OldIHC

Lucked out . Noisy start after changing spark plugs. Pulled head and found a broken piece of a spark plug washer stuck to squish chamber right above piston travel. Visible mark on piston but no gouge and nothing could be felt by hand.no marks on cylinder walls; looks like it never moved from that one position.
Waiting on new head gasket hoping all is quiet on restart.
BK

I was suspicious about this. This is not uncommon on flathead engines. Our local lawnmower shop get this several time a year.