Interestig advice from discussion on HK Support Center (user John):
" If you want a metal gear HXT900 you can make your own by combining the metal gears and top cap from a Turnigy MG60S or Hextronix MG-14 with an HXT900. All you do is swap the gears and cap over and screw it back together. (you need to swap the cap because of the bearing on the output shaft).
Forgot to mention that you need to use the metal shaft (that holds the secondary gears) from the donor servo and not the HXT900 because it's shaft isn't long enough to reach the hole in the new top cap.
As to why you would do this rather than just using the MG90S or MG-14 it's because while these servos have very strong gears their electronics suck compared to the HXT900 and they have overshoot and are slower. You can get the best of both by combining them. You can also put a blob of hot glue over the solder joints where the leads are connected to the circuit board as well as the leads to the pot and motor (circuit board side ONLY only NOT the solder joints on the motor and pot as these aren't likely to break and it's easy to mess up and ruin them with the hot glue) to support the wires and prevent vibration from weakening the joints and eventually causing a failure. Don't go crazy with it just a little blob over the joint to support the wire is enough."