Bicycle Mechanics - 27.4mm post in a 27.2mm seattube?

I just found a deal on a 27.4mm seat post.

I don't know what the tolerance is on most seat tubes - so I'm wondering if, generally speaking, I can fit a 27.4 post in a 27.2 spec'd seat tube.

Unfortunately I can't try before I buy.

I suspect I know the answer, but appreciate any experienced opinions on this subject.

Thanks.

As a gunnery sergeant once told me long ago "ya can't put 10 pounds of sh*t in a 5 pound bucket".

Not gonna happen. Just buy the right size post.

Impossible. Tolerance for seatposts/tubes are probably to 0.5mms.

wow--- that little bunny--- hes stuck in a 27.2 kleenex box---- get the liquid wrench--- If you have a old bike witha thick seat tube you could try to hone it out a bit? I wouldn't do it on a pinarello--- it the seat post is thick like my head --- you could try to give it a buff?!--- cute bunny!

I had a GT Backwoods mountain bike that had a very loose fit with a 27.2... but didn't quite work with a 27.4.

But, hey, if it's a good deal go for it - you are saving so much money over what you would be wasting if you bought the wrong size post for full price!

Impossible. Tolerance for seatposts/tubes are probably to 0.5mms.
I assume you mean 0.05 mm.

couldn't one sand file off .2mm off the post or lightly ream the seat tube?

Many seat tubes that take a 27.2mm seat post actually start out with an inside diameter of 27.4mm, but 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 while brazing the seat cluster, making it even less likely you could fit a 27.4mm post in there, but a 27.2mm fit nicely.

For example, the most commonly used Reynolds 531 single butted seat tube (butted on the end near the BB, but not the seat post end) had a 28.6mm O.D. with a wall thickness of 0.6mm at the seat post end. 2x0.6mm = 1.2mm, which, when subtracted from 28.6mm, yields an I.D. of 27.4mm.

couldn't one sand file off .2mm off the post or lightly ream the seat tube?


Conceivably, yes, I guess one could do that. I, personally wouldn't, though.

There are many parts of a bike that I am perfectly willing to fudge with and modify, just to see if something will work.

However, things like steer tubes, forks, stems, brake bosses, and seat posts, I generally don't eff around with.

I would sooner use a crappy old rusty seat post off of a gas-pipe special, if it was the correct size, than try to turn one into something it isn't, even if it was a primo Thomson that I got for free.

Just my take on it. Good luck.

Were only talking .008ths,2 pieces of notebook paper thick.Do the seat tube,not the frame,alot easier to fix should there be a problem.If you know someone with a lathe,should take about a minute to fix.If not,and you have the time,you can polish that much off by hand.You'll be at it for a while unless you have some way of spinning it,but it can be done.

couldn't one sand file off .2mm off the post or lightly ream the seat tube?

Hey look! Target has Wrangler jeans on for half price! Oh.. they only have them in 28" waist... I guess I will have to cut them apart and sew some more denim in then sew them back together... and do this over and over until they fit!

couldn't one sand file off .2mm off the post or lightly ream the seat tube?
ANYTHING is possible if one put enough effort (time) and money (tool) into it.

CAN yes, practical, no.


If a free carbon fiber bike falls in the forest.....

If there was that kind of tolerance, why would they sell 27.0, 27.2 and 27.4 posts? Buying the wrong size post because it's on sale is false economy. And this is coming from a serious cheap bast*rd.

You could try pounding it in with a hammer. Kids: Don't try this at home.

I assume you mean 0.05 mm.

Yes, I was doing math in another reply and got my numbers confused. On the other hand one of my new textbooks was originally authored by Ben Bernanke and I am excited to see how he puts things.

I walked under a tree the other day and a 27.2mm seatpost fell on me.

You could try pounding it in with a hammer. Kids: Don't try this at home.

I've tried the hammer method before and unfortunately the seat tube magically became a little larger and now the correct size doesn't tighten up. Good thing it was my roommate's bike.

