“The forward exhaust stud became loose and was forced out of position by the effects of exhaust pressures working against it,” writes the submitter. “When the threads were completely destroyed, both on the stud and in the cylinder head, the exhaust was allowed to exit the head from a position directly under the exhaust valve pushrod housing. The resulting heat burned a hole in the tube, allowing oil to escape and create a lot of smoke.

The flight was not interrupted as the smoke was not detected until landing, at which time the engine was secured and the passengers deplaned. (I) recommend at each 50 hour and/or oil change the torque be tested on these studs.” (The aircraft is a Piper PA31-350. The Lycoming cylinder part number included is LW-12966.)

Part Total Time: (unknown).