cstar 

Hi all,

I know that many people might think what im setting out to do here is dumb, and maybe it is, but please humor me

I bought an SR four sir seatpost on eBay advertised as a 27mm. It showed up and is clearly marked 27.2. So irritating. However, it's not a very common post and it's gorgeous, so I'm contemplating shaving it down to 27mm. The seatpost is for my Centurion ProTour, which definitively uses a 27mm post. The 27 SR post it came with reads 26.89mm with my digital calipers. The 27.2 Four Sir measures 27.09. So I figure realistically I need to lose about 0.2mm in diameter, which seems achievable to me.

I understand that a machine shop could do this but I don't want to pay (although TBH I don't even know what they would charge). I am very handy and love a good project. I did some mechanical engineering education and have been working on bikes and cars forever and have done some fabrication type work. So I recognize that this is kind of a dumb undertaking but I would like to do it anyway and I think I can figure out a way to do this.

What I'm really looking to know is, what are your ideas for methods to reduce the post diameter? I have some ideas but the creativity and genius of strangers on the internet never ceases to amaze me... Thanks in advance.

Iride01 

I'd ask the seller to exchange it or refund your money. Or if the seller doesn't give you any relief, then file a complaint with eBay. eBay might refund your money even if the seller won't.

thook

as long as it's not internally ovalized, you can do that much off quite safely. it takes a little time, but get some 60 grit paper and go to town. clean and polish it up with ultra fine wet paper. btw, if you need it to go down from 27.2 to 27, you only have to take off .1mm from the radius....which'll come to .2mm off the diameter

cstar 

Thanks for the reply. I thought about doing that but given the rarity of the post and my feelings for it and the minimal amount to remove I'm feeling pretty set on modifying it

CliffordK

For 27.2 down to 27.0, you can probably do it with sandpaper. Perhaps pick up a caliper so you can monitor your work.

I have seen posts that people have cut down thinner on a lathe, but personally I wouldn't do more than a couple tenths of a mm.

cstar 

Thank you, this is very helpful. I was contemplating different ways to sand it evenly. I could use some guidance on grits. I was afraid 60 would be too aggressive... I want to be able to polish it back out to the factory appearance which is quite nicely polished as many 80s high end Japanese components were

Do you think I will be able to polish out all the 60 grit marks? My plan is to work gradually finer, say 60-80-100-150-220 etc etc. I'm not that experienced polishing aluminum so I would love guidance re: the finish polishing

cstar 

Agreed, that would scare me as well. The tube of this post has pretty thick walls as older seatposts seem to have so I'm also feeling pretty safe about it.

When you say caliper, do you mean a digital vernier scale type caliper or something else? I do have the former

zandoval 

Taking off 0.1 mm with sand paper is a piece of cake. Don't be afraid...

cstar 

Thank you!! 👍

CliffordK

I'd probably start at 100 or 120 grit, and see how the progress goes. Generally sanding on a block.

If you work up to the finer grits, 320, 400, 600, and 1000 grit, then it will give you a reasonable polished appearance without needing to go to a buffer.

thook

tbh, i think i'd started with 80 grit, but i really don't think it's going to matter until you get closer the the 27mm diameter goal. i'd done it by hand and just kept rotating and moving the post up and down. doing it that way, each successive pass kinda cancels out previous scratching by the paper. iow's, it'll come out smoother looking than you'd think being that you're sanding in all directions. i was lazy and skipped doing a bunch of progressive paper grit. maybe move from 80 to 200 and on to the finishing grit like 2000 or so. it really didn't seem to matter because at the end you'll be wet sanding and rinsing and that will move much of the aluminum particles off the surface making the 2000 grit paper work to smooth rather than making more scuffing. i hope that makes sense. you'll see what i mean. don't complicate it

ps. i should add i didn't go for an ultra polished look. rather i wanted the more dull bright or aged look since none of the other aluminum bits on the bike were high polished. the post still came out very smooth, though

dedhed

A number of threads on this very subject to peruse.

https://www.bikeforums.net/classic-v...eatpost-2.html

https://www.google.com/search?q=sand...&bih=617&dpr=1

KerryIrons

Have you actually done this? I did it once and it was a true pain. I felt like I sanded (with emery cloth to get better "wrap-around" contact) forever.