I've tried fitting posts into the seat tube of a frame I didn't know the size of. That .2mm makes a HUGE difference. It's literally go or no go. Or if the right size is a little stiff then the next .2 mm down is just a super sloppy fit that WILL distort the upper tube cluster area far too much for my tastes by the time it is clamped tight and the post will wobble even when clamped. Not a good thing at all. And even on a post that fits with no slop but slides easily if you go the .2mm up from that it would require a sledgehammer to get it to fit.

Get the right size for the job. And if it is a hair snug that's fine. Now if there's a burr that scores the tube badly then THAT is worth reaching in and lightly grinding smooth. But that's a horse of a different shoe size......


A thick walled post COULD be machined down but there's no way to remove that much metal with a file or sandpaper and keep the outside truly round enough to fit properley. We're talking about a job for a proper lathe and an operator that knows proper lathe skills for removing that much metal over that long a distance and to do it accurately.

19 irrelevant posts later and the only answer the OP needed was 'no'.

19 irrelevant posts later and the only answer the OP needed was 'no'.

But where's the fun in that? If we can't have a feeding frenzy over a topic we can all dig our teeth into then we may as well give up and cancel our internet services.... :D

couldn't one sand file off .2mm off the post or lightly ream the seat tube?

You could but I would not recommend it.

An old buddy of mine fit a 27.2mm post in his 27.0 seat tube. It was a very tight fit; it took serious elbow grease to move the post around, even though it was well-greased. The seat tube was steel (which tends to be more willing to flex than aluminium) and the post was aluminium.

It would probably work, but it wouldn't work very well. The post would also be more prone to seize inside the frame, which can be a nightmare to fix. Since 27.2mm is a very common post size, I would recommend just getting a post in the correct size and be done with it.

couldn't one sand file off .2mm off the post or lightly ream the seat tube?

Yes, one could. I've done it a couple of times using a 1/4 sheet pad sander.

I recently fixed up an older MTB where the seat tube was originally painted black. It had mostly worn off and looked horrible. I polished it up with some fine steel wool and painted it with black epoxy paint. It looked like new, but it was too thick to fit in the seat tube. I ended up steel wooling off the paint, using generous layer of grease and it fit ok.

That's why I was asking about sanding and/or reaming.

I bought a machine to do this job once upon a time... I needed something that could fit down the seat-tube and bore it out precisely to 27.4mm...

http://i42.photobucket.com/albums/e346/DannoXYZ/AutoTech/CNCmill.jpg

In the end, I figured out it would've been easier, cheaper and faster to just by a 27.2mm seatpost...

o muy gooosh--- i forgot what kind o bike and what kind of post--- I like pinarellos and long walks on the beach.... I mean rides by the beach............ i need a coffee.................

no--- dude----- that is crafty--- in all seriousness --- wow I'm glad when we can all come to your house and work on our bikes.

I bought a machine to do this job once upon a time... I needed something that could fit down the seat-tube and bore it out precisely to 27.4mm...

http://i42.photobucket.com/albums/e346/DannoXYZ/AutoTech/CNCmill.jpg

In the end, I figured out it would've been easier, cheaper and faster to just by a 27.2mm seatpost...

Holy crap that is beautiful and while I have no clue how to use one, I just want to get one so I can figure it out.

AAWWW c'mon Danno, now that's just unabashedly cruel. Flashin pics of your very own CNC fab center in the mechanics forum is heartless and may even be grounds for banishment.

However, despite this blatant act of cruelty, you are now officially my BESTEST FRIEND EVER! :D:D


BTW: I got dibs on machine time for Thursday nights. Yeah baby!!!

.....However, despite this blatant act of cruelty, you are now officially my BESTEST FRIEND EVER! :D

I take it you live close enough to reap the benifits of this blatant butt kissing then? :D

Mine's bigger but it doesn't come with the built in TV set for watching reruns of Three's Company while doing long and fine cuts..... :D

That's one sweet setup you got there Danno.:thumb:

I take it you live close enough to reap the benifits of this blatant butt kissing then? :D

Mine's bigger but it doesn't come with the built in TV set for watching reruns of Three's Company while doing long and fine cuts..... :D

That's one sweet setup you got there Danno.:thumb:Why thank you. That's actually the computer-screen for the PC at the bottom that controls the thing. But now that you mention it, I could add a TV card... then I'd have no reason to run inside during long jobs... hmm...