Aluminum/aluminum combinations aren't as prone to issues as when dissimilar metals are used , ie. aluminum post in steel frame. Not being able to rotate it does make it tougher. Faced with a situation where I can't pull a post out, I usually use "reverse psychology" on it and try to push it in a bit more. Of course it has nothing to do with psychology, but it is far easier to drive a post in than pull it out. Tis also gives me a decent sense of how stuck it actually is.
To use this technique, remove the saddle and support the BB shell on something like a wooden block, so it can't move and diffuse any energy that way. Protect the top of the post with another piece of wood, and deliver a solid blow with a hammer. If the post isn't truly frozen, it'll move and you'll know that it can and will come out if persuaded properly.
Once you've shown that it can move, the best method is to clamp an old junk saddle into the post, and try tapping it up from underneath. Bracing the frame and/or hammering directly on the post also help, but aren't that easy on most posts. Soaking in penetrating oil for 24 hours before trying also helps, and on some bikes this works better if the BB is removed, and oil is poured into the inverted seat tube through the BB shell.
If the post doesn't move when you try driving it inward, you're in for a tough fight. Sometimes a long soak in penetrating oil, like Kroil or PB Blaster helps as does ammonia. But IME ammonia only does any good if it's the first thing used, since any oil will prevent it from getting to where it needs to be.
One other last resort method that sometimes works is to securely clamp the post in a heavy vice on a solid workbench, and hammer the frame down off it. This is a tricky process and requires that you find a place to hammer on the frame that won't damage it. Or you might try bracing a jack of some kind between the crank or BB and the underside of the saddle and sacking it up slowly.
Whatever you decide, I still find that driving it in is a good diagnostic tool which will tell you what you're dealing with.
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FB
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