T-Mar

That doesn't make any sense unless it has been shimmed. 25.6mm would down in the hi-tensile steel range and is typical of entry models. This bicycle has Campagnolo dropouts with an integral derailleur which you would not see on an entry level/hi-tensile steel frame.

3alarmer 

.
...the one I have (very similar in age, same model/color) takes a 27.0 :

marius.suiram 

The seat post is between 25.8... 26.0
It is weird, because you can see the frame is all chrome, where the paint is scratched.
I knew from this forum that the 531 Reynolds has a 27mm seat post, but somewhere, I don't remember somebody said that some frames take 26.6 or something like that

CO_Hoya 

From Sheldon's database:
Straight-gauge 531 tubes can have posts at 26.4 or so (e.g. Raleigh Super Course).
Butted 531 metric tubes can have posts at 26.4 or so (e.g. Peugeot PX-10).

Mid 70's Motobecane Grand Touring models (Vitus 172?) took a seat post at 26.0 mm

marius.suiram 

Raleigh Super Course, Reynolds 531 - 26.4
Mondia, Reynolds 531 - 26.6 (bad position to measure it)
Peugeot, Vitus 172 - 26.0
Peugeot, Vitus 980 - 26.4
Falcon - 25.85

T-Mar

Well, 25.8-26.0 mm is a little better but still severely under size for what would be normal on this frame. How did you measure the post? Did you actually remove the post to see if it was shimmed? To accommodate a post that small, without a shim, the cinch slot at the back of the seat tube would have to be pinched closed at the top.

marius.suiram 

I used a digital caliper and measure the seat post which was on the bike.
I try to replace it with the one I have on the Raleigh Super Course, but I didn't want to force it.
I measured the interior of the seat tubing. It is between 25.95 and 26.2mm

big chainring 

Looks like a steel seatpost with backwards clamp, and bars approaching the DUI standard. Original Campy post was replaced at sometime with whatever was available. Probably needs a little reshaping of the seat lug to get something "normal" in there.

T-Mar

Well, the cinch slot in the back of the seat tube must be pinched shut at the top, or very close to it. If it isn't, then somebody brazed the frame using the wrong seat tube or installed the seat tube upside down, both of which significantly lower the value.

3alarmer 

...looking at that horrific modification someone did to the seat post clamp, I, too, suspect that maybe you are not getting a correct measurement on the inside diameter of the seat tube.

To do this correctly, that poor abused seat post clamp lug needs to be carefully pried all the way open, and you need to reach well down past it into the tube to get an accurate measurement. I get the impression that it might be a benefit for you to find someone who has a little more experience in working on older bikes, and pay for their expertise in getting a reliable measurement of what you really need for a post. While it is possible that a bike of this quality level was manufactured incorrectly, I think it much more likely that @big chainring is more or less on the money here.

To my knowledge, these were sold as complete bikes, not frame sets, so any manufacturing error would have been weeded out during the assembly process. AS stated, mine (very similar in age and model) is reamed for 27.0.

marius.suiram 

thank you guys, I am sure the normal seat post was a 27mm.
I will try to open that lug careful.

eschlwc

my '73 falcon sr has a 27.2 seatpost.

my '76 grand record had one when i got it, its clamp making a permanent imprint on the frame. they're in such poor taste.

Markedgr

Whilst 26 mm is clearly not 'right' for this bike, 531 still came in various 'grades' well into the 60's and many larger sized bikes right through to the early 70's have 'thicker' grade 531 fitted to the seat tube in order to give a little extra 'stiffness'. My Carlton Professional(67/68), Mercian Professional (1969) and falcon 76 (1971) all take a 26.8mm seat post and all have seat tubes/lugs in perfect condition..... All these bikes are 24-25ins frames. Smaller sized frames of these models all seem to take the usual 27.2mm seat post....