In regard to engines, running-in involves the issue of moving parts compared to other parts that remain stationary. To be clear, the head, cylinder and crankcase are the static parts in a kart engine, while the piston, connecting rod and crankshaft are moving parts. When they are installed new in an engine, these elements need a settling phase to ensure maximum performance and, not least, long-term reliability. The settlement between the moving and static parts is called running-in. Each type of engine (mini, single-speed, shifter) needs a different running-in approach, and here we illustrate that of the KZ engine.