(Mistercrash)
I started by modifying the Traxxas TRA5415 and TRA5416 Traxxas output shafts. You cut the gear part off and have to drill a 2.6 mm hole for the spider gear pin. The tricky part is having the hole perfectly centered in the shaft.
Here are two of the TRA3985X Traxxas output gears, you see them with the flat side of the spokes up. The left one is stock and the right one has been drilled in two holes with a 6 mm bit. If you use the Axial posts I linked above, the holes will have to be 7 mm or 9/32 inch. There are two of the gear’s spokes that are thicker than the rest. The holes have to be drilled next to those thick spokes and which side of the spoke you drill is also important. I drilled according to the rotation of the gear inside the tranny, I wanted the forces exerted to the gear to be transferred to the posts from those thick spokes. The center hole has a little ridge that was cut off to allow the 8X12X3.5 bearing to sit a little deeper in the gear.
The two inner threaded aluminum posts that will be screwed to the diff cap. The length on the posts is about half a millimeter less than the thickness of the Traxxas TRA3985X output gear.
Now to start modifying the ring gear. I make the holes and I counter sink them first. To make sure that the holes were in the correct position, I used the Traxxas output gear and aluminum posts as a template. I slipped the two posts in the holes I made in the output gear and the 8X12X3.5 bearing in the center hole. The bearing is there just to center the gear perfectly on the ring gear. An 8X12 bushing would work also but I couldn’t find one.
I take the Traxxas output gear with the posts and bearing and I slip it on the ring gear, I rotate it to center the posts between the screw holes of the ring gear the best I can and with a 2.3 mm drill bit chucked in the drill press, I go through the posts and make contact with the ring gear. The 2.3 mm bit will not touch or damage the threads in the posts, this is just to make a couple marks on the ring gear to show me exactly where to drill the 3 mm holes with the carbide bit. I take off the Traxxas output gear and then I can make the holes and counter sink them.
Then I take a 6 mm threaded rod I have and with two nuts and two washers, I secure the ring gear to the threaded rod. I can then chuck the rod in my drill press and with an angle grinder, I grind off the teeth until the gear is left with a half millimeter lip that will fit over the diff case.
This step is optional; I chuck the threaded rod in a power drill and with a belt sander, I can clean up the marks left by the grinder.
Here’s the finished ring gear from both sides, now transformed into a cap. You can see the counter sink holes on the inside of the cap.
We’re ready to assemble the center diff. I start with securing the posts on the ring gear with the M3X6 mm flathead screws. I put blue Locktite on the threads.
