High Torque When Rebuild Starter?

Not being an expert in electric motor design, I can only speculate. Certainly our understanding of classical electromagnetics has not changed over the decades. Understanding of materials has greatly improved, and the density of magnetic fields due to a specific flux very much depends on the material properties of the core that shapes the field. Newer alloys with higher magnetic permeability would contain more of the field energy from a given current and increase the strength of the field. Newer machining could improve the tolerances between the laminations, and increase the field or require less current for a given power. With the advent of computers, our ability to analyze variations in design with methods previously impossible like finite element analysis could lead to design of more efficient motors.

 

Adding a gap cuts the eddy currents in the core. They use laminations instead of solid iron in transformers and motors to limit the size and direction of eddy currents. He might have been able to figure that out by inspection. Changing the windings probably required some thought and calculations. If he learned something by listening, that's some specialized voodoo. When an MRI scan is running, I can listen and hear what's happening in the scanner noises ... the field gradient "music." Likely he had listened to enough of these machines that he knew what it should sound like, and what condition makes what sound.