Bicycle Mechanics - need help with seatpost size

Crap. I want to order a seatpost online but I'm not sure what the size is, a 27.0 or 26.8. I'm pretty sure it's 250mm (about 9 2/3 inches) long but it's difficult to measure the width because the difference is so small. Is there any way I can measure it accurately at home? I currently have a Thomson Elite on my Waterford road bike. When I use the measuring tape, it comes to about one inch and one millimeter, measuring from the outsides. Hmm..anyone know of a better way to measure this thing or will I have to take it to a shop? Thx. :o

Most of them are marked.

If you look carefully on the seatpost the size is often marked on it. It's just difficult to see if the post is greasy. If you clean the post carefully you may find it. It is usually near the min insertion line.

26.8 if 1 inch, 1 mm. 27 puts it @ 1 inch, 2 mm+ a hair.

Are you sure there are no size no# stamped, all posts i've seen are marked.

What you need is a calliper, not expensive and pretty usefull for measuring tubes or any small increments.

I just did some quick measures to agree to 26.8. Not a Fact, I'm relying that you measured the tube right.

>jef.

Do you have another frame that takes 26.8 or 27.0 ? Easy way to find out.
, slide out the old, look for the size#, then, see if the Tompson fits.

>jef.

Thanks, you guys, for the suggestions. I can't tell if there's a marking on the Thomson because (and I forgot to mention it) the bottom half is too corroded (it got stuck into the frame a while ago and had to be wrenched out, hence why I'm getting the new post). But after reading jeff's idea I did try the seatpost from my other bike and it fits the Waterford frame perfectly, and that seatpost is clearly marked 26.8. :D Thanks again!

I feel your pain. My Gitane TDF seatpost was rusty with no hint of size markings. Resources said 26.4 so I tried my wifes seatpost from her gitane and it swam in my seat tube- too small. I have a Bianchi with a marked 26.6 that was a tight fit. Bought a 26.6 Shimano post and after some cleaning of seat tube it fit perfect. A bike shop should be able to help size it. If you have access to other bikes with marked seat post try them. Order a few cheap post from somewhere that can be returned and try the smaller one first (so it won't be damaged) to see which you need. I couldn't believe what a difference .2mm would make. If you're buying from somewhere like E-bay you want to be right because you'll probably be stuck with whatever you get.

Good Luck, Bob

Hey I came across the Sheldon Brown Seatpost Database http://www.sheldonbrown.com/seatpost_sizes.html

I was looking for the size for an old Peugeot curb find.

Of course any shop shoold be able to measure either the inside of the tube or the outside of a post that fits.