Ok, so at first I thought it was my imagination because it happens so gradually but now I really think my seat post is slipping down.

I have the seat post lightly greased to prevent corrosion from welding my seat post into the seat tube. The clamp is as tight as it will go. Is there anything else I can do? Is there some sort of supplementary seat post clamp they make for vintage bikes?

Like I said, it happens SO gradually that I thought I was imagining it but it really is slipping down.

It sounds to me like your seat post might be not be the right size for your frame. That gap in the back seat lug should not close up all of the way, it should be open evenly from top to bottom.

I've had this problem before and it was an easy fix for me.

The seat binder bolt was as dry as the desert so although it felt tight it wasnt. After lubing it, I could get a few more turns which solved the problem.

Good luck!

what's your seat post size? what's the measurement in your seat tube? has it ever been reamed?

Seat tube reamed? sounds painful.

Mutt

I had a simular problem on a 40 year old frame. I ended up cutting a shim out of a thin alum. beer can. Can't see it and it works perfectly.

I can't imagine that the seat post is the wrong size. It's the same one that came with the bike over 30 years ago. The gap is small but it is there. However, the clamp itself is pretty well closed.

I thought I remembered lubing the bolt before but I may be mistaken. I'll have to pull it out and double check.

I may have to try this! How big of a piece did you cut? Did you lube the aluminum? I can imagine that pressing aluminum so tightly between two pieces of steel would eventually cause it to weld itself into place if you didn't.

I have two Raleighs that came with the original posts that were too small for the frames. It was hard for me to believe it too, but I am pretty confident that is what happened.

My Raleigh gran sport has reynolds 531 tubing and takes a 27.2 post, when I got it it had a much smaller post that appeared to be original. The gap in the seat lug was almost entirely closed.

I cut a band about 2.5" wide by 3", it wrapped about 3/4 of the way around the post. Lightly greased all surfaces, inserted and tightened the clamp. Worked perfectly. I did this on a Raleigh Sport with a steel post.

if the post is the correct size I am not sure I would ream the tube but perhaps a flex hone to rough it up or burnish the inside a bit. reaming is mor for if there is a burr or something else causing it to stick

I have the exact same problem on my my wife's Centurion Turbo. I also thought that the seat post was the wrong size, but it is the same size as Chicago Al's, so I am guessing not.

One thing I noticed is that the both sides of the binder bolt spin on this bike. In most of my bikes, they don't. I have to get a screw driver to hold that side while I tighten the other side with an Allen wrench. Could this be part of my problem? Could the OP have the same problem?

I am not really sure if binder bolts are supposed to work in any sort of special way.

I bought a bike that had a piece of thick sandpaper wrapped around the post-rough side out. Not only does it not slip, it is a pain to adjust, but it is tight!

I am not sure what your problem is, however; a general look at how to prepare your seat post and the seat post cavity might prove useful. I always follow these procedures, when preparing a frame set for use, and rarely have seat post slip or gouge problems.

Hope this is a help.