Framebuilders - Reaming Seat Tube 26.6 to 27.2

How do you determine if a seat tube can be safely reamed to a larger size?

Good question, how thick is the tube? and what size do you want to make it? Is there a big thick lug at the topend??

I haven't done the measuring, but if a straight wall 4130 is .9 mm, and the thin middle on standard butted is .6, and if those are the same dimensions on a single butted seat post, then you could safely remove .3 wall thickness from a .9, but not from the single butted tube if that reduced it to .3mm.

The other part of this is can you ream out that much material with a reamer. That is also a little difficult. I have used chucking reamers to reduce tubing by about 7 thou, or about .2 mm. It isn't difficult to make happen, though technically reamers are supposed to be used to remove just a few thou. That can't be true of all reamers since chambering reamers are quite agressive. Still that is what I have heard said from a number of sources. Seat tube reamers are not designed to open up tubing from one size to the next. They ae designed to restore smooth surfaces to your SP after brazing. That is why you can get seat posts in so many sizes. I use a 26.8 for some applications, you can get a 26.6 from a bike shop.

I could have mentioned that if you do want to take the reamer to it, you might consider an adjustible reamer of normal format like the ones sold here, Hand Expansion Reamers:

http://www.victornet.com/cgi-bin/victor/index.html

That way at least you don't put the wear on a specialty bike tool, though you can use these instead of seat tube reamers for that mater.

I originally wanted to ream the ST to 27.2 in order to get a carbon post for the bike.

I forgot to measure the wall thickness when I was at the shop, but I better understand the purpose for reaming now. Thanks guys. Great link, Peter.