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Old 06-12-10, 05:18 PM   #1
peripatetic
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Advice on reducing the diameter of a seatpost

I had to replace an old Peugeot seatpost that was stolen with the only thing available, an alloy post. The size I need to fit is 25.6mm [edit:24.6mm], according to my calipers. The post I got is 26.0mm [edit:25.0mm]. I've been sanding the sucker down by hand--with the coarsest grit sandpaper I could get at the hardware store--and it's taking forever. I can just barely get the bottom of the post inserted into the frame. I don't have access to a lathe, but a neighbor said he had a hand grinder. I'm wary of using this, however, as I assume the grind will be uneven. It's not a lot to take off of the post, but I don't have the energy, free time or patience to keep doing it by hand. Any suggestions on how to speed this one up?

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Old 06-12-10, 05:26 PM   #2
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Sorry but you're barking up the entirely wrong tree. You'll never ensure that it's even enough and round enough to work well. Not to mention that you've still got an arm busting amount of work ahead of you doing it this way.

If you truly can't source the correct size post then the better way to do this is to go with a smaller size post and shim it out to fit your seat tube size. Common sources of shim stock can be had from some pop and beer can sides (it's actually a very high grade of alloy) or in the form of brass shim stock from model railroad or airplane hobby shops. Or really any source of tin or aluminium stock. There's even some commercially available shim sizes for just this sort of issue. Some measuring along with trial and error will soon find a suitable size.

To avoid the shim shooting down the seat a trick to use is to fit it in place with about 2 mm's of metal sticking out past the rim of the seat tube. Flare it out to an angle with a screw driver shaft or similar. Now insert the post ensuring that it doesn't suck away the shim. Now using various smooth tools you may have to avoid scratching the snot out of the seat tube to flare out that last 2 mm's some more and in repeated stages work it down until it's flat to the end of the seat tube. That'll form a rim that won't let the shim get sucked down into the tube.
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Old 06-12-10, 05:27 PM   #3
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"Draw file" the post with a fairly smooth cut, long, wide metal file. File lengthwise and rotate the post very frequently to make the diameter reduction uniform around the circumference. Switch to sand paper when you ger near to the proper fit.
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Old 06-12-10, 05:27 PM   #4
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just how coarse is the sand paper?
I assume it's an aluminum seatpost.
120grit should really speed things up.

and wear gloves and a respirator. Aluminum dust is quite toxic.
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Old 06-12-10, 05:28 PM   #5
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I would always try niagaracycle.com as they might have what you need.

I just managed to sand down a seatpost from 26.8 to 26.4 with a belt sander by holding it against the belt in a way that it could spin but with friction and at an angle, so stock was removed.

Before that, I tried building a little hand tool which consisted of a bore in a block of wood, with a screw through it which was ground into a cutter shape-I wanted to put the post through the hole, screw the screw down until it cut the metal, then spin the post around inside the bore. My work was too sloppy and it didn't work-I just slapped it together. Maybe if you made a stronger collar and fitted it to the seatpost with some epoxy but waxed the post so it could still rotate, then used a better cutter, it would work.
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Old 06-12-10, 05:30 PM   #6
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"Draw file" the post with a fairly smooth cut, long, wide metal file. File lengthwise and rotate the post very frequently to make the diameter reduction uniform around the circumference. Switch to sand paper when you ger near to the proper fit.
I tried that, but the stock just comes off too slow. Maybe a wood file would take the stock off but you risk wearing it out or clogging it, besides having to buy a tool you may not need for anything else.
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Old 06-12-10, 05:30 PM   #7
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Turning it down in a lathe would be the way to go - can you contact the local HS shop teacher or technical school to see if they could turn it down for you ??
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Old 06-12-10, 05:34 PM   #8
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Niagara has one for $5.50......
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Old 06-12-10, 06:22 PM   #9
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Sorry, I mis-typed. The seatpost size is 24.6. The smallest size now made or readily available is 25.0 mm, hence my need to actually just shave this down. (I think YellowJersey did sell some French sized seatposts; I'm not sure if they still do, but in any case, they were too expensive for this project, which is basically a friend's beater). I've done my research on the various options for something like this, and this is by far the simplest solution as recommended in various posts on this site and others. It's an 80's Peugeot mixte frame with skinny, HTE tubes. And yes, the post is a Kalloy aluminum.

