AEC Z drawings
Jesse,
I have all of Bob's AutoCAD drawings. There is no Z13-20 version of it that I'm aware of. That said, the Z13-8Q version is based on a dynamo (permanent magnet rotor). Is the 20 amp version also a dynamo? Or is it a conventional electro magnet rotor style alternator? If it uses a dynamo, the Z13-8Q drawing will work.
Charlie Kuss
PS Have you checked all the other Z variants to see if perhaps it's listed somewhere else? The B&C SD8 is similar in operation to the old Lucas dynamo alternator used on British (and smaller Hondas) bikes of the 1960s & 1970s.