AIRSPUR FLT 500, A DOMESTIC CARGO FLT, DEPARTED KALISPELL, MT, AT 1903 MDT WITH AN IFR FLT PLAN TO GREAT FALLS, MT. AT 1922, THE PLT NOTIFIED SALT LAKE ARTCC HE WAS RETURNING TO KALISPELL DUE TO A ROUGH RUNNING ENG. SHORTLY THEREAFTER, THE PLT REPORTED THE ENG HAD FAILED COMPLETELY. THE PLT MADE AN EMERG FORCED LANDING ON A MOUNTAIN RIDGE AT THE APRX 5500 FT LEVEL IN IFR CONDITIONS. ACCIDENT LOCATION WAS APRX 18 NM EAST OF KALISPELL. ENG DISASSEMBLY LATER REVEALED THE #4 CONNECTING ROD HAD FAILED CATASTROPHICALLY.