As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ENGINE STARTED TO RUN ROUGH IN FLIGHT, THE STUDENT BROUGHT THE PLANE BACK. START UP AND RUN UP REVEALED A DEAD CYLINDER. VISUAL INSPECTION REVEALED A CRACK GOING FROM ONE SPARK PLUG HOLE TO WITHIN AN INCH OF THE OTHER AROUND THE CIRCUMFERENCE OF THE HEAD JUST ABOVE THE CYLINDER. THIS IS THE THIRD CYLINDER TO CRACK ON THIS ENG. PROBABLY CAUSED BY A DEFECTIVE CASTING PROCEDURE. RECOMMEND GOING BACK TO THE OLD WAY OF CASTING CYLINDER HEADS.