From most likely to least...

Electrode fell off
Broken piston ring
Broken piston
Engine ingestion

Each have their own reason for happening.

"john collins" wrote in message
...
Had a tuneup on my 18 ft 1999 Larson 3 liter volvopenta and after 5 min.
in
the water had banging noise. Dealer took engine apart and found broken
ground electrode in engine and a scored cyilinder wall. Comments please??