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Seatpost sizes can be so frustrating

Old 02-21-21, 10:58 AM
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b dub 
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Seatpost sizes can be so frustrating

I own bikes with many different seatpost sizes. I have a box of seatposts with various sizes. And I use one of these cheap but accurate digital calipers.

And yet Iím often having the hardest time to figure out the correct seatpost.

Example 1: 1983 Nishiki Cresta. According to most of what I read the correct size is 26.6 or 26.8 (I found reports of both).

But on my Nishiki I canít get a 26.6 post to fit and a 26.4 post fits like a glove.

Example 2: my Van Herwerden (Chesini) Columbus SL frame should fit a 27.2 post but I can only fit a 26.6 post.

And I find it impossible to measure the size on the inside of the down tube.

With both examples I donít believe anything is bend or forced.

Ok, I feel better now 😀
Happy Sunday!

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Old 02-21-21, 11:06 AM
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It took me two Campy binder bolts to realize that my Battaglin needed a 27.4 post, not a 27.2. Luckily, I was able to find an American Classic seatpost in 27.4.
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Old 02-21-21, 11:10 AM
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Totally normal experience. It’s hard to rely on stated seat tube spec from the internet. I have a few posts in the parts bin I know to be more accurate and use these as reference posts when buying a new one. Buying used is tricky since you’re still at the mercy of someone else’s measurement. All part of the C&V treasure hunt.
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Old 02-21-21, 11:11 AM
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It sounds like the clamps are a bit pinched. I'm using a Trek 400 that couldn't fit a 26.6-26.8, don't remember, when I bought it. The 400 is a 27.2 post, I did a slight pry and used a trashed steel 27.2 as a ream and with a tad of filing sharp areas, wazam 27.2 post no problem.

I think the 27.2 should happen on the Italian, the others I think should be 26.6
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Old 02-21-21, 12:12 PM
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Originally Posted by b dub View Post
And I find it impossible to measure the size on the inside of the down tube.
You can get a cheap set of telescoping gauges for $15.

When checking the diameter of the hole, make sure the slot (if there is one) is not distorted; the top corners will be obvious, but sometimes the bottom of the slot can also be bent inwards a bit, and a post that almost goes in won't.
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Old 02-21-21, 12:41 PM
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The Stein style seatpost sizing rods are really the most useful. They provide a bit more feel for how an actual post would fit.

I had an older set in chromed steel, not aluminum. Those were even better, maybe because they got rid of some of the friction. They had an oddysey sticker on the end, not sure if that's who made them.
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Old 02-21-21, 12:44 PM
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I just keep old seatposts around in the common sizes to check. I find this to be more accurate than calipers.
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Old 02-21-21, 01:01 PM
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Posts at 25.4 mm aren't bad to have around. I've beer-can shimmed one to ~26.0 without trouble and used it that way for years.

I had another in my bin that I thought would fit my Ross MTB (Ishiwata 024 tubing, it turns out). I brought it on a trip from Europe to visit my family - turns out the bike took 27.0 mm, so was much higher quality than I thought! I rode it with the aluminum post + external clamp and returned the next trip with a Cane Creek shim, which was nicely done in aluminum for the price. I had thought the post would fit my Finnish Helkama city bike to replace the galvanized (!) post it came with, but it turned out to be 23.0 mm with a nicely done plastic shim to 25.0 mm. I probably should have just honed the tube out...

I also had a Canadian-made late '80s Peugeot with HLE tubing that took a 23.8 mm post (15/16"!). Luckily the clamp diameter was the conventional diameter, so I was able to reuse the post with a clamp that I got from an LBS for a couple bucks. That's a size that's not so easy to find.
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Old 02-21-21, 02:18 PM
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Originally Posted by wesmamyke View Post
The Stein style seatpost sizing rods are really the most useful. They provide a bit more feel for how an actual post would fit.

I had an older set in chromed steel, not aluminum. Those were even better, maybe because they got rid of some of the friction. They had an oddysey sticker on the end, not sure if that's who made them.
Originally Posted by bikemig View Post
I just keep old seatposts around in the common sizes to check. I find this to be more accurate than calipers.
...if I had kept on working at the bike co-op here, I was gonna make up a set of seat post sizing rods from all the old seat posts that came through there.
It really is the only way you can get a satisfactory feeling for the fit in a given situation. For myself, I just keep around a milk crate full of old posts in various sizes.

I know this is an inefficient use of resources, but none of them cost me very much to buy.


There is a tool you can buy to set to measure the interior of a tube, and I have a set of those.

They still do not work as well as having around a bunch of seat posts to try in the seat tube.
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Old 02-21-21, 02:22 PM
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...the main problem with the Stein sizing rods is that they only give you a feeling for about an inch near the seat lug.
But if you have access to them, they are a good starting point. If you don't use them intelligently, you will often undersize the post you fit.
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Old 02-21-21, 02:54 PM
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So if my Nishiki should be 26.6 what do I need to do to increase the size by 0.2 mm? Take a correctly sized old post (which I have) and hammer it in?
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Old 02-21-21, 03:10 PM
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Originally Posted by b dub View Post
So if my Nishiki should be 26.6 what do I need to do to increase the size by 0.2 mm? Take a correctly sized old post (which I have) and hammer it in?
... not sure if serious, but nicely played. If you have a post that seems to fit, don't try to change the seat tube just based on someone's number on the internet somewhere.

