Hi,
Often, when you have a contact between exhaust valve and piston, you have also a problem with camshaft bearing cap dowel.
Check, one by one, the state of bearing cap.
Sometime it is necessary to repair thread dowel with helicoil system.

Check torque setting :
6 mm. dowel : 10 Nm.
8 mm. dowel : 20 Nm.

Regards.
Claude