3alarmer 

...not certain what your problem was, but I've done it a number of times (5-10 ?) and it went pretty quickly for me. Maybe you were using very fine grit abrasive, which takes llonger ?

3alarmer 

,,,first, before you start sanding anything, test the post you have in hand for the fit in the seat tube you want to use it in. Make certain that the frame seat tube slot is fully expanded, and grease the post prior to insertion. Don't force it, but it might already fit, depending on who reamed the seat tube last, and how exact they were in setting the reamer.

Proceed according to your results. Seat post diameters and interior seat tube diameters are an inexact science, at best.

79pmooney

The science is exact. But scientists don't draw the tubes, machine the posts or braze the seat lugs. Don't make parts or bikes on Monday or Friday.

Second using emery cloth. I'd start sorta fine, measuring before I start, sand so many swipes, rotate post, repeat then measure again. This will give you a feel for how much coarser you are comfortable with. I'd also make up wood blocks for my vice, secured in place and with grooves to securely hold the post with a quick tighten of the vise.

krakhaus 

I've done it. It would have taken forever done entirely by hand. I used a belt sander laying on its side. It took the bulk of it off enough to do the fine polishing by hand. Looked great and worked perfectly.

Crankycrank

Why not just check if the seat tube can handle being reamed out to 27.2? The ST I.D. will be perfectly round, your SP will fit as is and you'll be able to find far more posts in 27.2 if you need another at some point.

zandoval 

That's right!!! Check the hole first! It may not be the seat post at fault. I forgot all about this. Before you start be sure to check the ID of that seat tube. It could be way off or just need reaming of the upper portion...

TPL

Calipers ....digital ....and ....vernier ....are 2 distinctly DIFFERENT types of calipers ....digital is easier to read for most folks

Removing 0.2 mm using sandpaper will go as fast as the grit allows ....fine grit = slower process = ( more patience ) ....heavier grit = works faster = ( will likely require more finish polishing, see below )

final polish by using a green scotchbrite pad = patience

jccaclimber

Taking a couple hundred microns off the diameter of an aluminum shaft is easy. A greater question is if the post has any anodize, including clear. That won’t return. Anodize is cheap on a per part basis, but the minimum order is usually more than the post is worth.
Mind you that probably won’t be a functional issue, I only mention it as you seem very cosmetically oriented and I don’t know what it looks like.

I’ve taken posts down in the past, ridden them through a salty winter, and had no issues so don’t worry about corroding it. There are plenty of bare aluminum bits on bicycles and it’s seldom an issue.

If you’re nervous I’d give it just enough with the coarsest paper you plan to use to get it looking scratched, then bring it to the finish/polish you want. This gives you a practice run on the polishing and if you measure as you go you’ll also find out how much diameter your cleanup steps remove.

pdlamb

Add another vote for try it first.

If you do need to take it down, I'd suggest aiming for 27.0 rather than whatever your current seatpost is. That would mean sanding off 0.05mm rather than 0.1 mm. If that turns out still to be too large, you can take some more off. It's a lot harder to put material back on.

urbanknight

Wouldn't sanding it down be horribly uneven? I can just see the post slipping at certain points and sticking at others. If you're a machinist with a lathe, sure, but I can't imagine something more crude doing anything more than causing headaches.

phughes

I did this to a Nitto stem once. I used emory cloth, it will work better on metal than sandpaper. I used the cloth from a roll, which makes it easy to sand a round surface like a seat post or stem. Go slowly, check the size often, then use progressively finer sanding material, then rubbing compound followed by polishing compound for a good finish, depending on the finish desired of course. When I was done, you could not tell it had been altered.

icemilkcoffee 

You should definitely not do anything goofy to it. Ask yourself this- would YOU buy a seatpost that's been sanded down by some guy in his garage? You are ruining the resale value of this part by doing something like this.