RFC

I have an American Classic 26.8 that I would like to reduce to 25.0. Can this be reasonably accomplished?

Thanks

mudboy

No. Even if you had a lathe, by the time you were done, the wall thickness of the post would be less than a soda can.

Andrew F

You could if you renforced the post from the inside...but why?

ColonelJLloyd 

I don't think that you can come up with a good enough reason to do such a thing.

rhm

I've reduced a 27.2 to 26.8, and I've reduced a couple of 26.8's to 26.4, because I had the former and wanted the latter.

The technique is pretty simple. Dip the shaft of the post in Drano for a little while, then rinse, measure, and repeat as necessary. When you get to the right size, polish up with fine grit sandpaper.

But going from 26.8 to 25.0 is basically going from having a post to not having one.

GrayJay

I had a very thinwall steel MTB frame that had to be repaired by brazing a sleeve into the top of the seattube. The original seatpost I had was a fairly heavy / generic aluminum post and the new deminsion of the ID of the repaired seattube was now an oddball non-standard size for which I couldnt find a replacement post. I took the original aluminum seatpost and was able to slowly reduce its OD down to fit into the new seattube sleeve diameter by wrapping coarse sandpaper around the post and twisting, slowly sanding it down until it fit. It is not machine shop perfect but 15 years later I am still using the same seatpost, no sign of imminent failure.
That said, I once bent an unmodified American Classic seatpost on a different MTB, I dont think I would try taking 0.9mm of material off the walls of an American Classic post as they are fairly lightweight / thinwall already. Sourcing a replacement 25.0 post shouldnt be too difficult.

RFC

Thanks. Just an idea, but not a very good one.

ColonelJLloyd 

Dura Ace
New Uno from VO just made available today $20

Grand Bois

I've done it more than once. I use a power sander and turn the post constantly to keep it round. It's pretty much the same as sanding a stem to fit a French steerer, but may involve removing more material.

TimeTravel_0

I have a Bianchi that takes a 26.6.

I cannot decide between a 26.8 and sanding the 0.2mm down or using 26.4 and just tightening the binder bolt just a little extra.

ColonelJLloyd 

You could buy a 26.6mm seat post for it.

https://cgi.ebay.com/DK-26-6mm-Alumin...item45ea7a3778

Or something a little nicer

https://cgi.ebay.com/Campagnolo-vinta...item4154311394
https://cgi.ebay.com/SHIMANO-600-ULTE...item43a28bf8b0

buldogge

I've turned down several on the lathe...but...as pointed out already...these needed only a .2-.4 mm change (.1-.2 mm off the wall).

A lot depends on the wall thickness of the seatpost.

You would have ZERO luck with something like a Thomson but might be able to turn down certain SR Laprades which seem to use a single casting to achieve the range of sizes.

There are plenty of seatposts out there...so...unless you have something you grabbed for super cheap AND you have access to machinery I wouldn't bother.

My 2c...

randyjawa 

The number one cause of damaged seat tube lugs. How many times have I spent time massaging a seat lug back into shape after it had been distorted by over tightening, which will be necessary to hold a too small seat post tight.

old's'cool

BTDT & generally concur with all the above. However, the word casting, also bolded above, is inaccurate, I think. All the alloy seatposts I've examined have a tubular part joined to the upper part, presumably pressed together. For the tubular part of the seat post, I'm pretty sure the raw material is either extruded aluminum tube (most likely) or possibly billet dowel (which I doubt). An aluminum casting would not have the tensile strength properties required for the tubular part of the seat post.

531phile 

Problem solved.

dbakl

I've done it too, but only to a French bike! No big deal in my case.

buldogge

Could be right...I would have to dig one out to look...regardless, they use a single extrusion/etc. to achieve their product range.

My bad.

toytech

I have reamed a seat tube out from 26.8 to 27.0 but never the reverse.

RFC

Many thanks. But too much setback. Looks like I'll have to go with the Thomson 25.0

x136 

It's a shame the 25.0 Thomson doesn't come in silver, or I'd have one already. The cheapie that VO released today is an interesting option.

Grand Bois

Examine these. The only one I have that is constructed the way you describe is a cheap Kalloy.

dbakl

I agree, most seem to be made from a single piece. Wheather they're cast or not, I have no idea, but they look like it. And I'd guess all sizes are machined from the same blanks.

BTW, you have too many bikes! What's the one with the Zeus seatpost?

old's'cool

Grand Bois, point taken, I'll take a closer look at what I have on hand.
From my understanding of aluminum properties and forming methods, the cheap Kalloy method actually makes a lot of sense as the most cost-effective solution.
buldogge, my apologies for calling you out with insufficient justification. Now the question is, are the apparently monolithic posts actually monolithic, or just cleverly disguised 2-piece designs, and if monolithic, are they cast or forged? I will maintain, if they are cast, they are heavier for the same strength than if they were forged, or 2-piece with extruded or billet post.