The engine study in 1968 revealed that the three main bearing VW crankshaft design was not suitable for the larger displacement engine project. In order to deal with the high stress created by the torsional and gyroscopic forces, a four main bearing crankshaft would be required.
The design also had to eventually include the installation of a series of variable and constant speed propellers with varying weights up to 23 lbs.
The production crankshaft flanges were to be machined from an E4340 forging. A large area 3° locking taper was chosen as the method to connect the flange to the crankshaft without keyways or splines. The locking taper area had to extend into the larger section of the crankshaft nose so that the forces would not be concentrated onto the smaller section near the edge of the crankshaft. By doing so, this would eliminate the distributor drive gear lock ring and spacer which were no longer needed.
The outside diameter of the shaft end of the flange provided the fourth main bearing journal. The fourth engine bearing was to be cast out of aluminum bearing alloy. A one-piece, ring type design was required; two internal rotary seals for propeller oil control and one outboard lip seal for internal and external sealing.
The design called for the flange to be pressed onto the crankshaft via 3/4″ left-hand threaded bolt at a torque value of 150 ft. lbs. For flange removal, a set of internal threads were provided which allows for a puller bolt to be threaded in to contact the nose of the crankshaft. This would allow the removal of the unit without deforming the O.D. of the flange.
The actual flange dimensions for mounting the propeller hub were integrated from the SAE standard #1 which also meets aeronautical standard 127-A. These same standards were used by Continental and Lycoming on their smaller engines. Another design requirement was for the propeller disc to be located far enough forward so it did not compromise the exhaust and cowling installations. The R-2100D-CDI engine manufactured today still uses this design.
In the early 1970′s aluminum ground adjustable propellers were put into service. By 1977 the oil-controlled constant-speed propellers were introduced. There were two and three blade versions with varying weights ranging from 14 to 25 lbs. In 1985, after several years of service, the propeller was officially grounded due to several random blade failures. Other than the blade failures, no reports of crankshaft flange problems were received. Engines that suffered blade failures were inspected and put back into service; the service history of these engines indicates that no long-term problems were caused by blade separation.