Thanks for the input. I guess I could ask around for a technical school, but I tend to avoid high schools here in NYC at all costs; it's just not my bag. I did think about a machine shop, but I have no connection to such places, and I've already asked a couple of friends who work in factory settings if they knew people, and neither of them could help.

I've used both a metal file and coarse grit sandpaper. I can't remember how coarse it is right now, but I'll double-check. I seem to recall the number was less than 200, though. The problem is it's just a lot of length of tube: I've already sanded a stem down (for another French bike) .2 mm, but that was only a hand's-width of tube I had to get down.

Anyway, thanks for the various inputs. I was just trying to confirm there wasn't something I had missed...I'm thinking my next step I might try and enlist my neighbor to hold his grinder while I just slowly rotate the tube as evenly as possible. I figure if I go a bit slowly, I can be a bit more even about it.

Oh, and thanks for the tip on covering my mouth, etc. I had forgot about that, but yeah, I don't want to be inhaling aluminum dust.

Last edited by peripatetic; 06-12-10 at 06:25 PM.
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Old 06-12-10, 09:20 PM   #10
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Originally Posted by peripatetic View Post
The problem is it's just a lot of length of tube: I've already sanded a stem down (for another French bike) .2 mm, but that was only a hand's-width of tube I had to get down.
You could cut away the excess length first. The Kalloy post is likely at least a few inches too long if not 4-5.
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Old 06-12-10, 09:31 PM   #11
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How about borrowing a buddy's random orbital sander, preferably with a vacuum attachment? If you use 100 or 80 grit it will go quite quickly, and the vacuum will suck the nasty dust away while allowing the sandpaper to work more effectively. If you make sure you are constantly moving the sander, you should be able to remove material quite evenly.
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Old 06-13-10, 01:24 AM   #12
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None of these methods is going to result in anything but an egg shaped and decidedly non straight seat post. Well, at least not without a lot more care in checking diameters and straightness very frequently. And since it won't be accurately round it's going to want to walk itself down in the seat tube unless the clamping screw is super tight.
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Old 06-13-10, 02:23 AM   #13
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Honestly this does sound like a job for a machine shop. Get some thick walled 6061 tube or even a solid bar, lathe most of it down to 24.6 and lathe the top section down to 22.2. Then stick on the clamp from an old style straight seat post and done. Simple enough job that it should be affordable even at full shop rates.

I can't envision manual grinding working out well at all. If you do want to try it manually sanding or grinding you will want to turn the seat post FAST (i.e. have it chucked in a drill) to have any chance of keeping a round shape. but a chuck that can take a 1 inch seat post is rare enough you might as well find a lathe.
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Old 06-13-10, 03:43 AM   #14
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A machine shop should be able to do this quickly and cheaply for you. Even a small metal-working lathe would be able to handle it.
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Old 06-13-10, 05:52 AM   #15
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Google found me this:
http://www.classical-bicycles.com/Seatposts.htm
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Old 06-13-10, 08:21 AM   #16
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I appreciate the suggestions. I think I'll continue my search for someone with a lathe and perhaps, in the meantime, continue the occasional sanding. For those who say the post will be too misshapen to fit in the seat tube, as I mentioned before, I've done this procedure by hand with an alloy stem that fit fine and well inside a French bicycle. In that case, I had to reduce the diameter by .2 mm (i.e. .1 mm around), and in this case, I'm doubling that. As I said, I've got the bottom part of the post fitting just right on insertion, and the shape doesn't seem to be too off. Using something like an orbital sander, I realize, could create a flat spot, I'm aware, but again, two tenths of a mm, from what I've seen in doing this, just isn't enough to magnify the errors to make it unworkable. And the reason I did this in the first place is, after doing a good couple of hours of searching online for "Peugoet, replacement seatposts, 24.6 mm seatpost, etc.," the 'standard' solution mentioned by just about everyone was this one. If it were more than this much, yes, I'd be more concerned about getting the proper slimming in a round manner, but I'm not worried about that. I have made sufficient progress to show I can get to where I need to, and I'm not in a position to now spend more money on a different seatpost (though if I had seen the link above before I started, I would have contemplated it--thank you, John).