But it's very possible that your seat lug has gotten distorted over the years by abuse and wrongly fitted seat posts. If that's the case, (and you can confirm by taking your caliper and taking a number of measurements of the clamp diameter at different points around the circumference...a distorted seat lug will show up as different diameter measurements), the proper remedy is reaming with an adjustable reamer. Pounding a seat post in with a hammer will only give you the additional issue of a stuck post.

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Old 02-21-21, 03:21 PM
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Originally Posted by 3alarmer View Post
... not sure if serious, but nicely played. If you have a post that seems to fit, don't try to change the seat tube just based on someone's number on the internet somewhere.
Iím glad you got me. Rarely is taking up a hammer the solution to fixing a delicate bike related issue.

I will do some measuring but honestly the current post I have fits nicely. No slipping ever so I donít feel the need to mess with it.
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Old 02-21-21, 04:46 PM
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I used a wood shim to straiten the clamp slot on the seat cluster, then I tried ascending sized posts they all were floppy until 27.2. I used a trashed 27.2 to check for burrs and lightly filed at the pinched area of the clamp.
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Old 02-21-21, 05:29 PM
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This is the 26.4 post. Fits perfectly.


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Old 02-21-21, 06:56 PM
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I’ve also found that posts are not always true to the labeled size. Most recent example is a brand new laprade-style post labeled 25.4mm for a low-end MTB I’m refurbing. Installed it, took a couple of rides, and realized it was rotating when I was looking over my shoulder. Got out the dial calipers, which show the diameter as 25.2mm. Great.
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Old 02-21-21, 08:41 PM
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The only question I have is this: Why do so many old bikes that I bring home have missing seat posts? Who is taking these old seat posts? Why are they taking these seat posts? Why, why, why******************************??
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Old 02-22-21, 01:28 AM
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Originally Posted by 3alarmer View Post
the proper remedy is reaming.
When is it not, though?
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Old 02-22-21, 01:32 AM
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Would you believe someone over on MTBR lately started a thread to complain how there are THREE seat post sizes?
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Old 02-22-21, 02:53 AM
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Originally Posted by SurferRosa View Post
When is it not, though?
Well, it may often not be.

Consider: the tube starts out straight, presumably the lug does so also, and the framebuilder does a proper job.

If any of the metal later on is not where it started, it's been bent. If steel, the best solution might well be to bend it back - not to cut away the parts you don't like.

And reaming is not without risks; recently there was posted a picture of a frame which had a seat-tube that had rusted through - but in an odd way; you could see that the tube had been reamed slightly off axis and at the bottom of the cut the tube wall was paper thin on one side and full thickness on the other. I bet neither the owner nor the worker noticed. Until later.
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Old 02-22-21, 06:08 AM
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Try Sheldon Brown. Comprehensive tables of seat post sizes and manufacturer's seat post/down tube sizes.
I found them very useful trying to pi down my 1968 Holdsworth Mistral. Previous owner had a Campa 26.4mm in a 27mm downtube!
Take nothing for granted with pre-owned bikes..
Hope this helps.
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Old 02-22-21, 09:00 AM
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It does look in the picture of the slot that the post has zigzag scratching. It maybe caused by the post sized used. I would use a fine semi circular/flat file the next time you need to service, or if the bugs of concern get to you.

This may be a point of, if it ain't broke don't fix it.
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Old 02-22-21, 09:17 AM
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Originally Posted by genejockey View Post
It took me two Campy binder bolts to realize that my Battaglin needed a 27.4 post, not a 27.2. Luckily, I was able to find an American Classic seatpost in 27.4.
I have that same seatpost - it's been sitting in a box for years I look at it once in a while and decide to keep it in case I ever luck into a vintage Klein, or a Trek 170!
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Old 02-22-21, 12:25 PM
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Originally Posted by DoubleClanger View Post
Try Sheldon Brown. Comprehensive tables of seat post sizes and manufacturer's seat post/down tube sizes.
I found them very useful trying to pi down my 1968 Holdsworth Mistral. Previous owner had a Campa 26.4mm in a 27mm downtube!
Take nothing for granted with pre-owned bikes..
Hope this helps.
Good resource, but be careful, It had my '73 Raleigh Competition labeled as 27.0 seat post. It's a 27.2. I had a 27.2 laying around and should have checked. I guess I have an extra 27.0 Campy seat post if I ever need it...
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Old 02-22-21, 12:40 PM
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Let's not talk about the 5 seat posts I bought to find out I needed a 26.6 probably because I didn't know 26.6 existed until last spring then finding a quality post was yet another joy.
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