Find out seatpost size?

Page may contain affiliate links. Please see terms for details.

bigfella

Über Member
Location
Essex
I have used a vernier calliper to measure the internal diameter of the seatpost of my old racing bike project. It seems to vary from 25.2mm to 25.5mm.

What could be the correct size? Was there any standard so I know which one to buy?
 
I very strongly suspect you are looking at an inch. (25.4mm)
 

brodie

New Member
Depends on the tubing. For a steel seat tube in standard 28.6mm diameter in a good quality tubing like 531, the most common seatpin size should be 27.2mm, but there are other sizes like 26.8mm etc. Old alu frames like the Alan or Vitus which had seat tubes in standard 28.6mm diameter but had thicker walls used 25mm seatpins.
 

Big John

Guru
I've found that doing an accurate measure is virtually impossible. I always take my old seat post into the shop if I'm after a new one and say "I want one this size please". Usually works. Or........they check it against another seat tube, tell you the right size, you say you'll think about it, then buy one off the net. Now I would NEVER do a thing like that ;O)
 

e-rider

crappy member
Location
South West
From the original post it sounds like you're measuring the internal diameter of the seatpost! You should try measuring the external diameter of the seatpost or the internal diameter of the SEAT-TUBE of the frame.
 

tyred

Legendary Member
Location
Ireland
Does sound like it is a one inch post that you need but if you're not sure, the easiest way (if it's convenient for you do so) is to take the frame to an LBS and get one that fits.
 
OP
OP
B

bigfella

Über Member
Location
Essex
tundragumski said:
From the original post it sounds like you're measuring the internal diameter of the seatpost! You should try measuring the external diameter of the seatpost or the internal diameter of the SEAT-TUBE of the frame.


What you take me for! The frame came with no seatpost and I used a vernier caliper to measure the internal diameter of the seat tube.

:biggrin:
 

dodgy

Guest
bigfella said:
What you take me for! The frame came with no seatpost and I used a vernier caliper to measure the internal diameter of the seat tube.

:tongue:

But you did say "I have used a vernier calliper to measure the internal diameter of the seatpost of my old racing bike project"

You probably meant seattube :biggrin:
 
OP
OP
B

bigfella

Über Member
Location
Essex
Yeah yeah!

You knew what I meant!
 

e-rider

crappy member
Location
South West
so, now we have that sorted it could be either:

25.0
25.4
25.8
26.0
26.2

25.8 and 26.0 will be very difficult to find these days - maybe impossible.

25.4 is still quite common and the most likely according to your measurements
 

dodgy

Guest
bigfella said:
Yeah yeah!

You knew what I meant!

I can honestly say, I didn't know what you meant. I just thought you were confused how to measure for a new seatpost.
But you've clarified it now, so don't get stressed :biggrin:

Got me doing it now, I just said seattube when I meant seatpost! Ahhh! :tongue:
 
bigfella said:
I have used a vernier calliper to measure the internal diameter of the seatpost of my old racing bike project. It seems to vary from 25.2mm to 25.5mm.

Hmm. A difference of 0.3mm. There is a pencil on the desk with 0.5mm lead in it, not a lot is it? It is tricky to measure accurately internal diameters with a vernier, holding it at 90 degrees to the tube and applying the same finger pressure for each measurement. If the tube is perfectly round as well. It has been subjected to heat when being built so there may be a tad difference there.
Personally I wouldn't fret too much over what it is to that degree, as long as it goes into the seat tube with using brute force and doesn't rattle around when its in there.
 

Gerry Attrick

Lincolnshire Mountain Rescue Consultant
It's complicated because the seat tube without a post is unlikely to be exactly round due to the pinch bolt squeezing it out of shape. Then if you take out the pinch bolt you end up with a duff reading. Strongly suggest trying likely seat posts to get a good fit. Don't try and make do with a nearest is good enough, you will cream cracker the frame or yourself.
 
Top Bottom