[MetalMan]

Go with the Kyosho spur. In the end it will be cheaper, since if you keep the steel spur and want to try different gearing you'll end up spending a fortune on hardened gears. I can confirm the Kyosho spur gear fits the Hyper 8 diff since that's the diff I picked up off of Ebay. But you might have to play around with the shims to get the spur to fit properly (with proper internal gear mesh).