It seems that Axial has never cared about giving us the needed shims to properly set ring/pinion mesh. I put together 1:1 ring/pinions, so I'm a bit OCD about ring/pinion setup.
If you just stack up shims outside the housing odds are good your pulling the pinion too far away from the ring gear and actually reducing gear contact and worsening mesh slightly.
You'll need 10x12x.1 to sit under the carrier bearings. If you pull off the bearing caps you'll likely find that the carrier/ring gear can shuck back and forth in the housing quite a bit. This significantly changes gear mesh. Adding shims behind the carrier bearings allows you to precisely adjust where the ring gear sits side/side in the housing.
You'll need 5x7x.1 shims for the pinion shaft. Oftentimes this requires so many shims I'll start with .2m thick shims. I also add solid spacers between the pinion bearings so that once everything is shimmed and the driveshaft is tightened there is literally no slop in the pinion shaft. Once your pinion depth is set, and you solid spacer is installed you put whatever thickness of shims on the outside so the driveshaft buts up snug against them once tightened. The solid spacer I use is from the original generation HPI Blitz rear hub spacer. It was originally used to hold the rear wheel bearings at the proper spacing, allowing you to tighten the wheel nuts without crushing the bearings together. It serves a similar purpose in this application. The solid spacer also helps force the bearings to share loads.
I've found that the stock sintered gears typically benefits from .1-.3mm worth of shims behind the pinion gear inside the housing to bring it closer to axle centerline. Do this before shimming the ring gear/carrier. It seems that for every .1mm you move the pinion towards centerline, you can ditch about .3-.4mm worth of shims behind the ring gear side of the carrier and transfer them to the other side and keep similar gear backlash. This relationship is important to understand, othewise you spend WAY more time swapping shims around to find the sweet spot. The deeper the stock pinions sit to centerline the more of the pinion gear actually contacts the ring gear. More gear contact=stronger.
The helically cut HD gears do not respond similarly, they have a prefered pinion depth to result in the best mesh that can only be found with trial and error. Seems some don't want any shims under the pinion gear, others want .1 or .2, but so far never more.
IMO the stock sintered gears are stronger than most think, but only if they are properly shimmed before being run. They wear quickly if the mesh isn't perfect.
