What was found
DIPSTICK DEPARTED AIRCRAFT ON FINAL. MAINTENANCE UNCOWLED ENGINE TO PERFORM COMPRESSION TEST. ENGINE CRANKCASE HAD ERUPTED BETWEEN NR 1 AND NR 2 CYLINDER. BOTH MAGNETOS TORN FROM MOUNTING PADS NR 1 AND NR 2 CYLINDER ROD CAPS AND ROD BOLTS SHEARED FROM RODS AND 1 COUNTERWEIGHT DETACHED FROM CRANKSHAFT. SUSPECT CAUSE: COUNTERWEIGHT PIN FAILURE. ENGINE MANUFACTURER NOTIFED AND REP TO EXAMINE ENGINE TEAR DOWN INSPECTION REQUESTED BY OWNER, OPERATOR, AND MAINTENANCE.