 Why are you making the sound? The bike should make the sound now. But my bike doesn't make the sound. Some bike make the sound. Why doesn't your? That is because of something called bore and stroke. You all must know that inside bike engines there are cylinders where the fuel vapor mixture explodes. Those cylinders will have a diameter and a diameter is called the bore. The cylinders have a height and that height is called the stroke. Stroke is the distance through which the piston moves in a single explosion and bore is the width of the cylinder. When bore is more than stroke, then the cylinder is called oversquare. When bore is less than stroke is more, the cylinder is called undersquare. Roil and field cylinders are undersquare which is why they make that characteristic thumping sound. Oversquare engines are used in high performance racing bikes which need to operate at high RPMs like Yamaha Kawasaki, other Japanese and German bikes.