seatpost diameter

david462 said:

i bought a size 268 seatpost for an old road bike. the original seat post did not have the diameter stamped on it so i kinda had to measure and guess (didnt have calipers i just used a ruler). turns out its barely too small. when i went to put it in, it looked like it was gonna work but the clamp didnt hold it. is it obvious that i need a 272 then? just making sure before i pay shipping charges.

The best thing to do is go to your local hardware store and buy a cheap pair of calipers. I think you can pick up a cheap pair for $15 or $20. This will give you the right size on all of the components that you purchase from now on. I really would be concerned about the seat tube diameter if I were you as 4mm is not much, and if the 268 was too small, there is a good chance that a 272 will be too small too.

david462 said:

i bought a size 268 seatpost for an old road bike. the original seat post did not have the diameter stamped on it so i kinda had to measure and guess (didnt have calipers i just used a ruler). turns out its barely too small. when i went to put it in, it looked like it was gonna work but the clamp didnt hold it. is it obvious that i need a 272 then? just making sure before i pay shipping charges.

Or measure the circumference of the old one using a strip of paper, and divide by pi (3.14). That should give you a close enough idea.

If its a common size, then its either going to be 27.2, 28.6, 30.9, or 31.8

Circumferences for each are:
27.2 => 85.5mm
28.6 => 89.8mm
30.9 => 97.1mm
31.8 => 99.9mm

n

hmm. i understand how to do unit conversions and all that. i am not buying calipers cause its a waste of money for me. i know the dimensions of everything else on the bike.

the seat post i bought only came in two sizes...268 and 272 and i figure 272 is a popular size.

but yeah when i put the 268 in i really thought it was the right size but then when i went to tighten the clamp it didnt hold it. theres really not any visible space between the post and the seat tube.

Sounds like you may need a 27.0...not a common size, but it's out there.

Measuring is the best way to make sure you get the right size, but maybe your frame is on this list:

http://www.sheldonbrown.com/seatpost-sizes.html

Uhl said:

Sounds like you may need a 27.0...not a common size, but it's out there.

Measuring is the best way to make sure you get the right size, but maybe your frame is on this list:

sheldonbrown.comseatpost sizes.htmlOpen ↗

could be i guess. its like an 84' schwinn prologue

david462 said:

i bought a size 268 seatpost for an old road bike. the original seat post did not have the diameter stamped on it so i kinda had to measure and guess (didnt have calipers i just used a ruler). turns out its barely too small. when i went to put it in, it looked like it was gonna work but the clamp didnt hold it. is it obvious that i need a 272 then? just making sure before i pay shipping charges.

I recall that someone observed that a soda can is 0.2mm thick (maybe it is, maybe it isn't). So, you may be able to MAKE A SHIM from a soda/beer can if you need a 27.0 or 27.2 seatpost ... it might seem a little dodgy, but it may work for you ... be VERY careful with the cut edges (round them off with some EMERY CLOTH or sandpaper ... rough up the smooth surfaces of the "sheet" of aluminum, too) ... a slightly BIGGER seatpost can sometimes fit due to the imprecision in the milling.

The other, "immediate" alternative is to go to your LBS and pay between $5-and-$10 for a shim ... call to see if they have any before showing up ... AND, BRING your bike & post otherwise you might end up with the wrong size, AGAIN!