All the Heng Long and Asiatam gearboxes are disassembled, cleaned, re-assembled with washers/shims if needed (usually are!) and broken in again during each overhaul. Then the motors are RPM tested free and under load. Then I test the rpms of each gearbox too to make sure they're as close as possible (within a few rpms of each other) and if not keep breaking in the offending gearbox until it spins the way I want it too. I use white lithium grease to lube the boxes and semi seal them with custom made covers that are RTV'd over the tops once they are screwed down inside the tank. Have never had a failure with a metal gearbox by following this PM routine.