Seat Post Clamp Diameter: The internal diameter of the seat clamp must correspond with the external diameter of the seat tube – too big, and the seat post may move in the frame, with the risk of cracking; too small and the collar can’t fit on the top of the tube (and you might damage your frame in trying to force it on). There are five standard seat tube diameters on the market: 28.6mm, 30.0mm, 31.8mm, 34.9mm, and 36.4mm, so once you know the size of your seat tube (measure with a calipers or check your manufacturer’s specifications) make sure to buy a seatclamp that matches.