Determine Seat post size from seat tube outer diameter?

Question

I'm tinkering with a welded steel frame beater bike which has an undersize seat post and a home-made thick copper shim. 
I'd like to replace these with a correctly sized seat post instead.
Trouble is, the shim is glued in, so I just can't pop it out and measure the I.D. of the seat tube.
I'm confident I can eventually remove the shim and clean out the seat post, but to limit the bike's down time I'd like to have the replacement part already available when I start that work.
Seat tube outer diameter looks to be 25.0 mm by Vernier caliper. Based on this, is it possible to say which diameter seat post the bike would need?

WleAtl-2 · Answer

Seat tube outer diameter looks to be 25.0 mm by Vernier caliper. Based on this, is it possible to say which diameter seat post the bike would need?
==not from outside
unless you can measure tube thickness

==why not use same as shimmed diameter?

eventually you have to get it out

or if you know something about the frame you may find someone who 'just knows'

or not
you said it was generic

maybe they were all the same..

maybe you can measure shim thickness
or take part of it out and measure that part, still use current seat

edit
i agree a 25mm outer seat tube seems very small
you probably are not measuring it right

wle

bikeworks · Answer

Wha... you think your local shop doesn't carry a large variety of seat post diameters, or maybe you can't wait the day or so to have them order the correct one?

There is no standard frame tube size. The only thing I might be able to suggest is to measure the wall of the tube, then subtract twice the wall thickness from the outside diameter. This will probably NOT be precise enough for a properly fitting seat post.

Mtrlpqbiker · Answer

Is it the seat tube that is 25.0mm in diameter? That is very very small, would require a seat post that is about 4 mm smaller. What bike do you have?

Determine Seat post size from seat tube outer diameter?

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