EMERGENCY DECLARED DUE TO VERY ROUGH ENGINE.

Narrative

ON A VFR FLT FROM PHL TO REEDSVILLE MIFFLIN CO OUR ENG BEGAN TO SUDDENLY RUN ROUGH; I TRIED THE PROCS ON MY EMER CHKLIST; MAGS; CARB HEAT; CHANGE TANKS; BOOST PUMP SET. AT FULL PWR I COULD MAINTAIN ALT AND 92 KTS AIRSPD. I CALLED LANCASTER TWR AND NOTIFIED THEM OF MY PROB AND THEY CALLED READING ATC AND GOT A TRANSPONDER CODE FOR ME TO SQUAWK. WITH THIS I WAS INFORMED THAT I WAS 12 MI SE OF LANCASTER ARPT. THIS WAS ALSO THE VOR I WAS TRACKING TO. I TOLD THE TWR OUR OIL PRESSURE WAS OK AND OIL TEMP WAS NOT CLBING; BUT I THOUGHT I LOST A VALVE AND OUR ENG WAS HAMMERING PRETTY BADLY. AT 6 MI OUT LANCASTER CHKED ON MY FORWARD VIS. I TOLD THEM WITH THE SUN IN MY FACE I COULDN'T SEE ANYTHING BEYOND A MILE. THEY TURNED THE RWY LIGHTS UP AND CLRED ME FOR A STRAIGHT IN ON RWY 26. I DIDN'T WANT TO DESCENDED TO A LOWER ALT UNTIL I HAD THE RWY IN SIGHT; SO AT ABOUT 1 MI OUT AND THEN OVER THE END OF RWY 26 I TOLD THE TWR I WAS GOING TO MAKE A DSNDING RIGHT TURN TO FINAL AND LANDED. THE #3 CYLINDER WAS FOUND TO HAVE NO COMPRESSION AND A BROKEN EXHAUST VALVE. THERE WAS NO PISTON DAMAGE AND NO OIL LOSS. I CALLED THE TWR AND THANKED THEM FOR ALL THEIR HELP.