DURING AN IFR FLT, THE ACFT WAS LEVELED AT 13,000 FT & THE POWER WAS SET AT 18 INCHES MANIFOLD PRESSURE & 2300 RPM. APRX 5 MIN LATER, THE RIGHT ENG BEGAN TO SURGE TO 3000 RPM & ALMOST 'PEGGED.' THE PLT ATTEMPTED TO CONTROL THE RIGHT PROPELLER. FINALLY, THE RIGHT ENG WENT TO 2300 RPM & STABILIZED THERE. THE PLT INFORMED ATC & BEGAN AN IMMEDIATE DESCENT TO THE NEAREST ARPT. AT ABOUT 7000 FT, THE LEFT ENG BEGAN RUNNING ROUGH & PROGRESSIVELY BECAME WORSE. AS THE LEFT ENG RPM DECREASED TO ABOUT 1500, THE LEFT PROPELLER WAS FEATHERED. AT ABOUT THE SAME TIME, THE RIGHT ENG SURGED AGAIN & THE RIGHT PROPELLER WAS SUBSEQUENTLY FEATHERED. AFTER DESCENDING BELOW THE OVERCAST, A FORCED LANDING WAS MADE IN A BLACKBERRY PATCH & THE RIGHT MAIN GEAR WAS DAMAGED. AN EXAM REVEALED THE NUT THAT HELD THE RIGHT PROP, SPINNER BAFFLE HAD BACKED OFF. IT HAD WORN GROOVES IN THE AIRVALVE WHICH ALLOWED THE PRE-LOAD AIR PRESSURE IN THE HUB TO RELEASE. ALSO, THE PRE-LOAD AIR PRESSURE ON THE LEFT PROP WAS FOUND AT 45 PSI, BUT SHOULD HAVE BEEN 70 PSI.