What are typical seatpost tolerances?

stevecoh1 said:

I'm riding a 12 year old Specialized Sirrus road bike. I'm a bit overweight, but hey, cycling is one way I try to stay on top of that. I like the bike well enough except for a seatpost that won't stay put. It always wants to slide down, slowly and if I have to stop short and dismount in a funny position I can easily wiggle it from side to side.

Several years ago, when I decided I wanted to raise the original equipment seatpost (5 inches long at most) higher than it wanted to go, I went out and bought another. It was a Zoom and it did the job for awhile. This year, however, the seat would no longer stay up. I broke the clamp trying to overtighten it. I just bought a new clamp, but really, the problem has persisted with the new clamp.

Today, I resolved to do something about it. In an earlier life I was a machinist and I still have some of my measuring tools. Most of the seatposts in catalogs seem to be 27.2 mm. (1.0708"😉. How did mine measure up to this? Hmm, only about (1.060"😉 which might make it a 27.0 mm. I still have the orgiinal and it, too, measures about 1.060". Did I buy the wrong thing years ago? Or has the seat tube widened over the years? I also removed the seatpost and tried tightening the clamp as tight as I could. It measured 1.065", so even at its tightest it looks like there's .005" of slop.

So here is my question. Is it going to be worth my while to buy a new seatpost? What, typically are the tolerances on seatposts? I can't find this information anywhere. If it's + or - .005", it might not do me much good. If it's more like + - .001", then that could be my answer. Does anybody know? Does this tend to vary from cheaper to more expensive? Anyone have other ideas?

Size should be marked on the original post. If it's 27.0 you can't use a 27.2, and post come in .2 increments.Both are common enough sizes. You could try roughing up the post or seattube interior with coarse sandpaper.You could also try a pop can shim. Tightening clamp with no post inserted is a no-no.

boudreaux said:

Size should be marked on the original post. If it's 27.0 you can't use a 27.2, and post come in .2 increments.Both are common enough sizes. You could try roughing up the post or seattube interior with coarse sandpaper.You could also try a pop can shim. Tightening clamp with no post inserted is a no-no.

Hmm, thanks for the advice. Original post was a 27.0. I don't see 27.0 advertised anywhere. In Performance they're mostly 27.2 with some 26.8s. I had thought that because I could measure > 27.2 at the top of the tube that I would be okay with a 27.2, but, of course, thinking about it, that doesn't measure the diameter further down the tube. So you have saved me from ordering something that probably wouldn't have worked anyway. On your advice, I have now tried the pop-can-shim trick. I got the shim in and was able to tighten on it. We shall see how it works!