Did a full tune up on a 5 L. (carbed) and took boat out for a quick water test before putting on CL. Was running a bit over 4K RPM, 45 PSI on the oil, temps where they belonged. It started making the loudest racket imaginable and I grabbed the throttle and pulled back. When RPMS dropped they got to maybe 2K and WHAM, engine locked tight. Sounded / felt like engine was coming out of the boat! Went home and pulled outdrive and it appears to be fine, engine is locked like it was forged as a solid piece. Breaker bar and pipe extension fails to even budge it. No clue what happened but WOW. Never seen / heard an engine just disinigrate with no warning and running so well.
