Originally Posted by drroebuck
Probably a stupid question but want to make sure I order the right thing. If I have a 32mm seat tube, should I order a 31.6mm seatpost?
Unfortunately, you can't always just go by the seat tube OD because tubeing wall thicknesses may vary by a small amount and, with seat posts, every tenth of a millimeter counts.
Do you still have the old seatpost? If you do, the quick, dirty way to know for sure is to look for a size stamped on the bottom. Another way is to take a caliper and carefully measure the old seatpost. You're measuring to a tenth of a millimeter so you have to be pretty accurate. If you don't have a caliper, wrap a piece of paper around the seatpost, mark the circumference, measure carefully with a ruler and divide by 3.12.
If you don't have the old seatpost you can measure the inside diameter with a caliper. Measure it in two or three diffferent directions and average the result. There actually are some slightly conical rods for measuring seat tubes, but few shops have them. Finally, you can contact the frame manufacturer and see if you can find somebody to look up the original specs.
As a last resort, you can order two or three different sizes and return the ones that don't fit.