Originally Posted by
bikingman
dsaul - Mind expanded - I just realized that I've not thought about butted tubes in relation to the cuts. And I don't know why, because that seems awfully obvious now...
Looking into externally butted seat tubes, I don't see one explicitly listed on Nova Supply, but I do see one that could work on Framebuilders Supply. I'm assuming that the external butt should be positioned at the top - but I'm a little worried about how that will fit together since I'm planning to use a lug between the ST/TT. It looks like the OD is a little larger than the lug ID. Perhaps a doubled butted tube intended as a ST, or a single or straight gauge tube would be a better choice?
Potential ST:
https://framebuildersupply.com/colle...1-2-length-620
It wasn't clear that you were using lugs, so the external butted seat tube is not an option. As already noted by others, a single butted seat tube with a .6mm wall at the top is usually used with a seat lug. I agree with Andy that the seatstay placement you have planned is a bad idea.