What was found
AIRCRAFT ENCOUNTERED SEVERE VIBRATION ON CLIMB OUT. REQUESTED TO RETURN TO AIRPORT AND LANDED WITHOUT COMPLICATIONS. INSPECTION OF VIBRATION FOUND THAT THE COUNTERWEIGHT FROM NR 2 PROP BLADE HAD COME LOOSE. INSPECTION SHOWED BOLT THAT HOLDS COUNTERWEIGHT HAD PULLED THREADS LOOSE FROM PROP BLADE SHANK. BOLT DID NOT BREAK LOOSE, STILL HAD TORQUE SEAL UNBROKEN. BOLT DID NOT COME LOOSE. DAMAGE WAS DONE TO SPINNER AND WIRE HARNESS. PROBABLE CAUSE COULD HAVE BEEN BOLT TOO SHORT OR OVERTORQUED