On May 10, 2004, about 1230 eastern daylight time, a Beech A-36, N3670S, registered to and operated by a private individual as a Title 14 CFR Part 91 maintenance test flight, incurred a heavy vibration during climb to cruise, in the vicinity of Daytona Beach, Florida. Visual meteorological conditions prevailed, and no flight plan was filed. The airline transport-rated pilot and one passenger were not injured, and the airplane was not damaged. The flight was originating at the time of the incident. The mechanic stated that he had performed maintenance on the airplane, and he and the pilot were engaged in a maintenance test flight, when the incident occurred. He further stated that after takeoff, and during climb to cruise, at an altitude of 1,000 feet AGL, as the pilot reduced power from 2,700 rpm to 2,500 rpm, all of a sudden there was a loud bang, and the airplane started to shudder and shake violently. He stated that the pilot reduced power, turned the airplane, and made an otherwise uneventful landing on runway 23. After shutting the airplane engine down, the airplane was then towed to the maintenance shop and examined. After checking various items, the mechanic said that he noticed that one blade of the three-bladed McCauley propeller appeared to be in negative pitch, and when moved, it revolved freely in the hub. The propeller had last received maintenance at another maintenance facility, about 678 flight hours prior to the incident. An initial examination of the propeller showed that the pitch change actuating pin for the No. 3 propeller blade had separated, with a threaded portion of the pin still remaining in the hub. The hub and pitch change actuating pins for all three blades, along with the spacer for the incident blade, were retained and sent to the NTSB Metallurgical Laboratory, Washington, D.C., for a detailed examination. On May 10, 2004, the incident McCauley propeller blade, and associated propeller parts, to include the ferrule, fractured pitch change actuator pin, along with the washer and spacer, were examined at the NTSB Metallurgical Laboratory, Washington DC. The examination revealed that for the affected blade, the ferrule hole into which the fractured pitch change actuator pin had been inserted, had threads that did not extend into the hole as specified by the propeller manufacturer. The threads extended only to about one half the way to the bottom of the hole, and fatigue cracking initiated in the threads of the pin and progressed through the pin before separation due to overstress. Records indicated that the propeller had last been overhauled on January 30, 1997, by a maintenance facility in Johnson City, New York, about 678 flight hours prior to the incident.