Confused about seatpost diameters

Your premise is a little floored. Both my Reynolds tubesets have seatpost diameters smaller than I had somehow ended up believing they would be. Unless your builder is able to ream material back into the tube, rather than take it away.

I wasn't talking about your ridiculous notion that 531 is always 27.2 I was saying that the idea that a tubeset always takes the same diameter seatpost is wrong as it can be made to whatever size the tubeset can be machined to.

but whilst you brought it up a butted section of 531 may be well below 26mm internal, so if cut at that point the builder would not have to 'ream material back into the tube' as you put it.

Your original notion that 531 is always 27.2 was wrong, except it, understand the reasoning, and go back to whatever you where doing last week before you discovered 'fixies'