Seat post size
 

I am going to use a Reynolds 631 seat tube 31.75mm diameter double butted 0.8/0.5/0.8 The nominal internal diameter of the seat tube is 30.15mm

It will be quite a large frame so there will be plenty of thicker butted section at the top to support the seatpost.

Build will be silver brazed lugs.


I am trying to source a seatpost and am not sure what range of sizes I could use. None of the sizes seem common so the more choice the better.

Would a 30.0 mm post fit? I know they have to be slightly smaller than the seat tube hole but would this work. It would be handy for me as I already have an adjustable reamer that will reach about 30.1
What would be the maximum size? Could I ream out nearly all of the butting to fit a 30.6 seatpost? Would this compromise the strength? I am aware people use single butted seat tubes 0.5 thick which makes me think it is fine, but then I also read you should use an externally butted tube.
Would 30.2 or 30.4 be a better compromise.

Not keen on using shims!

Thanks

Mike

Typically you want the seat tube ID to be about .2mm larger than the post size. You can ream to various sizes, even removing all the .8mm butt. Whatever you decide just be sure to check for available seat posts. There aren't many 30.0mm or 30.6mm seat posts around other than a Thomson and some elchepo's, but anything from 30.0 to 30.6 should be viable.

Not sure i'd want to shave a ST top wall down to .6mm if the stays were attached to the ST directly. But we don't know about this aspect yet, or whether the joining will be brazed or welded. Andy

Thankyou for confirming what I hoped to be the case.

I will aim at 30mm and start with an 'elcheapo ' post. I can upgrade or make my own post at a later date and when I know the fit is good.

Relax, Machined seat post sizing Shims work very well ,
I now have 3 bikes , over sized seat tubes, with sizing shims
to use common 27.2 seatposts , 2 are cane creek Thudbuster seat posts..

in fact one of them, 3rd example; a Bike Friday Tikit,
I took out the 27.2 Kalloy Uno
seat post, head slipped angle too easily..
and reused my 25.0 <C> SR* with a 25>27.2 USE seat post shim,
so its a Sleeve inside a sleeve, inside the seat tube.

* came out of my screwed & Glued AlAn..