What was found
AIRCRAFT NOSE GEAR COLLAPSED AFTER A LANDING GROUND ROLL WITH MINOR DAMAGE TO AIRCAFT AND PROP BLADES. INSPECTION FOUND NOSE GEAR UPLOCK VALVE, THAT HAD BEEN REPLACED PRIOR TO FLIGHT WITH A USED UNIT, MISSING A SMALL CHECK BALL CAUSING A LARGER CHECK BALL TO WEDGE WITHIN ITS SEAT. AIRCRAFT ON JACKS PERFORMED AS REQUIRED AND LOCKED. IN-FLIGHT, THE DOWNLOCKS WOULD NOT FULLY LOCK, ONLY PART WAY. USED UNITS SHOULD BE INSPECTED CLOSELY BEFORE INSTALLING IN AIRCRAFT.