Paul, the 51 was set up for a hand crank. You want to check the harmonic balancer to make sure that it has the proper lug hole in it to allow the hand crank to release when the engine kicks. There is a possibility that the balancer was replaced in the past. If you are trying to hand crank it over prior to starting you can pull the plugs and use the fan as a crank or pull the pan off of the fly wheel and use a pry bar or screw driver to turn it over.
Tom
