Reply To: 1971 Evinrude Ski-Twin 40hp spark plug gap closed

[Don]

The only other thing that comes to mind is a worn crankpin or wrist pin bearing creating extra slop allowing the piston to rise higher and crush the gap.  It is easy enough to check for piston/rod slop though without pulling the head/covers.  Usually you can hear the knocking noise at idle, sometimes goes away when the throttle is raised though.