Originally Posted by
base2
You want to use the proper sized seat post. It's not so much the clamp you want structurally sound, it's the moving around & flexing you want to keep in-check. Too small & the result is like flexing a paperclip with every turn of the pedals.
If you can, & you have a snap-gage or a ball-gage & measure the internal diameter & go from there. One good measurement is better than a thousand guesses.
Honing, reaming, or very careful spot treatment with an abrasive & a drill or spoon/round file may be required.
I agree with the snap gauge. If you don't have one, a local machine shop should be able to measure rather quickly, hopefully for free.
Just be sure to clean the I.D. thoroughly before attempting a measurement.
Or you could probably measure the seat tube OD circumference and deduct the wall thickness x2.