So six months ago, I have a little problem. The seat is too low. Not a bunch, just a smidgen, need to raise it maybe a quarter inch (6mm). So I loosen the seatbolt screw, and the seatpost refuses to budge. Problem!
Turns out, the frame is steel, the seatpost is aluminum, when they're in contact and wet, you get corrosion, and the corrosion products are bulkier than the original metals. So the seatpost gets packed in tighter and tighter when it corrodes.
So I run it down to the bike store where I bought it. They jiggle it a bit, nothing. So they turn the bike upside down, clamp the seatpost in a vise, and twist the frame back and forth with all the gusto they dare to use. Nothing. "Sorry, nothing we can do!" Translation: "We can get this thing loose, but there's a distinct chance that we'll ruin a frame doing it, so we dare not try any further if we'd have to pay for your bike." So that's the end of that. By the way, grease on the seatpost is supposed to prevent this problem, and the bike mechanic assures me that he greased it when he assembled it.
Fast forward a few months. The seat height is still bugging me. It's nothing I can't ride, just not quite right, either. And in the meantime, I've researched the topic and found I'm not alone. Unfortunately, people are usually faced with this problem with junk frames, where they don't mind ruining a frame or letting it soak in penetrant for a month, and I don't have that option. Thus, I apply my ingenuity to the situation.
First, I seek out the PB Blaster. Now, you're supposed to dump this down the seat tube with the bike upside down and the crank removed, and then let it set for a couple of weeks while you repeat the process. Only problem is that I'm riding my bike every day, and the crank needs to stay in place. So I hose the seatpost down with PB Blaster several times over several days from the top side. No change.
I get a brainstorm, or maybe read about it, I forget which. Aluminum expands and contracts more than steel with temperature changes. Chilling the frame ought to shrink the seatpost more than the frame and make it looser. So I buy $10 worth of dry ice, apply it to the frame, nothing, no change whatever. Scratch that idea.
So I make up the Seat Post Impact Puller. And finally, yesterday, I applied it to the problem. I took an old steel bike frame, split tubes lengthwise, cut out notches to fit around the top of the seat post thus:
Then I attached it to the seatpost thusly, using multiple hose clamps torqued down pretty good. (I put some foil in between to try to avoid scarring the seatpost too badly.)
Then I attached the top part, a steel bar with a 2 or 3-lb hammer head slid over it, with cross bolt at the top.
Voila! I can now hammer that pesky seat post right out of the frame. So I start in WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM WHAM etc. Nothing. Zilch. It doesn't move a millimeter. All that work to make that contraption and it's totally useless, unless you just enjoy hammering on a bicycle. Grrrrr. (On a positive note, the hose clamps held the thing on the seatpost without slipping, so this does have SOME possibilities!)
So in a fit of desperation, I resort to the dreaded hacksaw. I cut the seatpost off about 3/4" above the clamp, and started slitting it lengthwise inside. I carefully cut it on one side, nothing. This is a slow and tedious process, by the way. When I sawed the seatpost off, I discovered that it is fairly thick, too, it's not sheet metal that you're going to pry out of there. Anyway, I got it sawed on the inside of one side, with no change, so I flipped the bike over and started sawing the other side. Got it sawed through, I thought, and still no change. I knew if I actually got it cut all the way through on both sides, two pieces ought to just fall out of there, so I kept working at it on one side and then the other. Finally, as I was sawing, the blade suddenly seized up, and I knew I had it. I see-sawed the blade loose, locked the vise grips on the stub of the seatpost and I was now able to turn it. A few minutes later, I had it worked out of there.
This sawing process took about 4 hours for both cuts. When I got it out, I found that one cut wasn't all the way through at the bottom. So I think if you actually cut it all the way through, it would only take the one cut. Sawing away at your good bike with a hacksaw will sure enough make you feel like you are doing something mighty stupid. I kept wondering if any minute, the blade wasn't going to come popping through the side of the seat tube. I envisioned myself taking the bike down to the bike shop and saying "I accidentally sawed my bike in half, can you put all these components on another frame please?" But when it was all done, there seemed to be minimal damage to the steel surfaces. I would still recommend any other method you can think of over this one. One of the problems is you can't see down in there to check on progress. So a borescope or whatever you can come up with to enable better inspection of progress would help considerably.
The new seatpost is now in place, and a few adjustments and I should be riding again. I post this so you can marvel at my mechanical ingenuity and dogged determination or laugh at my folly or whatever is appropriate.