For all you "racers" out there that are using EMAX MT1806, or SunnySky X1806 or something else similar on your 250-class quad or hex:

You have probably noticed that when using 5X3 props, your motors run fairly cool with 3S batteries, but when you move up to 6X4, the motors run too hot.

I have a solution for you -

FPV Model MC2204 motors (2300KV).  These little motors are larger diameter than the ones mentioned above, and aren't as tall, but they are significantly better than the ones they replace - and they have more power, my thrust stand confirms it. 

The FPV Model motors have a larger bolt-pattern - a full 16X19MM, and have M3 holes, rather than M2, but if you can fit these on your craft with little modification, do it!

These motors spin a 6X4 prop on 3S batteries just fine, and they barely get warm.  They are well-made and look to have good bearings.  The leads are better as well.  Instead of simply providing strands of fine copper wire like some motors, the FPV Model units have "real" wires. 

They don't have shafts that extend past the motor, instead they have CW and CCW adapters that bolt onto the top of the motor (with four M2 capscrews).  The motors cost a bit more - about $16 each, but they are worth it.