A slew of things to look at. The other cylinders look like they have good ring seal. Would like to know more about the pistons, ring setup, spark plugs used (heat range), timing, ignition system and fuel system/type of fuel. Carb(s), EFI, added water injection? Would like to see that piston and spark plug when they come out. Looks like maybe too much timing and detonation from bad ring seal/oil in that hole for starters.
