The 31cc & 28cc Ryobi's are pretty much the same. You can use the 31cc cylinder and piston assembly on the 28s and vice versa. Most Ryobi differences come year wise rather than through displacemet. The earlier ones have a slightly smaller crankcase and there are also differences in the reed back plate shape. Early pistons will hit the later cranks but later pistons will fit the early style. And so forth......Bob