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seatpost size question

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seatpost size question

Old 06-16-14, 08:03 PM
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seatpost size question

My 68 Carlton had a 27.2 mm post and it was really tight when I removed it... I measured the inside of the seat tube on the frame and it measured 26.8 mm. I am going to replace it and believe I should get a 26.8 mm. Any thoughts ??

KF
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Old 06-16-14, 08:45 PM
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If your bike did originaly come with a 27.2, then the clamping area of the seat post might have been pushed in. Best to research and see what was original to the bike. If it is indeed a 26.8, let me know. I have a Birmalux alloy post in that size.
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Old 06-16-14, 09:19 PM
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If the 27.2 seatpost fit into the seat tube, even though it may be a very tight fit, it may be surface rust on the inside of the seat tube that's making it tight. I'd do some research into the original seatpost diameter for that particular model before assuming it takes a 27.0 or 26.8 seatpost. If it's a tight fit, the seat tube may just need reaming with a 27.2 reamer to clean out any surface rust and ensure it's round. Generally speaking, it's not a good idea to use a smaller O.D. seatpost than originally intended as this can put stresses on the seat stay brazing at the seat cluster.

What is the frame tubing?
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Old 06-16-14, 10:43 PM
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Butted seat tubes are thinner up top,,thus the 27.2 post .. a straight wall seat tube tube may have a 26.8..


of course my Ouija board cannot guess what you have.. Or how accurately you are able to measure it.
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Old 06-17-14, 03:44 AM
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Thanks guy's i will try to find out the original size before i purchase a new one. My local shop is really good i will bring it to them this weekend and see if they can clean it properly.

KF
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Old 06-17-14, 05:55 AM
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Originally Posted by kflagg
Thanks guy's i will try to find out the original size before i purchase a new one.
KF
^ A wise course. (There is a world of difference between 26.8 and 27.2.)

Originally Posted by kflagg
....My local shop is really good i will bring it to them this weekend and see if they can clean it properly.

KF
Many people clean out the ST with a brake hone - but just a couple quick passes so as not to remove parent metal with the rust. The thing is though, that you want to install your bb and bearings AFTER this operation, or the slag goes down there. If your bike shop does this correctly it could pay you to get your own brake hone.
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Old 06-17-14, 06:05 AM
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I recently read on another forum that it was not unheard of in the early 70s to use 27.2 rather than a 26.8 as the latter were hard to find. Apparently close enough that if you can't squeeze the larger one in, it only takes a few passes with a reamer to make it fit.
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Old 06-17-14, 08:32 AM
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Which Carlton model is the bike?

The 1968 Carlton catalog shows a number of models with 531 tubing which would ordinarily take a 27.2mm seatpost.

1968 Carlton models with 531 frames:

L'avenir
Flyer
Competition
Longfellow
Giro d'Italia
Grand Prix
Team Carlton

Possibly other models where the tubing isn't specified.

EDIT - Never mind; I see from your profile it's a Team Carlton/Giro d'Italia. That model has a 531 frame and almost certainly takes a 27.2mm seatpost.
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Old 06-17-14, 11:59 AM
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my '73 531 english bike's 27.2 seatpost fits the same way, really tight. no way i'm scrapping the original record post though. and since i'm its only rider, there's little need for height adjustment once it's set.
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Old 06-17-14, 12:27 PM
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As a refresher for oldtimers and a sort of "Idiot's Guide to Seatpost Sizing" for relative newbies, Lennard Zinn explained the reason for seatposts to be sized the way they are in one of his technical columns in VeloNews several years ago.

What’s up with all these oddball sizes?

Originally Posted by Lennard Zinn
Seat-post size often is dictated by the tubing manufacturer. The outside diameter of the seat tube will often be an English size, say 1.375″, and the I.D. will be dictated by the wall thickness, which the frame manufacturer selects as being appropriate for that particular rider and frame size. That’s why there are so many seat-post sizes – lots of framebuilders are trying to find the best balance between stiffness, strength and weight, while they are stuck with the English-size O.D. to make sure that a front derailleur clamp will fit on, and front derailleur clamps only come in 1-3/8″ (34.9mm, a.k.a. 35mm), 1-1/4″ (31.75mm, a.k.a 32mm), and 1-1/8″ (28.6mm).

27.2mm became the standard seat-post size because most high-end road frames in the 1970s and 1980s were lugged and were almost universally made out of Columbus SL or SLX or Reynolds 531. These seat tubes were 1-1/8” in diameter, or 28.6mm. The single-butted seat tube was 0.9mm thick at the bottom and 0.6mm thick at the top. Well, 2◊0.6mm = 1.2mm, which, when subtracted from 28.6mm, yields an I.D. of 27.4mm. However, the tolerance on the wall thickness and roundness of the seat tube made it so that you rarely could fit a 27.4mm post inside, even before brazing. And then, the seat tube always got distorted during brazing, making it even less possible to fit a 27.4mm in there, but a 27.2mm fit nicely. The same goes for why a 27.0mm seat,post was often used on the bigger sizes, which were made out of Columbus SP or SPX, whose seat tubes had 1.0 X 0.7mm wall thicknesses. Because of tolerances, the predicted 27.2mm post (28.6 – 2◊0.7 = 27.2mm) never fit, but a 27.0mm fit nicely.
Lennard
Although Reynolds made 531 seat tubes in both metric and Imperial outside diameters and with different wall thickness, in 1968 most 531 performance oriented frames with 28.6mm O.D. seat tubes were single-butted with 0.5mm walls at the seat cluster end and took 27.2mm seatposts.

Here's a late sixties or early seventies Reynolds catalog page showing the standard 531 diameters and wall thickness gauges. 24 gauge (Imperial Standard Wire Gauge) is actually .0559mm, or closer to .06mm than .05mm, but Reynolds documentation rounds down to .05.

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Old 06-17-14, 01:31 PM
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Thanks for all the great info everyone.... looks like it is likely 27.2 it's going today to be honed at my lbs. Will keep you posted.

KFKF
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Old 06-17-14, 02:49 PM
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Make sure you coat the seatpost with a thin layer of waterproof grease before you install it in the seat tube to prevent it from getting stuck from electrolysis.
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Old 06-17-14, 03:43 PM
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Always lube everything. Dropped the frame off at lbs bottom bracket shell chased and headset reamed seat tube cleaned tomorrow should be in good shape soon. Some new parts on the way hopefully start to look like a bike again soon. Thanks to everyone for the help.
Originally Posted by Scooper
Make sure you coat the seatpost with a thin layer of waterproof grease before you install it in the seat tube to prevent it from getting stuck from electrolysis.
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Old 06-19-14, 04:16 AM
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Frame back from lbs. Seat tube cleaned up nicely 27.2 as everyone thought. Seatpost fits perfect now thanks all for the help.

KF
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