They are six Cubes, or roughly 100cc's. Ball breaker of a saw. More torque than anything I have ever ran for the size stock, hands down. You will have problems with the new gasoline with ethanol in it! There is a small "window" that is made of some sort of rubber/plastic between the bar oil and gas tanks. I was told that they sold these for cut off saws as well, and the window could be taken out for more gas capacity. The problem with the new fuel is that it eats this window, making the bar oil spill into the gas. I bought one of the last 10 655's ever made around 1995 for my Dad, and it lasted two months with the new fuel. Hope you have better luck, some don't seem to do it. Three of his have. I got him running a 390xp now, so it's all good but not the same torque.