Mac Motorcycles are powered by the Buell® single cylinder, air-cooled, 2-valve, push rod, 492cc, 5-speed 'Blast' engine. They're called "thumpers" and with good reason, with one combustion for every 720° of crankshaft rotation when that cylinder fires, it's really an "event".
Valve adjustment is automatic, using hydraulic lifters. The transmission is down low giving a low centre-of-gravity that helps great handling. The fuel is enriched en route to the pot. Mac bikes exploit the power-to-weight ratio principle with this great little motor and a very light rolling-chassis.
Stock motors produce 34 HP at 6500 RPM but a bolt-on big-bore kit (515 cc), Andrews Cams, a Mikuni HSR 42 carb', Screamin' Eagle ignition and a Supertrapp exhaust will produce around 50 HP at 7200 RPM. A Blast running an NRHS 515 kit set a record at Bonneville in the 650 class, whilst much of the tuning potential of the motor has been exploited by the short and flat track racers in the States.
*Buell is a registered trademark of H-D Michigan, Inc.