The blackhead (AKA silverline) is cross scavenged and had a piston with baffle top. The Speed engines have a plain piston top, and finning right down to the crankcase.</p>

Most popular props for the .60 were 11x7.5 for screaming performance (speed) and good performance (silverline) </p>

The speed was no good at larger props as needed for more silence, but the Blackhead could use 12x7 props very well on standard mufflers. To persuade the speed engines to lower rpm than 16,000, long header tuned pipes were the way to go.</p>