A process of elimination will make your life easier with a stuck post.
1. Get post in sturdy vice and nip up tight. Hold frame at either end for big leverage, rock (twist) back and forth gently, see how it goes. Could be loose now...
2. IF not, carry on with more pressure. still wont budge? I've heard of people bunging up exits at the post end and pouring anything from coke to plusgas fuel down the seat tube via bottom bracket shell and leave overnight to soften up the oxidised metal that's causing the problem, then repeat step one. May well work.
3. IF not. Try freezing it out as alu will move twice the rate of steel in either direction, which may shrink the post enough to release the bond.
see here for tips......viewtopic.php?p=1706925
Brilliant example of mechanical intervention before destroying anything.
4. Still there? bloody thing! Now, can you see how the above has been low-cost and making chemicals work for you? that's the idea. So, from here you will have to decide whether or not the frame needs a repaint or to spend £20 on tungsten carbide hacksaw blades and days of stress and blasphemy. been there, done that, it has been done successfully but what an ordeal!
IF you have 60mm of post in there, fine. but you will probably have 150mm+. it's very hard to get pressure at lower point.
frustrating and time consuming with no guarantee of success.
Caustic soda. melt it out, hence "need a respray" as paintwork will be sacrificed.
see here for tips...viewtopic.php?f=1&t=192660
By stage 4 you can see how it gets messier and more expensive so, always worth trying the first options, first. It all simply reflects how badly seized in the post is.
Ride yorkshire and the north east
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