ok, the center diff gear. basically I put the gear against a bench grinder while holding the bearings with the old diff supports. this ensures it stays round as it grinds the teeth down. I don't have any technique for measuring this, I just grind it down a bit at a time untill the edge of the teeth snap into a 66 tooth e-maxx spurgear with the center removed.
this technique is brutally effective and eliminates the need for any sophistocated measurements while making a complete unit that has consistant mesh with the motor pinion. infact the mesh is more consistant than when the e-maxx spur is installed in an e-maxx(oridginal).
anyways, once the fit is made, pop off the new spur and drill 2-4 holes in the x-ray diff gear to bolt the 2 pieces together. I used 3. those metal spurs are hard stuff and difficult to drill, even with a drill press and a good bit. pop the new spur back on and use the diff gear as a template to drill 3 holes in the new spur. the e-maxx spur never stripped on the ofna BL but its enevitable that someday a stone is going to do enough damage that it will. so I repeated the last step on 2 more e-maxx spurs as "spare parts". I tapped the holes in the diff gear and the new spur just to eliminate as much slop as I could and put it together with 3 short 3mm socket head screws and red lock-tite.
I'm doing it this way because, plastic 66 tooth e-maxx spurs are durable, cheap and easy to get. so are small 32 pitch pinions. the 66 tooth spur is also bigger than the typical stock 1/8th buggies spur and helps bring the gear ratio to a more suitable range.
and heres the pics.