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NOS frame - seat post sizing query

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NOS frame - seat post sizing query

Old 04-14-15, 07:53 PM
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NOS frame - seat post sizing query

Hello knowledgable folk,

I have a NOS unbuilt late 80's reynolds 531 stickered frame here.

I've measured the seat tube (using internal callipers and a vernier, multiple measurements averaged) and come up with a solid 26.8mm.

Great! I have two 26.8mm posts already ,

However attempts at installing them are met with immediate scratchy resistance from the 'top' of the lugwork (yes the clamp is loose).

Question: is it just that is has been unfitted and there is a bit of overspray and other unknown but safe factors - and I can proceed to use force ? (never a hammer, but a bit of mass in the form of me) maybe 'clean' it out first ? but how?

I don't know what I don't know about new frames...

Or is that if the frame measures 26.8 unclamped than perhaps 26.6mm is the correct post dia ? (the 26.2mm I have is obviously too small) - intuition tells me this is the solution, but for the sake of ebay prices on period correct campag posts, I'm here asking
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Old 04-14-15, 08:33 PM
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Ideally you want a rat tail file for this, but other shapes might work. Odds are there are burrs or a turned edge at the top of the lag or along the inside of the slot. File all those corners to break the corners and try again.

BTW- 26.8 is small for most R-531 tube sets, so don't be surprised if it turns out you need a larger post. Once you smooth out the edges and slide the post in you might find it rattles a bit. The tipoff will be when you clamp it. If the post is undersized the slot will close more than normal. If all is right the sided of the slot will be very nearly parallel because the tube was a kiss fit for the post before the slot was cut. If the top of the slot pinches in by over 1mm or so, the post is too small.
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Old 04-14-15, 09:03 PM
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Since the seat lug area sees significant heat it often can be distorted from a perfect round ID shape. Agree with Francis in that most Reynolds seat tubes (of 531) take a 27.0 or 27.2 post if the tube is still round.

But the default in fitting posts is to use the largest diameter that allows complete clamping and still slides in and out. Since this is a feel thing it's very hard to recommend in words what is the best fit. An experienced wrench can "play' with the fit and explore what the best size is. But damage can be done which is EXTREMELY difficult to repair. Andy.
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Old 04-14-15, 10:07 PM
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Take the frame to the lbs they will have a rimming tool to clean out the hole or home depot has a honning tool that fits on a drill but is kind of expensive for a one time use
Good luck
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Old 04-15-15, 07:58 AM
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Reaming tool?
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Old 04-15-15, 08:16 AM
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One you clean out the seat tube (ream, file, whatever) make sure to put some grease in it before test fitting seatposts. I made the mistake once of trying to test fit posts dry and I thought a post was too big when it really just couldn't slide in due to lack of grease.

I agree, 26.8mm sounds wrong for Reynolds 531. Unless this is perhaps a French frame with smaller tubing diameter?
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Old 04-15-15, 08:51 AM
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26.8 is the size of a seatpost in an 0.9mm wall, straight gage seat tube, with a 28.6 OD,

just because the steel alloy tube is 531 from Reynolds tube mills , does not mean it also is Butted.. *

Butted seat tubes vary from 0.7 to 0.9 top to bottom thereby the larger top ID is leaving 27.2 for the seat post fit..
(DB is thick -thin-thick on the tube wall ... ends consistant OD. )

* they make miles of steel tubing for other uses, aircraft, etc.

Last edited by fietsbob; 04-15-15 at 09:00 AM.
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Old 04-15-15, 10:59 AM
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if it's NOS someone may have put a too small seatpost in it and tightened the clamp or something similar, causing the problem. i think i would double or triple check the ID of the seattube before i removed any material.

and yes, i agree, there are bikes out there with non-butted Reynolds 531 labels on them (the label on mine stated the fact that the tube was straight gauge), that may have a smaller ID than the butted versions in the seatpost section.

Last edited by hueyhoolihan; 04-15-15 at 11:05 AM.
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