go the whole way into the lugs acting as a liner making the lugs irrelevant to the inside diameter of the tube? CJ's answer almost says that but I just want to be fully sure.

[CJ]

Do the tubes actually go the whole way into the lugs acting as a liner making the lugs irrelevant to the inside diameter of the tube?

Yes.

Although some manufacturers (e.g. Peugeot I believe) have occasionally used internal lugs, this is extremely rare.

Quite a few modern aluminium or titanium frames however, have a liner welded into the top of an "oversize" seat tube in order to fit a "standard" 27.2mm seatpost.

Going even further off topic: I've recently had a call from a chap who had such a frame custom-built (in China) where the liner wasn't long enough - given the extension of the seat-tube above the top-tube - so it's lower end coincided internally with the underside of the external welded joint. The resulting double stress concentration exceeded even the wonderful fatigue resistance of titanium, so the seat tube had cracked through after only a short period of use.

[Jezrant]

I had a 27.2mm Campag seatpost in an old Bob Jackson 531ST frame for years until one day I tried fitting the same seatpost in two other frames (531C and Columbus SLX) and discovered much to my surprise that 531ST is slightly narrower. As Bob Jackson are still making frames, I asked them what the inner diameter of the seat tube on 531ST is. Their reply: 27.0mm.

[PW]

There seems to be a difference in the final reaming of the tube which varies with the builder. All my current frames are 27.2mm. A recently deceased 531C from a different builder was 26.8.