Some time ago a visitor bought a steel frame road-bike to do up – nice bike, in good condition, maybe 30 years old. However the seatpost was too low , and stuck.
Seat posts fit frames snugly – 0.2 mm - but usually are adjustable. The trick is bolt a saddle on top, and use that to get leverage. Once you can get the post twisting – you can then adjust height. Seat posts sometimes get stuck if they're not greased, or they're the wrong size and hammered in, or someone has bent them. But this one, seemed straight. It was however not original - an aluminium post in a steelf rame.
Aluminium and steel (iron) are reactive metals and rust (oxidise) readily. if you make a bike out of one or the other, it should last – specially if painted and kept dry. A bike of one with parts of the other, is OK if the metal surfaces arent in contact (ie they are greased or painted etc or otherwise sealed).
But here, the bare aluminium of the post was close up against bare steel frame, probalby damp between, helping ensure good electrical contact.
As most of you know, when two different metals are in contact, corrosion can speed up. Electrons flow between the metals, and oxidation (corrosion/rusting) speeds up.
The 0.2 mm space between post and frame, was now jam-packed with aluminium oxide. Looks like talc, but is also used as an abrasive on sandpaper.
We did give it a go. We tried RP7, oil. We tried the saddle trick. We tried a large wrench,on top of the seat post, an even larger plumber’s stilson to get leverage . Two of us tried. Not a mm of budge.
Dave S – who understands a lot about metals and metal work – took one look, then said, cut the post, file the aluminium out from the inside..
But volunteers and owner, wanted a last try. They took the bike into the workshop, turned it upside down, and clamped the seat post in a vice. Two people held the work bench, 2 more tried to turn the bike - using the frame to get leverage.
There was an awful silence .. then an awful grinding screeching noise …. then another awful silence. But the bike had turned; the post must have loosened. So we worked it to and fro, then stood back, to think what to do next…
It then became very clear, why the bike had turned. The steel and aluminium had stayed firmly stuck, and when we turned the bike, the steel tubing of the frame, had simply torn, about 3 cm below the top.
Moral:if you're looking for a bike to do up - most things can be repaired - but things rusted solid one inside the other, is going to be difficult!
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