Originally Posted by
Hormel
I've measured the stock seat post diameter and found it to be 31.0 mm +/- 0.5mm. ..... My question is, how accurate do I have to measure my seat post in order for it to fit my bike?
Ideally, you want to measure a post to within ±0.1mm. You're right that the clamp can pinch down quite a bit. the slot is about 3mm, and can close about half that, so that's 1.5mm of circumference or about .5mm in diameter. But the post needs to be a snug running fit into the tube below the clamp or it's rock below the place where it as clamped.
Oversized posts simply can't be inserted below the slot, and undersized ones rock and creak, so you want the right size.
BTW- as a frame of reference, the actual tolerance for fit is 0.05mm which is a full order of precision greater than what you are measuring now.