Also with a 2 stroke they (being the rings) have to suck (or have compression) on both the up and down strokes. As to pull the fuel air mix from the carby and through the crankcase to allow for the bottom end to be lubricated. Then pull the fuel mix into the combustion chamber. Where as 4 strokes only needed compression on the backwards stroke. Due to having a sump with oil to lubricant it's bottom end. So less work is needed by the rings in a 4 stroke.
