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Wheelers are tough on temps with the bodies being so small and having poor ventilation. It also doesn't help that the batt, motor, and esc are so close together.

I run a Novak GTB with a 7.5R motor (5800 kv), in my ZX-5 SP, and it runs a bit hotter than I like geared 78/17. I'm thinking of going to a 16T because I'm still accellerating at the end of the staight and would like to keep the temps down. In general the CC motors run hotter than the Novak stuff so you may need to gear down more, due to your 6900kv motor, and see if you can get any more venting in the body.

Also check your gear mesh. The ZX-5 o-ring motor mount seems to suck the motor closer to the spur when you tighten it down. I start a bit loose and once the screws are tight the mesh seems perfect. Put your finger on the pinion and make sure the spur has some play when the pinion is held still.