How can you tell the difference between the 31cc & the 28cc Ryobi.
I have looked all over the crankcase but nothing gives me a clue as to the capacity.
Does anyone know the bore and stroke of these two engines

I think the 31cc is 1.375"x1.250"

Cutarug,
I may be wrong but im pretty sure the 28 and 31cc engines have different cooling fins at the spark plug. The 28s have either one or two cooling fins that are much smaller than the others. I know there are pictures in here somewhere showing the difference