Seat post sizing problem

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Seat post sizing problem

Postby pherron » 18 May 2011, 7:36pm

I have bought a new seatpost for my bike and am having problems getting it to fit. I have a 531 frame tourer which has had a cheap alloy post in it for many years. The post is stamped 26.8. I purchased a beautiful new seatpost, also of 26.8mm, and find that with the binder bolt fully tightened it isn't quite tight enough. If I shove hard on the nose of the saddle it moves. I measured both posts and find that while the new quality post is 26.80 on my digital vernier gauge, the old post is 26.91. So it was inaccurate to 1/10th mm. So what do I do? I don't think a 27mm will fit (I haven't actually tried) and I have a very expensive seatpost that I'm not confident to ride.

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Re: Seat post sizing problem

Postby meic » 18 May 2011, 8:02pm

If you can push a 26.8mm down after tightening by hand and had a 26.91mm stem in, I would expect that you could get a 27mm in.

I imagine that it would be too difficult to shim for just .2mm.
Which means buying a new seatpost or taking a file to the slit in the lug and widening it out so that you can deform the tube into a tighter oval shape.

I would check that somebody hasnt already done this to your lug.
Take measurements of the tube internals at different places. Somebody could have already squeezed it together to take a smaller post.

This post is just my ideas, I wouldnt act on it without something else to support it.
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Re: Seat post sizing problem

Postby Mick F » 18 May 2011, 8:19pm

Generally, but not always, 531 is 27.2mm.

If you assume that 27.2mm is correct, you won't be wrong.
Just sometimes you may be. :oops:
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Re: Seat post sizing problem

Postby fivebikes » 18 May 2011, 8:50pm

Depends on the 531 tubing. I have a Claud Butler made of plain gauge 531. The seat post size is smaller than 27.2 but my memory lets me down as to actual size. I vaguely remember 26.8. I am sure someone will correct if wrong!

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Re: Seat post sizing problem

Postby 531colin » 18 May 2011, 8:52pm

meic wrote:I imagine that it would be too difficult to shim for just .2mm.

I have Engineers steel shim marked 1 1/2, 2 , 3 , 4, 5 & 10.
That's not millimetres, so its thousandths of an inch?
0.2mm divide by 25.4 = 0.007874 inch, that's seven or eight thou' ?
So try 3 or 4 thou' shim?

or even a bit of Coke can?

As already said, 27.2 is the "usual" 531 size, 27.0 is not unheard of. Seem likely the frame has been distorted by somebody using an undersized post?

EDIT....can you return the new 26.8 post and get a 27.0, or is the post marked and therefore can't be sold as new?
Last edited by 531colin on 18 May 2011, 9:24pm, edited 1 time in total.

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Re: Seat post sizing problem

Postby Mike Sales » 18 May 2011, 9:13pm

As others have written, 531 DB usually takes 27.2, though some builders did not ream it out to this size. One I know, if I remember correctly, would fit 27.0 but this would be unusual. Look at the slot. The sides should be parallel.If the tube has been pinched the sides will show it. Some bike shops will have a couple of stepped guages which will measure the tube internal diam. best done after making sure the slot sides are in fact parallel. I would expect your new post to be accurate as sold, and guess your measurement, digital though it is, to be out. It is plainly undersized, but don't commit to a 27 mm. without checking. The chances are you need a 27.2.

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Re: Seat post sizing problem

Postby pherron » 18 May 2011, 9:49pm

Thanks for he replies.
I bought the frame from new. It is 531 ST. The shop that supplied it also supplied the seat post which I fitted without any concern at the time. That was back in 1989! Apart from occasionally re-greasing it's been in place ever since. I can't see any obvious distortion to the frame. The seatpost is a cheap Triplex brand with some nasty moulding seems. I always meant to replace it sooner.

The new post does measure accurately at 26.80mm so I don't think my gauge is out. I can't push the new post down easily but I can push the saddle round side to side without too much force. I am starting to think that a 27mm post would be correct because the 26.9mm one has fitted quite well with no obvious damage to the frame. It sounds good to get it measured, but I don't know if my LBS have the correct tool. I'll have to pop in and ask them.

Unfortunately the seatpost is marked from inserting it and riding a couple of miles, so could only be sold second hand. An expensive mistake to believe what it said on the old post then! I guess I'll put the old post back in for riding for now and try and get the frame measured, buy an other expensive seat post and try and cut my losses selling the old slightly scored one.

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Re: Seat post sizing problem

Postby Malaconotus » 18 May 2011, 10:09pm

I had the same problem with a 27.0 post. New one is slimmer than the old. I used a beer can. (A coke can was too short) No problems now but the beer can has slipped down with adjustments of saddle height and now none is showing above the seat clamp. Not sure how easy it will be to get out if I ever have to.

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Re: Seat post sizing problem

Postby emleyman » 18 May 2011, 11:00pm

I've got a 531 framed Falcon that has a 27.0mm seatpost. I've also refurbed a 531 framed Raleigh Touriste that my sister is riding, and that is also 27.0mm. I'm not saying MickF and 531Colin are wrong as they are much more expert than me, but you certainly do get a few exceptions to their rules.

If you have a set of calipers, can't you measure the inside diameter of the seat tube?

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Re: Seat post sizing problem

Postby Mick F » 19 May 2011, 7:21am

I'm no expert, just speaking from experience on here, and remembering the many many threads about seat post diameters.

More often than not, 531 is 27.2mm.

