I have a single key stock forged crank in mine at 9 lbs of boost. The only time I ran into trouble with it was when I switched my pulleys to get 12 lbs of boost. Although it was great for power the stock balancer didn't like it and broke due to the extra torque on it. Took out the timing cover as well. I don't know where the limit is on a cast crank but I wouldn't chance it, I would go with the double key.