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I'm trying to free my aluminum seatpost corroded to a CrMo frame. One of the methods is to heat the aluminum seatpost and then to let it cool. In theory, the aluminum will expand twice as fast as steel and hence the expansion/contraction should crack the bond.

Some recommend using a blow torch or even boiling water. I don't have access to a blow torch, but was wondering if there was something better than boiling water.

Recipe for disaster?

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Metals are elastic, and homogeneous expansion plus compression for the sake of it, in this case, most probably won't crack the bonds, I think. –  heltonbiker Sep 28 '12 at 18:37
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Have you tried simply taking the seat off and whacking the end of the post with a 3-pound sledge? –  Daniel R Hicks Sep 28 '12 at 22:12
    
With all these stuck seat posts, you'd think that someone would just design a seat post that fits inside the already stuck seat post. Chop off the old one about 0.5 - 1 inch above the frame, cut a slit in the old seat post, install a clamp, lube the new seat post, and drop it in. Or use something like an old quill stem so it could adjust to fill the gap, as you would probably end up many different diameters due to the varied seat tube thicknesses. –  Kibbee Sep 29 '12 at 2:40
    
that 3 pound sledge trick.. i will have to try .. i have the same problem.. i tried pulling and twisting.. but the sledgehammer didn't come to mind.. –  marc-andre benoit Dec 29 '12 at 11:52

2 Answers 2

up vote 2 down vote accepted

I have an identical problem, having been rinding the bike without seat clamp for more than two years now.

I suppose you have already read something about it, if not I recommend Sheldon Brown on the subject. But I must warn you that the ammonia theory didn't work for me, and I tried ammonia on some aluminum sample parts without any visible effect (corrosion, oxide removal, whatever).

The problems are:

  • Aluminum oxide occupies more space than pure aluminum. This, oxidation of aluminum posts create EXTRA VOLUME inside the seat tube, and this can create huge forces;
  • Aluminum and steel create a GALVANIC PAIR of metals, inducing electrolytic reaction between them, with dire consequences as you have already noticed.

I believe the solution, failing the chemical approach, must be oriented towards two possibilities:

  1. Dilation, a drastic one, but TO THE COLD! Since aluminum dilates more than steel, you should FREEZE the seatpost, by pouring some very very cold substance inside. Liquid nitrogen (frostbite danger), dry ice (perhaps not cold enough) and CO2 cartridges (doesn't remain inside for too long) are good candidates. Perhaps if you wrap a towel around the seat tube, pour boiling water on it, then pour the freezing stuff of your preference inside the seat tube, you'll crack the bond indeed, as you planned.
  2. Total mechanical disintegration. You'd look for some heavy machining tools (lathes and all) and eat the post from inside. Some damage to the inner surface of the seat tube is very probable.

I don't believe too much in brute force if the seatpost is frozen inside for too long (more than three months since last "movement"), but skilled shop owners have been asuring me that it is possible (I didn't go all the way with this for now because fortunately the current seat height is working and I don't want to risk being too many days without the bike).

Hope this helps, send some feedback if you get some result!

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"riding the bike without seat clamp for more than two years now" - ditto. –  Jacob Sep 28 '12 at 18:52
    
Yes, ammonia didn't work for me either and hence, I'm trying the heat differential method (and if that doesn't work, time to break out the hacksaw). –  Jacob Sep 28 '12 at 18:53
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Along the lines of Total Mechanical Disintegration, there's the old hacksaw trick. See option #12 on Sheldon's site (sheldonbrown.com/stuck-seatposts.html) –  Kibbee Sep 29 '12 at 2:24
    
@kibbee this link is THE correct answer, –  mattnz Sep 29 '12 at 9:05

The problem I had was getting the ammonia to the corrosion. I hung the bike upside down by both tires so the seat post was vertical. You can either remove the bottom bracket or as I did the bottle cage screw and fill the seat post with straight ammonia. If possible plug the hole so the ammonia doesn't evaporate. I let it hang for 2 days. I then laid the bike on its' side and put a 36" pipe wrench on the seat post and cranked. It broke loose luckily before the seat tube bent. The key is time. The ammonia has to seep down the entire length of the seatpost. Let the ammonia soak for at least 2 days so the chemical reaction has a chance to happen.

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Good point about leaning the bike and stepping over the lever. Someday I'm gonna try to get repeated overnight seepings of ammonia while riding the bike daily without seatpost. If it works I won't regret riding home without the seat if necessary! –  heltonbiker Sep 28 '12 at 20:27
    
Yes, I tried it overnight ; perhaps I should try it over the weekend! –  Jacob Sep 28 '12 at 20:54
    
@Jacob I think the needed strategy is actually to alternate soak and ride, soak and ride, soak and ride for many days. I think soaking alone can only break a limited amount of bonding at once, and the flexing motion provided by riding MIGHT allow for the ammonia to go down deeper and deeper as time passes... Not sure, though... –  heltonbiker Sep 30 '12 at 16:57

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