I had built a couple of fillet-brazed 853 frames back in the mid 80’s with 853 seat tubes (externally butted, no less) and couldn’t get a reamer to work. The 27.2 id seat tubes ended up taking 27.0 posts…
Ever since then, anything 853 and fillet brazed in my shop gets a 725 seat tube. But with lugged and silver brazed 853 frames, no problems with 853 seat tubes and reamers, I guess silver braze doesn’t get the steel to the point where it air-hardens. In fact, I reamed the seat tube and cut the binder slot on my latest build literally yesterday - 853 Pro Team and stainless Llewellyn Manorina lugs - and no issues.
Mark Beaver
Tamarack Cycles
halifax, NS, Canada