I have an OS120AX in mine, mounted in the normal,(forward), location. I mounted the battery as far back in the radio area as I could. This 6 volt 2500mah battery is just above the wing bolt plate. It balanced pefectly with no lead. I have since moved the tank back to the CG and added a pump that mounts on the engine mount between the engine and the firewall. I guess th weight of the pump exactly countered the weight shift of the tank as this plane still balances at the recommended spot with no lead.