Doh, I think I understand now. The diode chassis is actually in 2 halves. The lower half has the B+ terminal for power and the lower mounting holes are insulated from the engine casing. The top half of the chassis is grounded through the upper mounting holes and connected to the lower half of the chassis through the diode circuitry.
So if the diode board is letting 13+V go to the engine casing when it is bolted into position - Does that mean the board is fried?