My seatpost on my single speed keeps sliding down while I am riding and its pissing me off. The original seatpost was way to short and I had to opt for a new one that was one size to big because the tightening thing on the frame was stripped and beyond repair. Now the seatpost has taken a liking to moving back and forth while I pedal and slowly move down. I have tried the cutup pop can method but it always seems to work in the first few miles them fails. Anyone else got any ideas or proven fixes for this. The frame is a really old raleigh so I doubt I can find the right components to fix it.

does the seatpost match the seattube diameter?

obviously, you need to fix that.

A hose-clamp might keep it from sinking into the tube........but i don't know if it would do much for the side to side action...

What do you mean by "the tightening thing on the frame"? If it is what I think it is (a binder bolt for a seatpost lug type clamp) they're easy to find. Here is an excessively fancy one: https://www.teamcyclist.com/store/Cam...m-ID_P1376.cfm---you can get em really chea p though. If the lug itself is ****ed, there are fixes.

Also, what do you mean by " one size too big"? If you have the wrong size seatpost in there, there's just about nothing you can do that will make it work acceptably. Get it out and get the right one in quick, you may have already damaged your frame.

agreed, a kalloy or good cheap post is like $20 brand spanking shiny new, no sense destroying a frame tube for $20.

the seat clamps are available from all kinna places, like these:
https://www.cambriabike.com/shopdispl...olt-on+Collars

the surly constrictor at a whopping $12.95 gets my vote.