The Phoenix gears self-shim through the use of springs on an axle of each gear. This allows them to independently move up and down as required, but that implies that there is potential for contact at some point. That could be how that one tooth on my spur gear snapped off. It's sad to see that such an expensive gear set is made from pot metal. It would've been nice if they were case hardened steel...
Here's an installation diagram:
The red circled item is a thrust bearing - note, not all Phoenix gear sets come with that thrust bearing. AFAIK, it was only offered when the Phoenix gear sets were first introduced as limited edition items.
Full installation instruction located here:
http://www.pbase.com/illusive_airsoft/phoenix
The SystemA Turbo is the new anodized ones with the neodymium magnets... those motors are awesomely nuts. I had a chance to play with a Magnum motor (the red high torque version of those new motors)... ANY ferrous object within a 2" vicinity will gravitate towards the motor and hang on with a death grip. I installed it in a PSG-1, but the magnets are so powerful that they won't allow the spring/piston to decompress through mechanical release of the anti-reversal latch.
The magnets are so strong that it's difficult to manually turn the axle, even by hand. But besides that, I think they're great motors for standard AEG applications. The Magnum motor worked REALLY well for cranking that beast spring I had in that PSG-1. The OEM Marui motor would heat up to the point where it was HOT to the touch. The Relay circuit I installed would also heat up to the point where it would literally burn you if you touched it. The Magnum took all the torque with ease with barely any increase in temps.