I had the same engine at one time and the instructions said 7mm. I could not get this engine to run well no matter what I did. I also have a friend who had the same engine and had similar problems. RC Showcase allowed me to trade it in and for the difference between purchase prices I got a ZDZ40 RL. The new engine has had none of the problems I experienced with the B Sport model. I'm thinking it was a problematic design. A few have had luck with them but from what I know, very few.