As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING 100HR COMPRESSION CHECK, AIR WAS FOUND TO BE LEAKING AROUND TOP SPARK PLUG AREA. FURTHER INSPECTION NOTED CRACK ALONG TOP COOLING FIN AREA. UPON REMOVING CYLINDER, CRACK WAS FOUND TO RUN FROM AREA AROUND TOP SPARKPLUG HOLE THROUGH EXHAUST SEAT AND THROUGH TO EXHAUST FLANGE MOUNTING STUD.