I have both motors and the reason why I didn't use my 40cc 6.2 air cooled Polini is I didn't want to use such a thin chain (25) and the clutch on the Polini isn't as durable as the Morini. Also in order to get the polini to run right you need to set the clutch engagement at 6k+ rpms to get the motor to perform. For cruising around town, 6000+ rpm engagement isn't relaxing and will turn a lot of heads. The other 50cc Polini motor are water cooled. I have a 15hp 50cc BZM laying around and I'll experiment with the motor when Lean Dean starts making full suspension frames for the motorized bicycles. The good thing about the BZM is there is an air cooled conversion kit available the bad thing is...it's not cheap. A series 2 a/c polini goes for around $850 and the BZM retails for $1300+ while the conversion kit is about $350.