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I seem to remember reading that the Model A engine with a stock 4.2 head can only utilize 28-32 degrees of crankshaft advance. High compression heads use even less. 20 degrees of distributor advance will be ~40 degrees of crankshaft advance. If the spark advance lever on the upper plate is just touching the distributor body at full retard, then no more than 7 to 8 notches are required, and wouldn't that be at high speeds of 50 plus or there about????
I've wondered about this. If you're using a 2 tooth Gemmer, a small window can possibly work fine if you're never driving at max speeds.