L
Looked like an old type piston to me with the radial ribbing underneath the head of the piston. Specialloid?What make of piston was it ?, There have been some bad ones, Not good for Vin's.
Always run a bit on the rich side, To keep cool.
Run bigger clearance than the makers say.
Use the highest octane petrol you can get.
Poor old Comets , Have a hard life !.
Been out 7 times this year on mine, Love it to bits, But it's Knackered, Like me.
Cheers Bill.