I have a carbon frame with a 31.8 seattube and I want to replace the seatpost.

My favorite seatpost that I have is a 31.6 carbon post. Will I have any problems using this in a 31.8 seattube. I have tried it in the frame and it moves about a little bit until the seat collar it nipped up. I am concerned that the small movement may damage the frame/post.

Or do I get a 27.2-31.8 shim and buy a 27.2 post to be on the safe side?

It's a bad idea.

make yourself a fizzy drinks can shim? just make sure it's trimmed back enough that you can't cut your legs on it.

27.2 in a 30mm frame…it is not a bad idea, it was advised by the designer……… 8)

Carlosg, I have thought about that and may give it a try but don't want a metal can shim dropping into the frame and not coming out.

ton, please expand on you comment 27.2 in a 30mm frame

got a 27.2 thomson in my new mm bop frame which takes a 30mm.
when used with a correct shim a thiner post offers a bit of comfy flex on a harsh stiff frame…

Time to find a shim then. CRC seem to do some cheap ones.
Thanks all for the help as I was a bit concerned about ruining a great frame but was hoping to be tight and not splash out on a new seatpost.

Carlosg, I have thought about that and may give it a try but don't want a metal can shim dropping into the frame and not coming out.

fold the top over a couple of times to form a lip?

or cut it neatly, very slightly trim the corners so that they are rounded and sandpaper all the edges. Have I done this before? . . . 😉

Oh, and a little 3M spray mount adhesive will stop the shim from sliding down the seat tube (easily loosened with GT85 once you can afford a proper sized post).

"A special shim from the private stock of Baron Alfred Krupp, who had to sell it at a great sacrifice."

You want a coke can 0.1mm thick?

"It's a bad idea. "
I know I wouldn't, especially being carbon