Bicycle Mechanics - Determining Seatpost diameter

Hello,
I picked up an old MTB frame (16.5" 1992 TREK 970) and I need a seatpost. The 27.2 seatpost I have is way too small.

What is the best way to determine the correct size?

thanks!
-Mo

Measure the inside diameter at the clamp.Or, have a big box full of all size posts and try till you puke or find one that is the right size.Or hope someone here has the same frame and can pull theirs and read you the size that is on it.

This may not be the most practical way of finding it out, but if their is a machine shop or Tool and Die shop close to where you live or work, take your bike there and it could be measured by a machinist with a dial caliper. (Like I said not the most practical).

If you don't want to do that, you could buy a plastic dial caliper for I'm guessing around $20 bucks. Make sure it measures inside diameter. (I.D.)

Or just take your bike to the LBS and try various posts until you find one that fits! :D Just my 2¢...

Seat Post diameters run in .2mm size increments. I bit difficult to measure inside tube diameter with a caliper to that fine a tolerence (I know, I've had to do it). It will get you in the ball park but may not be precise. I found a gentleman on the Web who has a 1992 Trek 970 and sent him an e-mail query for Seat Post Diameter. Will forward info if he responds in a timely manor. Best of luck building up your Trek frame!

go to sheldon brown's site-he has a seatpost diameter database

Originally posted by faith
Seat Post diameters run in .2mm size increments. I bit difficult to measure inside tube diameter with a caliper to that fine a tolerence (I know, I've had to do it). It will get you in the ball park but may not be precise. I found a gentleman on the Web who has a 1992 Trek 970 and sent him an e-mail query for Seat Post Diameter. Will forward info if he responds in a timely manor. Best of luck building up your Trek frame! ............. Not to be contrary,but I have and old trek frame sitting in the basement that takes a 272 post, and that is what it measures.That is what good calipers are for.one pitfall is that if the clamp has been tightened with out a post in the frame, the measuerment lwill likely not be accurate, and even the correct size will be a too tight fit.

Trek publishes a record of specs for its bikes through the 1990's.
Your local Trek store should be able to look it up. Also you could e-mail Trek. They'd help.

Nice frame. Have fun!

29.8 in case you still need the size

Good dial calipers can easily measure 0.001"
The good digital ones to .0002" / 0.005mm - http://www.mitutoyo.com/cat2000a/r96-97/index_1.htm