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Putting and 8.5t Motor in my XX but no idea what pinion to use. Had a 17t on my brushed motor but that sounds low, any suggestions?

I typically start out with the brushed motor gearing and go from there. I don't typically need lots of speed, though. It all just depends on how fast you want to go and how hot your motor gets. I'm sure you can probably bump up a few teeth on the pinion, though. Just keep aware of your motor temp.

What Matt said. Generally speaking, motors are independent of gearing, which is more a factor of pinion-to-spur gear ratio, internal gear ratio and tire diameter. All of those things are variables, but motor diameter stays constant. Your ideal theoretical gear ratio, and rollout won't change if you change your motor.

Of course, new brushless motors can be way more powerful than old brushed motors, so some people might play with pinion gearing to compensate, but it's not really necessary as you do the same thing by adjusting punch with your ESC.

Thanks for the replies I'm more worried about destroying my gear box than anything else

The XX transmission is pretty robust. If you are concerned about it, however, as mentioned, you can dial down the punch on your ESC, at least until you get a good feel for the new level of power. Also, if you don't have one already, I might recommend a transmission brace for where the transmission attaches to the rear bulkhead. You can find them on eBay or 3d printed ones through www.oldrc.com. The XX is a great buggy. Have fun!

I run a 6.5BL motor in my XX - from memory I'm using a 20t pinion on a green (88t) spur & too be honest I could probably go up a pinion tooth or two - on an 8.5 motor I'd be inclined to start on a 22t pinion (or equivalent depending on which spur you have) & go from there

Don't worry about the gearbox internals... the same internals were carried over to the XXX buggy series - which in turn was used on my Mid-Motor Atomic Carbon CR2 which handled the power of the LRP X20 motor that I still use, on a high grip astro turf track without any issues. You'll just find that with the quicker motors that you'll need to re-build the diff more regularly in order to maintain its smooth action.

Thanks guys my brushed motor had a 17t pinion on it so will up that I think