As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE FLIGHT ENGINE BEGAN TO RUN ROUGH AND MANIFOLD PRESSURE INCREASED. PILOT WAS ABLE TO RETURN TO THE HELIPAD AND MAKE A NORMAL LANDING. EXHAUST VALVE STEM WAS FOUND TO BE BROKEN AT THE BELL CURVE AND THE VALVE WAS IN THE CYLINDER BORE. SEVERE CYLINDER DAMAGE AND METAL CONTAMINATION OF THE OIL OCCURRED.