So I bought a "complete" G1 jetfire off of ebay only to find that the left shoulder had been glued in place and was missing the spring, the gear and the screw. Tomorrow I'll go for the spring and screw at a hardware store, but this gear turned out nicely. You might need to shave off the inside ever so slightly to get it to smoothly go on the toy's post, and you might need to sand/file down the exterior slightly to get the gear to fit in the should slot.

The gear works great. You will have a much harder time finding a screw and spring if those are lost. I went to a local ace hardware, but it was very difficult to find an exact match. I ended up getting a screw that had a much tighter threading and drilling the hole out a bit to fit the new screw. The washer was barely large enough to attach. If anyone has a suggestion of where to get small screws and springs for toys, I'm all ears.

I printed at 0.3mm, but I imagine 0.1mm would be better for a part like this, since I can imagine the grinding eventually rubbing the gear smooth.

Overall the project was a success and instead of a glued in place left arm, I now have a jetfire that is fully transformable again.

Finally, I'm happy to make this for free as I needed it for my own projects, but if you find it useful, please feel free to tip to let me know you appreciated the effort. Thanks!