I have a Merlin Malt-1 from 2003 and I want to replace the seatpost with one which has a decent clamp. I measured the post and seat tube: 31.4 as far as I can tell.

Nearest standard size seems to be 31.6, so I ordered a nice Thomson one. But it doesn’t fit – too big. Duh!

So I order the next size down: 30.9. Too loose. I have to really crank down on the seatpost clamp to hold it anything like firmly and even then I can turn it with my bare hands.

Am I just being a wimp, and should just crank it down harder, or is there an intermediate size I ought to know about before I damage something? Alternatively, where can I get a shim that’s 0.2mm thick? (How thick is a coke can?)

You need to get a 31.4mm post

Problem solved. Coke can sidewalls are 0.2mm thick. With the top rolled over to stop it disappearing inside the seat tube and hidden under the clamp, it makes a perfect shim 🙂