After testing 001, a number of problems serviced. Experimental work continued with 002 and 003 but the incremental improvement was not enough. It was clear that case rigidity was an issue. Also getting porosity free castings necessary for the pressurized oil passages proved challenging. A complete redesign was done for 004.
The 004 crank case was cut from solid 6061-T6 aluminum billet. The case was split at the crank centerline so the caps could be made one piece. The caps were designed to more than double the stiffness of the top case. A separate sump was retained in the redesign so to be maintenance friendly.