 Piston skirts and rings risk being extruded into this port, so it is always better to have them pressing hardest on the opposite wall (where there are only the transfer ports in a crossflow engine) and there is good support.In some engines, the small end is offset to reduce thrust in the intended rotational direction and the forward face of the piston has been made thinner and lighter to compensate; but when running backward, this weaker forward face suffers increased mechanical stress it was not designed to resist.This can be avoided by the use of crossheads and also using thrust bearings to isolate the engine from end loads.