I've done a search and haven't found anything that really satisfies my question.

I have a 1978 Motobecane that has a continuous slipping seat post. The diameter of the seat post is 25.6mm and is supposedly the right size. When I had this problem with my old seatpost I was using one from a British standard bike, and it was a few mm too small. I went to a LBS and they said the original post was too small and this is the right one. The problem is now back. The bottom bracket on this is Swiss, but I assume the standard everywhere else is French, which is the diameter seatpost that I recently bought for it. However, it seems that no matter how hard the seatpost nut is tightened that either the seat is loose laterally or once a month it will become just loose enough to start slipping down when I go over some rough road.

What's semi-unique about this, or maybe just bad luck, is the seat post is full of scratches. Zig zags from how i put the tube in, and at first I did use grease, but then the grease became a suspected cause for it to slip. So I wiped it down, reinstalled and had no problem. However, now it's started back up again (I went to raise it today). So are there any other things I could do to keep the seat height maintained? The bike is fixed gear, so there's nothing worse than riding to class to have the seat bottom out and my knees in pain. My last resort, is if there's no real remedy to either weld or JB Weld the seat post into place.

Shim it with a shim from a Coke can?

This is a non issue. Get a digital caliper and measure the seattube size and your seatpost.

I had this issue with a newish Trek atb. Switching out the stock quick release binder bolt and putting in an allen one solved the problem. It now works so well I don't take it out and grease it as often as I should. But I hear you, no matter how hard I cranked down that quick release, one solid bump on my ride and the seat went down an inch. Very scary at speed on a descent off road.

Make sure that the top of the seat-tube is not crimped inwards from previous attempts at cranking down on an undersized seatpost. Remove the seatpost and binder-bolt and carefully look at the slot in the tubing, both sides should be straight and parallel.

The zig-zap scratches on the post is a sign that the top of the seat-tube is actually crimped inwards. The points at the top on each side of the slot is then aimed inwards, scratching the post. This also reduces contact surface area between the post & tubing, it's just pinched at the top and isn't in contact anywhere else. To fix, stick a big flat screwdriver down into that slot and pry it apart so that it's slightly bowed outwards. This will then let you measure the true ID of the seat-tube about 1.5" down from the top. Use precision calipers. Then get a seatpost of that diameter.

When you slide the post down about 2", you SHOULD NOT be able to rock it back & forth with the binder-bolt removed. Then with it fully inserted to operating depth, it takes minimal pressure from the binder-bolt to hold it in place. I suspect that the true seatpost diameter you need is probably closer to 26.2 to 26.6mm.

OMG! Don't even think about welding.

If none of the solutions suggested above work, secure a hose clamp to the seatpost to keep it from sliding down farther than you want it to.

They come in all sizes. Marine grade clamps are stainless, and thus won't rust.

Bob

As always, check here:

https://sheldonbrown.com/seatpost-sizes-m-z.html#m

Take note that none of the Motobecane's in the '70's had that size seatpost.

I had this problem and the only solution I found was to take the seatpost out and put some tape round it

Thanks to all the helpful tips. This is a roadbike, not a mtb, I'm not even sure that was much of a thing in 1978, much less a motobecane thing. I was able to secure the correct size seatpost today. It's a 26.0 for those counting. It seems all is well now, and this should be the resolution. Gotta give props to DannoXYZ, who had the most practical tips. Hard to believe the shop would steer me wrong by 0.4 mm.

Either way I made out pretty well as I was able to pick up the correct seat post and found a new bottom bracket which solves all my Swiss BB problems and for only $2!