Originally Posted by
Bigbus
When the clamp is tightened properly, check the gap in the seat tube. I've seen instances where the gap is closed and the seat post is still loose. If that's the case, find a larger seat post or create more gap. Good luck
+1 and my first thought. There should be room for (say) a sheet of thin cardboard between the inside faces of the clamp at the bolt when tight. If not, file or sand or put in a vise and saw with two blades on a hacksaw those insides and try again. Or get another clamp. (I learned about the 2-blade trick from a framebuilder. This happens on steel frames as well.)
Edit: Another approach that has been done. Clamp as you have been doing but add another smaller clamp to the seapost itself and it slid down to contact seatpost or clamp top.
Second edit: You have calipers. Try measuring the seattube inside. (Take a bunch of measurements, Inside diameters are tough to get accurately and tubes may well not be round after welding.) The mickey mouse fix if the tube is too large for the seatpost, sheet aluminum shims. I used DIet Coke can shims on my namesake for around 15 years running an MTB seatpost n my 27.2 seattube.