MAX-21XR-B Version 3 is designed for 1/8 off-road and 1/8GT car categories. An inner head is redesigned with a new shape, a crankshaft counterweight and port timing is also refined for fast throttle response. A piston and a connecting rod in this engine are the same as we use in OS SPEED series, top quality. Two carburettor restrictors are enclosed as standard accessories. The Orange one (6mm) is for quicker throttle response and the yellow one (7mm) can make the engine more powerful.