I’m planning to replace the bearings in a radio-controlled helicopter’s main and tail rotors. When using Bell Hammer Gold Grease, should I choose No.0 or No.2? Also, which grease is suitable for the rubber drive belt?

For the bearings in both the main and tail rotors, No.2 (standard type) is recommended.
No.0 is very soft and designed for low-torque or automatically lubricated points, but it may not provide sufficient retention under high torque or high-speed conditions.
No.2 offers better grease-holding capability and centrifugal resistance, making it ideal for high-speed rotating parts like rotor bearings.

As for the rubber drive belt, grease application is not recommended.
Bell Hammer greases have a strong friction-reducing effect, which can cause the belt to slip.
If your goal is to reduce belt resistance or prevent squeaking, use a silicone-based or PTFE-based non-oily spray that is safe for rubber materials instead.