Hormel

I've measured the stock seat post diameter and found it to be 31.0 mm +/- 0.5mm. I have found that seat post diameters available are very fine. The particular seat post I am looking at comes in 30.4, 30.8, 30.9, 31.6, 31.8 mm. The seat tube uses a seat post clamp to secure the seat post. I would think that clamp would change the diameter of the seat tube by about 1mm, but I could be wrong. My question is, how accurate do I have to measure my seat post in order for it to fit my bike?

I've tried to look up the seatpost diameter on bikepedia, but it doesn't have the diameter for my year model, and the later year models of diameters of 27.6mm, which I am sure is too small.

Specs:
Raleigh M20 mountain bike
Year: 2001 (at least it matches the pic in bikepedia)

FBinNY 

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.

cny-bikeman

Yes, you need to measure with vernier calipers, which can easily manage .05-.1 mm.

bobotech

You want a seat post that fits in the hole perfectly with nearly zero play. If I get a frame with a squished seat post clamp, I research it fully and then spread the clamp enough so that I can get a real idea if a new seat post fits in there with no play.

Any play will lead to rocking seat and possible frame damage. Tightening the clamp down super tight does NOT help at all.

Spld cyclist

Dial calipers measuring to 0.001" can be easily found in the $20-30 range. (metric ones are also available, but I just multiply by 25.4 to get mm). It's fun to measure things just for the sake of measuring. I'm such a nerd I have two.

rtz549

Find one of these sizing tools somewhere cheap:

https://www.bikepartsplace.com/discou...st-sizing-rod/

https://www.ebay.com/itm/Cyclo-Bike-T...item4ac4cff389

fietsbob

Best Fit.. As I've found... is a specific post, [slightly ] Oversize: and the seat tube reamed to it that particular post..

+ for Adjustable reamers

mass produced frames, may be coarser, less accurate, seat tube bores.. because of production volume..

GeoKrpan

I had a bike where the nominal size of the seatpost was 27.2 yet a 27.2 seatpost was too small, it would slide down. I solved it by getting a 27.4 seatpost.

oldbobcat

There's always one in the crowd.

GeoKrpan

Man, it was a lot easier than reaming or some hocus pocus grease AND it couldn't have fixed the problem better.

FBinNY 

Actually this is more common than you might imagine.

The tubing supplied by the mill is very accurate, but things happen.

The builder may ovalize the tube, making the major diameter larger, and minor smaller. Then the shop has to ream the tube to get a post to fit, but nothing brings the enlarged major diameter back to size and you end up with a sloppy fit. However the most common issue is a shop mechanic who gets carried away and reams the tube oversize.

If you're lucky, the next size up is close enough, but usually it isn't and folks live with sloppy creaky seatposts and there's little that can be done.

I lump overly reamed seat tubes in the same category as over faced BBs, where the record that I know about is a 68mm BB faced down to 63mm.

FBinNY 

I can imagine it would be, since you say the "correct" post was too small and a larger one was needed. There's no way to ream a hole smaller, and grease that makes things tighter hasn't been invented yet. (rhetorical, so don't post about carbon assembly paste)

techsensei

Maybe that person was trying to reduce the Q factor.

operator

I can think of all sorts of interpretations taking this quote out of context. Aw yeah

FBinNY 

I like to set up straight lines so the next guy can get the rim shot.

FBinNY 

Actually, I spoke to the mechanic. He was having problems with chatter marks, and was trying to get a smooth finish, which BTW, he never did.

oldbobcat

I'm sure. That's why I always measure before inserting. Sometimes with the caliper as well as the cylindrical tool. And if I use the caliper, lengthwise and crosswise.