As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ACFT WAS BROUGHT IN FOR INSP WITH NO DEFECTS NOTED BY PILOT. INVESTIGATION OF A LOW CYLINDER COMPRESSION DURING THE DIFFERENTIAL COMPRESSSION INSP, REVEALED A CRACK THAT STARTED AT THE FWD SPARK PLUG HOLE AND ENDED AT THE EXHAUST VALVE SEAT. THE CYLINDER ASSY WAS PREVIOUSLY OVERHAULED, O/H FACILITY HAS TRADITIONALLY PUT OUT A GOOD O/H CYLINDER, THIS IS THE FIRST CRACK THAT THIS AIRLINES HAS HAD IN MANY YEARS OF OPERATING THESE CYLINDER ASSEMBLIES.