The one thing I can say, is that the seller of the engine to me has been very cooperative. I have triple checked the model number, and called about it. Same outcome every time, 25cc. Everytime someone see's the engine, it's a 35, or 41cc. I'm glad i'm not the only one confused. I would like to know what engine I will be putting in my plane. If I'm not sure of the engine size, it's not going in.