Billski said in his "Springs,etc" post that a reduction drive frequency must be 25% away from the engine's critical frequency. Den Hartog fixed a Pratt engine by using a softer element in the gearbox. The post above shows why that works. I really want to have a pusher prop installation so until I change my mind I am stuck with long shafts. The budget on this project kind of forces me not to buy new parts.