This is a gearbox to connect one 550-size motor (can be brushed or brushless) to one propeller shaft. The 'GearboxD' file is for a 5/16"(8mm) stuffing tube, and the 'Quarter-Inch' version is surprisingly for a 1/4" (6.4mm) stuffing tube. The shaft size is whatever your preference is.
**You will need: 2 gears (one for the motor, one for the prop shaft), a 550-sized motor, a 5/16"(8mm) or 1/4" (6.4mm) O.D. stuffing tube with its bearings and prop shaft. Prop is on you, along with any batteries, grease, propellers, sundries, etc.
-Print the file to match your stuffing tube, in ABS. I used 3 shells, 0.3mm layer thickness, 110C bed temp, and a 5mm brim (got some lifting without it).
-Some fettling may be necessary to fit your stuffing tube; do so as needed, and insert your stuffing tube into the largish hole on the non-motor side. You can use a slurry to glue it in, or hit it with CA glue. I didn't do either, as the gearbox is glue or screwed to the hull, and the stuffing tube is also. They fit together tight enough that they're good without heroic measure being taken.
-Put your driven gear on the proshaft, engage the setscrew to the flat on the shaft. Some people also have a brass collar on the prop shaft. If you do, you'll figure it out ;) [hint:put the collar on, then the gear!]
-Put the smaller drive gear on the motor shaft, and tightent he setscrew.
-Thread two M3x8mm screws into the mount holes in the motor, then slide the motor down into its mount.The gears should engage but not tightly. An easy way to set the clearance is to take a strip of printer paper and put it between the gears as you fit them together.
-Tighten the two M3 screws to cinch the motor in place. Rotate the gears to remove the paper, if you used it.
-Finally, fit the printed cover over the gears. This fits pretty tightly, and keeps wires and other things out of the gears, which is a Very Good Thing.
-Connect the motor to your ESC or other electrics, and test it! don't get anything valuable caught in rotating parts! It hurts!!!
*Special Thanks to Chris Kessler, genuine rocket scientist, for inspiring certain parts of the design!