My point is, it's not helpful nor correct to say it won't work--I've done it, I'm doing it, and it's working. The problem is, as I mentioned before, that it just takes too long to do by hand.
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Old 06-13-10, 09:07 AM   #17
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I bet you could git-er-done in an hour or two with a random orbit and 100 grit, with our without vacuum. Keep it moving and put on your headphones. Good luck!
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Old 06-13-10, 09:29 AM   #18
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I bet you could git-er-done in an hour or two with a random orbit and 100 grit, with our without vacuum. Keep it moving and put on your headphones. Good luck!
I just double-checked my paper, and it's 36 grit aluminum oxide. Obviously, the grit's correct, but I'm wondering if the alumunim oxide is hard enough or I'm just defeating myself with that. But yeah, I'm thinking I'm going to see if I know someone with an orbital sander; and if they do, I think it'll likely take more like 15-20 mins. As I mentioned, it's really not a lot to strip away, given some amount of powered help. Thanks for the suggestions and encouragement, all.
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Old 06-13-10, 12:18 PM   #19
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None of these methods is going to result in anything but an egg shaped and decidedly non straight seat post. Well, at least not without a lot more care in checking diameters and straightness very frequently. And since it won't be accurately round it's going to want to walk itself down in the seat tube unless the clamping screw is super tight.
It's actually easier than you make it out to be, if you keep the post going in circles.
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Old 06-13-10, 08:03 PM   #20
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The problem is that the more you take off the more room for error there is. But a lot of issues can be solved with care and regular measuring. Like you say though, it's a shopping cart full of work.
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Old 06-14-10, 05:59 AM   #21
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I just double-checked my paper, and it's 36 grit aluminum oxide. Obviously, the grit's correct, but I'm wondering if the alumunim oxide is hard enough or I'm just defeating myself with that. But yeah, I'm thinking I'm going to see if I know someone with an orbital sander; and if they do, I think it'll likely take more like 15-20 mins. As I mentioned, it's really not a lot to strip away, given some amount of powered help. Thanks for the suggestions and encouragement, all.
You might be pressing too hard and making the grit crumble off just under your fingers. maybe wrap the paper in a layer of old innertube or foam.
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Old 06-14-10, 07:17 AM   #22
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What about cutting a belt sander's belt to make a long strip of sandpaper, then use a shoe polishing motion by grabbing both ends and pulling back and forth. This would probably sand the post more evenly.

36 grit sounds pretty rough. Maybe try 80 grit with the belt.
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Old 06-14-10, 07:33 AM   #23
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I'd create a mandrel which fits into the post on one end, and the chuck of a drill on the other. Then use a sanding block, then file, to bring down the diameter as it spins.

Edit: No I wouldn't. I'd buy the post in John's link.
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Old 06-14-10, 07:37 AM   #24
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If the sanding doesn't work out then perhaps you can use a 22.2 bmx seatpost and shim up.
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Old 06-14-10, 09:57 AM   #25
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Humans built the pyramids without electricity,you can do this easy.It's only a few thousanths,just time consuming.

Start with a bastard or dbl bastard file,mill file,sandpaper, done.You can do it all with sandpaper,just takes a little longer.

Use WD 40 or kerosene when cutting aluminum and it won't clog your files/sandpaper/grinding wheels/carbide/rotary files ect.

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