AN 998.30 DECLARED AN EMER DUE TO ENG FAILURE CAUSED BY A BROKEN OIL FITTING BTWN THE ENG AND OIL PRESSURE XMITTER.

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ACFT WAS FLYING FROM ZZZ1 TO ZZZ2. 20 MI N OF ZZZ AN ENG VIBRATION WAS NOTICED BY PLT AND PAX. PRECAUTIONARY LNDG WAS MADE AT ZZZ. PLT CALLED ME AND I SUGGESTED CHANGING LOWER SPARK PLUGS. AFTER THE CHANGE; ENG RAN SMOOTH AND FLT WAS CONTINUED TO ZZZ3. NEAR ZZZ3; OIL PRESSURE GAUGE NEEDLE BEGAN MOVING ERRATICALLY. PLT CALLED ME AND I SUGGESTED A LOOSE LINE IN OIL PRESSURE INDICATING SYS. A MECH AT ZZZ3 FOUND A BROKEN WIRE IN OIL PRESSURE SENSOR AND REPAIRED IT. (SEE MORE DETAILS ON ENCLOSED LETTER.) PLT THEN OPTED TO RETURN TO ZZZ1. ON THE FLT N; PLT NOTICED SMOKE; OIL PRESSURE GAUGE INDICATED VERY HIGH PRESSURE; AND A FEW SECONDS LATER THE ENG STOPPED. AN EMER WAS DECLARED ON 121.5 AND A SAFE LNDG WAS MADE IN A COW PASTURE. THERE WERE NO INJURIES AND NO DAMAGE TO ACFT EXCEPT FOR THE ENG FAILURE. THE REASON FOR THE ENG FAILURE WAS A LOSS OF OIL DUE TO A BROKEN FITTING BTWN THE ENG AND THE OIL PRESSURE SENSOR. THIS FITTING HAD BEEN CHANGED 10 HRS PREVIOUSLY WHEN A HOBBS OIL SWITCH WAS INSTALLED. THE STRENGTH OF THE FITTING; IN THIS APPLICATION; WAS EVIDENTLY INSUFFICIENT. A NUMBER OF FACTORS CONTRIBUTED TO THIS INCIDENT: THE PRESSURE TO COMPLETE THE JOB IN A TIMELY (AS DETERMINED BY OWNER) MANNER; UNAVAILABILITY OF ADEQUATE HARDWARE; THE PERCEIVED NEED TO KEEP COSTS TO A MINIMUM; AND FINALLY THE POOR JUDGEMENT AND CONSEQUENTIAL ACTION TAKEN AT THAT TIME.