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I've recently found a nice bike with a steel frame. Well I noticed the aluminium seatpost was stuck but had no idea how heavily. To be honest I never had a similar problem and underestimated that beast...

So here's my question. I'm pretty sure the problem comes from the galvanic corrosion between the two materials, I understand that the post gets thicker due to this and therefore is stuck. Now I made some measurements of the seatpost diameter which gave me 27 mm which is correct. Then I moved on to the frame, measuring the diameter just at the top and on a few positions further down. The result is, there's quite some variation: 29.7mm at top, 28.7mm further down. I've attached some photos: https://photos.app.goo.gl/Yt3s3ycS74GfBagA6

to me this sounds quite heavy and I would like to have your opinion what to try next: a) try some chemicals b) cut out the seatpost c) sell the bike again (I wouldn't like to because I really like the frame, it's a Fuji Feather)

concerning a) I already tried WD40 without luck. I've got the hint to try ammonia: Remove bottom bracket, having the bike hanging upside down fill the tube with ammonia and leave it for at least 2 weeks. Then there's hope to get it out.

concerning b) what tools would you suggest for such an act? How can I prevent to ruin the frame?

thanks in advance!

  • do you know what diameter seatpost the frame was designed for? Many older Fuji models in the database at the end of this comment list seatpost diameters under 27.0mm. If the seatpost is mechanically stuck, then I don’t think ammonia will work. What tips me off is that the frame has a wider outer diameter at the top of the seat tube than in the center. If a too-large post was forced into the frame, that could explain this. sheldonbrown.com/seatpost-sizes.html – Weiwen Ng Jan 22 at 17:34
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    Extreme approach is to use lye rather than ammonia. If all else fails, that’s your best option. But first I’d try something thinner than WD40. Get a proper penetrating oil. Then use a pipe wrench to try to turn the post. Second level is cutting a vertical slot into the post and trying to crush it. – Andrew Jan 22 at 18:48
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    Corroding away the seat post is actually a rather gentle method as it completely avoids the mechanical damage that large wrenches or cutting could do to the frame. I've heard stories of frames being busted when wrenching away a really, really stuck post. – Walto Salonen Jan 22 at 21:05
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    @WaltoSalonen I'd call that an advantage. If a frame fails on the workbench, its not robust enough for the road. – Criggie Jan 23 at 2:28
  • @WeiwenNg the frame has a wider outer diameter at the top of the seat tube than in the center A full mm difference is pretty drastic, though. I doubt a too-large seatpost was forced into this frame to the point the seat tube "grew" an entire mm in diameter. Paint would likely crack, and the top tube weld joint would pretty much have to be distorted for the seat tube to change diameter that much. Nevermind how, umm, perversely determined someone would have to be to get such an oversized seatpost fully inserted in the first place. – Andrew Henle Jan 23 at 10:54
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What I've generally done is first pry the top of the seat tube (the tube of the bike that accepts the seat post) apart a little bit, at the slot where the clamp bolt tightens it, then work some lubricant (WD40 is good) down into the joint.

Next, use a pipe wrench to twist the post back and forth. Slowly work the post up as you twist it. It can take 5-10 minutes of twisting back and forth to get it all the way out.

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    Use the saddle to twist the seat post. You can also remove the bottom bracket and spray some WD40 in from the other end while the bike is upside down. – Michael Jan 23 at 7:50
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Use a really long tube as leverage on the end of a pipe wrench for example (used to grip and twist the seat post (around it's axis). combined with wd40, should get it out, if not the seat post will snap off most likely, if so you can either saw it into pieces using an iron saw blade and take the pieces out individually or as a last resort use chemicals to dissolve the aluminum.

If it's not too stuck you can use the seat for leverage but if it's quite bad you'll destroy the seat (pull out the seat rails) this way. Next step could be to install two big screwdrivers/Allen keys or something similar in place of the seat rails (after having taken off the seat) and tighten it back up again. Then you can use the screw drivers/Allen keys etc. for leverage without damaging the seat post.

Hitting the top of the seat post with a heavy hammer (in axial direction of the seat tube) whilst putting twisting force on the seat post can also help 'loosen' it.

On some seat posts (for example some older SR seat posts) you can use an adjustable wrench to grip the seat post after removing the hardware for mounting the seat. I've tried to illustrate how to place the wrench on the seat post in the image below (forgive the terrible drawing): enter image description here Using this method you can potentially get great leverage on the seat post without damaging it so far you'll have to throw it away (the same method can be used with a bench vice (as suggested by @Criggie ) if you have one (grip seat post on the same 2 planes with the bench vice)).

There is a guy on YouTube (RJ TheBikeGuy) who has some good videos on it... example:

Here is a video of the 'sawing out the seat post' method:

  • It looks to me like this guy has the bike resting on the chainring! – Daniel R Hicks Jan 23 at 0:38
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    This guy needs to remove saddle, flip bike over. and clamp the seatpost hard in a well-secured bench vise. The seatpost may get distorted and will be junk. Then use the entire bike frame as leverage to spin the bike around the stationary seatpost. I had to have an assistant to provide the rotation while I worked on lifting the frame using a rope and pulley going upward to a ceiling beam. Surprisingly, after polishing the chrome seatpost and honing the inside of the frame, the original seatpost fitted perfectly and I rode that bike+seatpost for a year. – Criggie Jan 23 at 2:33
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    @Criggie has it here. The more leverage the better. I once had the pleasure of using a WAY oversized crescent wrench to remove such a seatpost (see tinyurl.com/thuwa3l for example). We used it because someone had it, and it was fun. Realistically, its easier to flip the frame and stick the seatpost head in a clamp and go to town. Let some penetrating oil soak in for a bit first. – Deleted User Jan 23 at 15:30
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I've used Finish Line Chill Zone for the last many severely stuck seatposts I've extracted. It cold shocks the material to break up corrosion and get its penetrating oil component worked in. I've been impressed with it and I don't believe I've seen it fail yet, although sometimes the seapost itself still winds up being sacrificed. The big trick with it is to use way more than you think you need, continuously blasting it as you gradually work your way around the seatpost opening. That's how you get things super cold so it can do its work. A 6oz can does one stuck post in my experience. Use it in a very well ventilated spot where you can get oil everywhere, put cardboard down, and prepare the frame by tying rags around the top of the seat tube, seatstays, top tube, etc. It won't hurt to first get some penetrating oil around underneath from either the BB or the bottle cages too, because the thermal expansion/contraction will help work it in.

Either a vise or a pipe wrench is the way to go for gripping the post once you're ready. Using a vise on the clamp part of the post with the bike upside down is my favorite for really tough ones, with the qualification that the post may be destroyed at that point and also it may break and make you change your approach.

Ammonia does work and it has the upshot of being a cheap, non-specialized, less toxic (althouh plenty caustic) product, but it takes a long time and is less effective than Chill Zone, plus it has the ability to eat your frame, although that's more a concern with aluminum frames.

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Use a Slide Hammer to drive/pull the post upwards. Pulling naturally tends to reduce the post diameter, as opposed to twisting which does not.

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