Seatpost Won’t Stay Put!

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I’ve been commuting with my converted-to-fixed ‘81 Voyageur lately and something that is driving me nuts is that the seat post will not stay where I clamp it. After a couple rides it slides down about an inch or more. I’ve tightened it as much (or more, really) than I feel comfortable with. It’s a 25.4mm post in a 26.2mm (I think?) hole with a shim to make up the difference. I assume/hope there has to be a simple solution to this that I’m not yet aware of... I appreciate the help!
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Take the shim and post and clean them so there's no grease or anything. Also the inside of the seat tube. Then tighten. If it has no more room to tighten, then another shim or thicker shim is needed.

The bike looks good with those wheels.
 

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You can also file a bit of material out of the inside of the tabs the seat post bolt goes thru. Take the bolt and the seat post out, then you can file a bit out of the inside. Often the tabs actually touch each other when tightened, effectively stopping any further tightening. This works on handlebar stems too.

Carl.
 
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Two things that worked for me with the same problem with a solid seat post (I think the tube posts distort a little, helping the grip, which the solid post takes a lot more force to do): a gritty type of grease that increases friction and grade 8 hardware to apply more force. Not sure if I actually needed the grease, but I had it, so I used it.
 

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Why not just get the correct size post? They usually aren't that expensive.

I chose that post because I like that post ;)
I think I have the original post lying around somewhere but it was too short for my needs. I also remember not liking the style of the fluting as much as this one, which is from an '81 Le Tour and was in stock. I'll have to have a dig for it though.

Thanks for the kudos, it's a fun ride but pure stupidity in a place like Seattle :) That picture is literally from the moment I finished it 2 years ago. I knew it would never be as clean as it was then so a picture was mandatory. Since then I've changed the bar end plugs and just recently added a black B15 with copper rivets that really took the bike to the next level. Other than that, it's just dirtier. I'll try to get something new up in the next couple days.
 

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I tracked down the original SR post from the bike. It’s 26.8mm, shaft is about 6 1/4” (too short for my needs), has no fluting, and a needlessly complicated saddle mount.
8759CFE1-FD1C-426E-8848-2710C8141DA2.jpeg

I need 26.8mm, flutes, and more length. I stopped by a LBS this ayem that has a lot of used parts but came up empty.

Does anyone out there have what I’m looking for?
 
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Have you considered knurling the post to obtain a better "bite" into the shim material? A local machine shop ought to be able to perform this for you, since the tool can be rather uncommon in home workshops.

 
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Red Lock-tite or steel wool fibers wrapped around the seat post B4 inserting it into your frames seatpost opening/shim setup. You will solve your problem straight away...skpc
 

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Hey ya'll...
Knurling was the first thing to cross my mind, but I figured a machinists time was much more valuable than other possible options.

It is the correct sized shim (supposedly). How can we be sure the shim is at fault? This is the first time I've used a shim, so I don't know for sure. I obviously figured it would work without fault.

In any regards, a correctly sized post is probably the best solution. It will at least eliminate variables. I've searched on eBay for the post I'm after (SR, 26.8, fluted, long) but not surprisingly every one of those that is in the ballpark is priced like solid gold. I know of a great community bike shop up north that has heaps of used parts, so I'm thinking that's a great excuse for the missus and I to take a little road trip up there this weekend. If that fails, I'll get creative with steel wool or pop cans.

Much appreciated!
 

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Last weekend the missus and I took a rainy day trip up to Bellingham mostly so I could raid the seat post stash at The Hub community cyclery :) There are other community bike shops that are closer but I really like the guys there and it made for a fun and very scenic day trip. Anyway, once there I rummaged around and found a proper length fluted SR post that was a little scratched up and stamped 2#.8?? The main guy there Kyle measured it and I thought he said it was 26.8mm... For the whopping sum of $5 I took it home, after lunch at Aslan Brewery, of course. Once home I cleaned, polished, detailed the flutes, installed my B15, and dropped it in the seat tube without the shim only to realize that it was a hot dog in a hallway! I took my calipers to it and it measured 25.8mm. Whoops! So I tried it with the shim I already had, and it fit. I’m happy to report that I’ve put over 40 miles on it in the last 3 days and it hasn’t budged :) So, if I happen to trip over the correct 26.8mm post I’ll swap it out, until then, I’m calling it problem solved. Amazing the difference .4mm (.016”) can make...
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Thanks to all for the advice!
 
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GTV:
Typically, after I've gone thru my box of seatposts without luck, I check niagaracycle.com. I know that they stock several 26.8 mm posts because I recently ordered one.

Tip: Because I would usually have to go thru the whole box and put a caliper on most of the posts in order to find a suitable specimen (man, aren't there an awful lot of sizes?), I got a bit smarter and started writing the diameters on the posts with a marker.

Have fun, Andy
 

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GTV:
Typically, after I've gone thru my box of seatposts without luck, I check niagaracycle.com. I know that they stock several 26.8 mm posts because I recently ordered one.

I know the easy route would to just buy a new post, but I didn’t build a custom bike because I like taking the easy route ;) I like the look of the original SR fluted stem, and I prefer period pieces as well. We’ll see if I find one...
 
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