On certain well brazed frames, no distortion of the seat tube results and an ordinary size post (27.2 mm for Reynolds 531 and 27.0 for Columbus SL) does not clamp well. For 531, you are then in pursuit of a 27.4 which, to quote our British listees, is close to rocking horse manure.
Having had a few sessions of sliding down seat post on the Dave Tesch I brought to Cirque, I resolved to solve the problem today. I eventually figured out the seat post ears were meeting solid.
The trick to opening up the slot is to use TWO hacksaw blades in the hacksaw frame, side by side. Works like a charm. After opening up the slot between the ears, grease the slot to prevent rust.
Joe Bender-Zanoni
Great Notch, NJ