the correct seat post size

General cycling advice ( NOT technical ! )
mattsccm
Posts: 2215
Joined: 28 Nov 2009, 9:44pm

Re: the correct seat post size

Postby mattsccm » 2 Jul 2011, 7:14am

Lent my copy of Tony Oliver but I believe he says that 27,2 for db tubes but they should really be 27.4 if properly reamed to the correct, non distorted by heat , size.

ConradM
Posts: 9
Joined: 20 May 2011, 5:59am

Re: the correct seat post size

Postby ConradM » 12 Sep 2011, 2:33pm

Interestingly, I have a Reynolds 501 (yes, 5 0 1) frame, when bought new in '89 from Shorters was definitely all-tubes and forks with butted frame tubes that takes a 27.2 seatpin. "Everyone" knows 501 is thicker than 531. (It was recently repainted and decals gone - but was too cheap for 531!).
I have a similar-era cheap Chesini which was sold to me as plain-gauge Columbus Zeta which takes a 26.8 (2-bolt Gran Sport - that took some finding...).
O'course, butted seat tubes are single-butted anyway so I guess there will be more variety of thickness at the BB end rather than seatpin end - where the strength will be augmented by seatpin insertion, big lug, stays etc.
The plot thickens.

9494arnold
Posts: 593
Joined: 21 Jan 2007, 3:13pm

Re: the correct seat post size

Postby 9494arnold » 14 Sep 2011, 1:13pm

Get it measured.
In past I have had :

A 531 DB Higgins Trike : 27mm
A 531 DB Peugeot : 26.8 mm (my mechanic reckoned the seat tube was upside down and apparently not uncommon on a Peugeot)

Currently have

531 ST Dawes Ladies : 27.2 mm
531 C Raleigh 27.2 mm

Paul Arnold

BigG
Posts: 976
Joined: 7 Jun 2010, 4:29pm
Location: Devon

Re: the correct seat post size

Postby BigG » 14 Sep 2011, 3:54pm

pablito wrote:I have a french built 531 frame (constuit avec Reynolds 531). The existing seat post is some sort of plastic (nylon?) and marked 26.4mm. It seems to fit well, but is too short, so I'm looking to replace it.

Measuring the seat tube with some verynear callipers, I get 26.4mm, but it is possible that the top of the tube has been slightly compressed. Can anybody tell me with confidence what post size I should get, or how I can accurately measure inside the tube? Thanks.

I have an old Holdsworth frame (model unknown) dating back to the early '70's. This is badged as 531 tubing (no mention of "butted") but has been resprayed at some time so the transfer may not be correct. It takes a 26.4 mm seat pillar. It rides well and I believe that it is made from 531 plain tubing.

The Mechanic
Posts: 1922
Joined: 23 Jul 2010, 1:38pm
Location: Scotland

Re: the correct seat post size

Postby The Mechanic » 14 Sep 2011, 4:43pm

Well anyway...... :) I have a late 70s, early 80s Revell frame that is 531 DB - original stickers intact. It had a 27.2 seat post but I had a heck of a job getting it out. There is no sign of corrosion but the seat post is heavily scored. I have a number of 27.2 seat post and none of them seem to fit. I am inclined to think this should be a 27mm post. Either that or the reaming was poor but, since it was made by Mercian (confirmed by them), I am not sure of that.
Cancer changes your outlook on life. Change yours before it changes you.

mattsccm
Posts: 2215
Joined: 28 Nov 2009, 9:44pm

Re: the correct seat post size

Postby mattsccm » 15 Sep 2011, 7:38am

Sadly my bible (Tony Oliver book) is on loan buuuut........... Deoes he suggest that 531DB is 27.2 because of manufacturing quality and it really should be 27.4. The the 0.2 is distortion. I believe he base's this on the external diameter minus the wall thickness and says that he reams seat posts out to match.
Anyway IME 27.2

David9694
Posts: 51
Joined: 10 Feb 2018, 8:42am

Re: the correct seat post size 531 variants

Postby David9694 » 10 Feb 2018, 8:57am

I’ve restored (I say that, I’m sure a professional paint shop would snort in derision at my rattle can efforts) three 531 frames in the past few months. The 531c one followed the general 531 rule already discussed.
But the two others were 531 ST (Super Tourist, right?). One the vendor told me took 25.4 (those thicker walls I guess) but it needed a bit of love from my half moon file to actually make a new seatpost fit. For the other one, 25.6 was too lose, and a 25.8 has tightened up ok. If I was made of money, I’d buy and offer up a 26.0 just to see what happened.

A new Planet X Viner allegedly took 30.6, but that showed no hope of going in, but a Cane Creek shim with the requisite seat tube tightened up ok. Then I found that a standard star nut just slid on through the steerer tube so managed to use an expander, but then that’s another topic.

You can buy a metal gauge for this and I have got a birthday coming up soon...

pwa
Posts: 7081
Joined: 2 Oct 2011, 8:55pm

Re: the correct seat post size 531 variants

Postby pwa » 10 Feb 2018, 9:17am

David9694 wrote:I’ve restored (I say that, I’m sure a professional paint shop would snort in derision at my rattle can efforts) three 531 frames in the past few months. The 531c one followed the general 531 rule already discussed.
But the two others were 531 ST (Super Tourist, right?). One the vendor told me took 25.4 (those thicker walls I guess) but it needed a bit of love from my half moon file to actually make a new seatpost fit. For the other one, 25.6 was too lose, and a 25.8 has tightened up ok. If I was made of money, I’d buy and offer up a 26.0 just to see what happened.

A new Planet X Viner allegedly took 30.6, but that showed no hope of going in, but a Cane Creek shim with the requisite seat tube tightened up ok. Then I found that a standard star nut just slid on through the steerer tube so managed to use an expander, but then that’s another topic.

You can buy a metal gauge for this and I have got a birthday coming up soon...


Through experimenting with a couple of bikes, looking for the best position, I gradually acquired a small collection of not-too-expensive threadless handlebar stems. At the time this seemed like money wasted, but since then I have enjoyed just being able to have a rummage in a parts box at the back of the garage when a family member needs a new stem. It is often worth hanging on to these parts that we don't have a use for at the moment.

Brucey
Posts: 29225
Joined: 4 Jan 2012, 6:25pm

Re: the correct seat post size

Postby Brucey » 10 Feb 2018, 2:17pm

pablito wrote:I have a french built 531 frame (constuit avec Reynolds 531). The existing seat post is some sort of plastic (nylon?) and marked 26.4mm. It seems to fit well, but is too short, so I'm looking to replace it.

Measuring the seat tube with some verynear callipers, I get 26.4mm, but it is possible that the top of the tube has been slightly compressed. Can anybody tell me with confidence what post size I should get, or how I can accurately measure inside the tube? Thanks.


You can measure inside the tube itself (well below the seat cluster) using a bore gauge

Image

this is the traditional sort; you set the tool in the bore, lock it, tilt it and withdraw it. The measurement is then taken with a set of verniers or a micrometer. You can get fancier tools that measure directly (there is a micrometer head built into them) but these are just a PITA to keep calibrated IMHO.

But having said that if it is a French frame (esp with French threads) then it may have been built with a French spec Reynolds tubeset, in which case the correct size seat pin should be 26.6mm. It may be necessary to ream the frame to achieve this, if it has not been done before.

cheers
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