A mechanic describes the following engine discrepancy: “One number 4 cylinder base stud failed in tension (upper left stud). Both the nut and 30 percent (of the remaining) stud length were found between cylinders 2 and 4 on the inner cylinder baffle. Number 4 cylinder was removed, and the remaining partial stud removed with a stud extractor.” A new stud was installed, P/N 50-15, then the original cylinder. Studs were torqued to the recommended value of 600 inch pounds as per the Lycoming “direct drive manual.”

“(I believe...) the probable cause is metal fatigue due to the age of engine crankcase studs and the number of cylinder removals and replacements during the engine’s lifetime. This aircraft has spent most of its life as a training aircraft.

“Prevention: thorough inspection of and/or replacement of cylinder base studs during engine overhaul, with mandatory replacement time periods or cycles.”

Part Total Time: unknown.