As above please hes forgot what size..
Im guessing 31.9 ! ? LOL

If I remember right it's 31.6mm.

31.6, the only sizes you'll ever really see ending in '.9' are 30.9 (Spesh, some Giants and various other brands) and 34.9 (Scott carbon bikes only).

Some treks have a 30.9 post, which confuses matters, but being a full susser the Liquid is 31.6mm.

No they don't. Absoultely none, and never have.

Some of the 4000 series had an odd size, 28.6 I think, in about 2002, and early models were often 27.2, but all the more expensive aluminium models and all the carbon models, at least since 2000 have all been 31.6

Well the one in my 2003 Liquid 30 is 31.6mm & I guess they didn't change the size of seat tube.

Thank You all 31.6 it is then. Ill pass it on as i think he will be selling it on soon as hes just got a SC nomad (lucky git). Cheerz all