CliffordK

Before you try the lye, you need to verify what type of metal the seatpost is.

My guess is that you have a chrome steel seatpost similar to this, and lye won't help. Unless you tried electrolytic derusting which may change how the post is bound inside the seattube, but wouldn't necessarily fix the problem.

CliffordK

With a tapered top like the image above, you have two places to grab the seatpost. Either at the top where the edge curls in, or about 1 1/2" lower where the top of the post tapers.

I like the allthread approach above, but would do both allthread + cable + modified washer.

Do you have a welder?

I'd modify a washer so that it will fit inside the seatpost, and catch where it tapers down.

Either slot it, or attach a cable + allthread. With the slot, just the allthread. Trim edges the washer can slip inside of the post at an angle, the go straight when in place and catch the post.

Pull out with the Allthread as previously suggested.



Use a straight rod, and perhaps some tape to manipulate the washer down in the seattube/seatpost.

jccaclimber

1. Clean out the seat tube as best as possible first.
2. Find an adhesive that can be easily softened with heat, a chemical, etc. Make a tube that just fits inside the stuck seat post and glue it in. Use that to pull the post, and if it fails use the dissolving solution on the glue to extract the tool. This could go very wrong.
3. Plug the stuck seat post from above. Plug the BB openings to the other frame tubes. Weld up a set of BB cups to seal the BB shell and put a grease zerk in one. Pump grease until the seat post is forced out.
4. Find a reamer just a tiny bit smaller than the seat tube. Put a guide bushing the diameter of the seat post ID on the nose and diameter of the seat tube on the base, then shave it out axially.
5. Use a toggle bolt, but put a string in the top of each tab. This way you can push on the center rod while pulling in the strings to collapse the toggle and lift it out if it doesn’t work. Test this outside the frame a few times first. Make sure the toggle stops getting wider before reaching the seat tube diameter of you might damage the frame, depending on the contact angle.
6. Find a tap just a bit bigger than the seat post ID and try the thread the inside. If it just spins, then the toggle method will probably work. If not, then use the new internal threads to pull it once threaded.
7. Expand the hole in the BB shell until you can catch an edge and shove it out from below.
8. Get a ball whip hone and just hone it away to near nothingness from the inside, then pour out the dust. Note, this would take a really long time, and might do bag things to the frame if you kept going too long.

There are a few other options, but they become very labor intensive after the above options.

degan

Thanks all for the discussion! So I tried a couple simple "snag the bottom and pull" methods, one with a thin tube, one with a rod, both cut and massaged with a grinder and hammer into having a flat edge sticking out. Unfortunately I don't think I ever got good purchase on the bottom of the seatpost. I believe there is a good bit of rust and gunk down in there sort of filling in whatever tiny gap is between the bottom bracket and seatpost, which I believe is pounded down as far as it will go. My brother is back so I'll talk to him about some of the options that involve possible welding and taps and whatnot.

One of many failed tools iterations.

Camilo

Looking forward to hearing how it goes.

Crankycrank

An adjustable reamer would probably work. If it doesn't grab onto the seat post and break it loose you can still ream it out of the seat tube. Can be used to clean out a rusty seat tube as well. I have one of the cheapies from here and works OK for a few reaming's if you only need it once in awhile. Advanced Search Result - Wholesale Tool (wttool.com) (These are all the same type of reamer, just different sizes)