[frizzen]

More boring scratch-build notes:
Picked up the slewing ring gear, an Align trex 500 main gear, 162 teeth, 1/4" tall, 3.986" od. (Thanks again jim)

It seems to mesh ok with a servo spline mount gear i picked up several years ago. Hitech hs-645mg use C1 spline, this gear is 18 teeth, 1/8" tall, 0.426" od. I forgot if it's 32 or 48 pitch.
Gear ratio comes in at 9:1.