This is an RC-scale replacement main gear for the XRB-SR series that fits a 4mm main mast. It is intended as a direct drivetrain spare to support routine maintenance, fit checks, and tuning tasks on electric RC helicopters.
Focus when installing: ensure the gear seats squarely on the 4mm mast, confirm axial alignment with the pinion and spur gear mesh, and verify smooth rotation of the main shaft assembly before test flight. These checks help limit vibration and maintain consistent rotor tracking.
For tuning, use the part to restore correct gear engagement after wear, and re-check head geometry and blade tracking after replacement. This gear is described strictly as an RC accessory part for model helicopter drivetrain work.
Use this part as a like-for-like spare when the original main gear shows wear or during scheduled drivetrain servicing; confirm fit with your XRB-SR setup and perform basic mesh and tracking checks after installation.
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