As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON ENGINE START-UP TO IDLE, ENGINE OIL PRESSURE GAUGE SHOWED NO OIL PRESSURE. FOUND OIL PRESSURE RELIEF VALVE RETAINING RING WAS NOT SEATED IN HOUSING AND RELIEF VALVE HAD FALLEN PART WAY OUT OF HOUSING. SUBMITTER STATED WHEN INSTALLING PRESSURE RELIEF VALVE CHECK FOR PROPER ENGAGEMENT OF RETAINING RING WITH SMALL MIRROR AND FLASHLIGHT.