xiaoman1 

Some have inserted a large, flat screwdriver or small pry bar into the rear slot to spread the top of the seat tube at the clamp and this can also bend back out the clamp rings which may no longer be flush. This can be a tricky operation if the metal is already fatigued and cracked. You could ruin a seat lug, so it isn't without risk. When the sides are too narrow and the rear is elongated, I have taken a hammer to the clamp with sacrificial 6mm binder bolt inserted and nutted and used a flat, dull chisel to strike the pin to flatten the rear seat lug area a little to get it back into round with success.

Check to see if round not oval ....if oval use 2 screwdrivers inserted into the slot at the top tube and "gently" open up the tube. 1 driver will often times close the opposite side of the tube causing more distortion. open a little and check....open and check...dot try to get it all the first time.
JM2C's
Ben