Use a steel rule to make sure it's not deformed by over tightening. They are simple to correct with a brass drift and a small hammer. You can test the 'nip' when refitting with a new gasket by cutting a strip of one inch wide cellulose (something like overhead projector acetate) and trapping it between the cork gasket and the head at the suspect area, it should grip the cellulose/acetate before the screw is fully tightened. A little patience and you can sort it.
