Abstract: A piston for use in the cylinders of internal combustion engines of either spark ignition or compression ignition type. A succession of interrupted ribs, aligned along arcs of increasing radius, stand proud of the piston crown surface to promote turbulence in the unburned charge as the flame spreads to meet them. The solid parts of all the ribs are preferably arranged in a regular pattern, and may be of such arcuate extent and so staggered from arc to arc that no sector of the flame can spread straight across the surface of the piston crown without being deflected by at least one solid part. The surface of the crown may be divided into upper and lower levels by a step, with ribs standing up from both levels.