There are other sizes, but 27.2mm is by far and away the most common size.
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Re: Seat post sizing problem

Postby The Mechanic » 19 May 2011, 10:06am

I have had a similar problem but the other way round. I bought a second hand 531 db frame that had a 27.2 seat post in it. It was not seized but was very tight to remove. I have since tried to put other, newer 27.2 seat posts in but they were so tight I stopped for fear of damaginf something. The project has been on hold for a few months due to a Masters dissertaion so not resolved yet but will get back on track in the next couple of weeks. I think it may be just a question of a tight fit becasue I wouldn't envisage a 27.2 post going in a 27 tube at all.
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Re: Seat post sizing problem

Postby rjb » 19 May 2011, 2:15pm

Both my dawes galaxy 531st and super galaxy tandem 531 tandem tubing came with 27.0mm seat pins. It looks like 531st is 27.0mm as opposed to 531 which is generally 27.2mm (internal diameter). You can try cleaning up the internals of your tube using emery cloth. I did this on one frame which appeared to be a bit tight by wrapping emery cloth around a short section of broom handle drilled through to accept a long bolt and nut. Mount this in the chuck of an electric drill. Cut a small slot to start the emery cloth and then wrap it around clockwise until it is roughly the diameter of the seat tube internal. Then use it to clean the tube ( you can stop the dust and debris going down the tube by pushing a rag down first with a corner tied to a nut by a length of string - turning the bike uppside down the nut drops out enabling you to pull the rag out with all the dust etc.)
At the last count:- Focus Variado, Peugeot 531 pro, Dawes Discovery Tandem, Dawes Kingpin, Raleigh 20, Falcon K2 MTB dropped bar tourer, Longstaff trike conversion on a Falcon corsa. :D

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Re: Seat post sizing problem

Postby snibgo » 19 May 2011, 2:48pm

rjb wrote:It looks like 531st is 27.0mm as opposed to 531 which is generally 27.2mm (internal diameter).

Sorry to blow a great theory, but my 1987 Raleigh 531st tourer is 27.2mm.

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Re: Seat post sizing problem

Postby pherron » 19 May 2011, 7:52pm

After a thorough inspection tonight I am now 95% certain I have 27.0mm seat tube. I can actually see some slight distortion in the frame slit but not much. After 22 years I find out I was sold the wrong seat post! I think it hasn't caused huge damage due to the cheapness of the old post. Carefully measuring it I found that it varies in diameter from 28.84 to 28.96. Some of it is so close to 27 it hasn't caused much of a problem. It certainly fits much better than the new post which is consistently 28.80 all round. I'll order another and put the scored 28.8mm post on ebay. Hopefully I might recover some of my loss.

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Re: Seat post sizing problem

Postby GregLR » 19 May 2011, 10:48pm

Looks like the answer has been found, but it's interesting to note the information and comments on this issue in the book by former framebuilder Tony Oliver, "Touring Bikes - a Practical Guide" (1990), that includes a table detailing the internal diameter and seat pillar sizes for a range of Columbus, Reynolds, Tange and Vitus seat tubes.

For 531C tubing the table says that the internal diameter is 27.5mm and takes a 27.2mm seat pillar for a "brazed frame" (ie, brass brazed, with a little distortion) and 27.4mm for a frame "silver brazed to the highest standards" (ie, at lower temperature, and so minimal distortion). For 531ST the table says that the internal diameter is 27.2mm and takes a 27.0mm seat pillar for brass brazed and 27.2mm for a frame silver brazed.

The wall thickness is shown as 0.55mm for 531C seat tube; for 531ST it is 0.70mm (though in the text he says that most 531ST frames have a wall thickness of 0.55mm !).

In the text Oliver comments as follows:

"I disagree with many on the subject of seat pillars. For years 27.2mm seat pillars have been put in most quality frames without problems. I believe 27.4mm pillars should be used in most 531 frames, although not all....taking away two wall thicknesses [of 0.55mm from 28.6mm] gives an internal diameter of 27.5mm. A 0.1mm clearance is ideal and 27.4mm seat pillars fit beautifully in the above frames [ie, 531C, most 531STs and the old 531db] when the seat cluster is silver brazed, ie, not distorted.

Most 531 frames are brass brazed, a process at a higher temperature where the thin seat tube distorts just a little. This does not hurt its strength but effectively makes the internal hole size smaller. The tube is then reamed with a 27.2mm reamer, hence the normally quoted size is used. This is fine but the fit is only second rate and inevitably water creeps into the gap. Alloy seat pillars have a nasty habit of corroding into seat tubes...

...I have found examples of tube with slightly thicker walls than the quoted value. They still lie within the tolerance range of the tube spec but they require a size smaller seat pillar...

...Another confusing factor is that seat pillar diameters vary considerably. A pillar may be marked 27.2mm but it could be as much as 0.2mm out, either larger or smaller. For this reason I like to supply a seat pillar with all my new frame sets so I can select the best one to fit in the seat tube."

BTW, I found it difficult last year to find silver 27.0mm seatpost to fit my old 20 yo touring frame with Columbus SL tubing. Eventually I bought a very nice polished silver Nitto Crystal Fellow S-65 - see what Peter White has to say on this page I've since found it's also available from Alex's Cycles in Japan ( ) where nice matching polished silver Nitto handlebars and stems can also be found.

Back in 1997 I fitted a new 27.0mm Ultegra seat post to this touring frame but found I couldn't tighten the allen key bolt enough to stop it from slipping down. I had to give up and used a fluted Campag Chorus 27.0mm post instead. I had to replace that last year because it began tilting up under my weight.

Apologies for another long post !