What was found
WHILE IN IFR FLIGHT, PILOT REPORTED LOSS OF MANIFOLD PRESSURE AND EXHAUST GAS TEMPERATURE INSTRUMENTATION READINGS. HOWEVER MP STABILIZED AT LOWER THAN NORMAL READINGS. MADE PRECAUTIONARY LANDING AT CLOSEST AIRPORT, WHICH MET IFR MINIMUMS. TECH PRESSURE CHECKED THE EXHAUST SYSTEM AND FOUND A 2.5 INCH HOLE AT THE EXHAUST DATA PLATE ON LT INSIDE EXHAUST RISER. THIS WAS NOT VISIBLE WITHOUT MIRROR. CAUSE WAS POSSIBLY BECAUSE OF UNEVEN HEATING AND COOLING OF PIPE UNDER THE DATA PLATE. WHICH IS WELDED TO THE PIPE. SUGGEST THAT THE DATA PLATE BE WELDED ON THE OUTSIDE OF PIPE TO BETTER INSPECT. OR ELIMINATE DATA PLATE AND ELECTRONICALLY ETCH INFORMATION ON TO THE RISER, AS DONE ON SOME JET ENGINE PARTS.