CosterVFW

Forgive me... I changed the rings with the new piston. I still have the old piston, and just this evening, noticed a significant diameter difference between the new piston/cylinder and the old. I am using the original head, so I think I've narrowed this problem down: head diameter will be too small, and explains my serious head pressure leak. I love these Chinese ATVs...

Daughters quad started blowing oil. Pulled top end, found piston and sleeve marred. Replaced piston and cylinder, upon verifying TDC using my thumb to hold pressure, notice loss of pressure against thumb while hearing a "bubbling" sound from inside the head. 1) While I had compression loss from old piston/cylinder. I did not notice compression loss from head. 2) All Top End gaskets are new. Head gasket is in excellent condition. 3) Head and Cylinder mating surfaces were prepared, cleaned & slightly sanded to ensure no foreign objects & smooth surface. 4) Did not use any type of extra gasket material (RTV, etc) on head gasket. Although, head gasket came with some sort of (rtv style) seal. Link to show example of this head gasket: Cylinder Head Gasket for Coolster Mountopz ATVs - $3.95 I am beyond frustrated... What am I missing?