As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LOWER SPARK PLUG HELICOIL BLEW OUT ON NR 3 CYLINDER RESULTING IN LOSS OF THAT CYLINDER COMRPESSION AND LOSS OF ENGINE POWER. AIRCRAFT LANDED ON HIGHWAY WITHOUT INCIDENT. SUSPECT CAUSE: OVERTORQUE OR UNDERTORQUE OF SPARK PLUG ALLOWING PLUG TO MOVE IF UNDERTORQUED, OR PULLED THREADS IF OVERTORQUED. THIS ENGINE HAS STC SE792NW AND SA1008NW - HIGHER COMPRESSION PISTONS AND WOULD AGGRAVATE THE SUGGESTED CAUSE.