Aircraft engines are somewhat of an oddity to me in that the thrust is carried by the crank and transferred to the engine case on its way to the airframe. In an auto engine, only the torque is transferred to other components that make the thrust to move the vehicle. I could imagine that the original application had no allowance for the case being used to pull the vehicle around. In a direct drive I don't think that it could generate enough thrust to matter much but with a reduction drive I could see it being more significant. Being as the cases are already prone to cracking in the car form, has anyone tried to make a structure or girdle to help carry the additional loads and make it more ridged?