RE: K&B 3.5 outboard help
The stock 12mm head was designed for a short plug. If a long plug is used, it "may" come in contact with the piston. Also, the long plug has an effect on compresion ratio. Hence the need for two washers.
I've run 12mm with a aftermarket head, which allows a long plug. When I get home tonight, I'll measure the thinkness of the two.