To increase efficiency, engine compression ratios have been increasing, bringing the distance between the piston's top dead center and the valve closer together. This raises a problem: the piston, moving upwards, might collide with an open valve at top dead center. Therefore, internal combustion engine engineers designed recesses on the piston crown to accommodate the valves.
Due to differences in the number of intake/exhaust valves and valve lift, the recesses on the top of some pistons are asymmetrical. This design necessitates that these pistons be installed in the correct orientation; otherwise, the piston crown cannot avoid colliding with the valve.
Some diesel engines have a notch machined into the piston skirt to avoid the piston cooling nozzles located near bottom dead center. These pistons also have an orientation.
