I'm the other half of the tool design and replacement process development pair. Pullerguy & Pirate
RZN 4 Life,
The blue you are seeing the the "nylon" in the Nylok pinion shaft nut. Its a 13/16" diameter threaded shaft. As a long time Mechanical Engineer in Power Plants, I've never seen anyone try to use a Nylok nut on a shaft that big. Very early RZR 800 differentials used a steel nut that was staked to the shaft. Within about six months Polaris moved to the Nylok nut. It's not staked.
The nut requires 250 ft-lbs of torque. It keeps the inner and outer tapered roller bearings in place and positions the pinion shaft and ring gear mesh correctly. Sort of, the differential is not a design in which one can set the backlash using shims. What you get is close enough...so says Polaris. But they work, so long as the nut stays tight.
The problem occurs when the pinion nut begins to back off, that is loosen and begin moving to the spline end of the pinion shaft. As the nut backs off, the bearings get become loose and start wobbling a bit. As the nut continues to back off, eventually it gets to where the pinion gear has moved too far into the ring gear, the bearing roller cage inside the case comes apart, all the rollers and pieces of the roller cage drop into the differential case. And boom, the rollers get up into the gears and the pinion is too far into the ring gear...the pinion gets shoved downward and the differential house shatters spewing pieces of gears, bearings and housings on the road. Done!
Now what causes the Nylok nut to come loose? That's a very good question. Our work " doing the "loose nut dance" and replacing the nut with a steel nut, led to me tracking differential SN, date of manufacture and miles driven. By SN there are some 430,000 of these differentials out there (if the number are sequential). Most all of the differentials that have had the "dance" done have pinion nuts that are in various stages of getting loose. Some still have some torque on them, some are looser than finger tight. And we have seen a few that still have full torque on the. We have done the "dance" on every one of them, just to be certain there isn't trouble in the future.
What' the cause of the nut getting loose? How many miles do they last? NO one knows. Pullerguy has posed the a probable cause. A harmonic that causes perhaps a micro vibration that works on the shaft. Thus its impossible to measure, at least without a fortune in diagnostic equipment. But a harmonic could be occurring literately from Day 1, but depending on a differential casting, the fit of the ring and pinion gears, the assembly of the bearing components and the Nylok nut. With that many variables, no one knows if or when a differential will fail. We do know that there is a possibility that the differential will fail. We have come up with a "dance" that eliminates the issue and the differential should not fail...to date we haven't had one fail after doing the "dance".
Pullerguy and myself, Pirate, both have tool sets we let out to let you do you're own "dance", we can provide all the needed parts and a good set of instructions. There are also any number of sets owned by others that may be available.
Hope this helps you understand.
Pirate