If ANYTHING is going to be considered "weak" on the 8.5, and it's still pretty stout, it would be the axles themselves. Increasing spline count would help, but that means installing a new center section as well. Going from 28 to 30 splines may not seem like much, but it will really strengthen it up. The thing to watch for is the axle eater wheel bearing surfaces on the axles and the spline wear itself.
The ACTUAL weakest point on the rear ends is the pinion shaft retainer bolt. Almost will guarantee that yours is broken if it's got 170K on it. Yet it's captured so it won't allow the pinion shaft to move much. You can find new ones everywhere. And you can still find GM pinion shafts for the G80 8.5" as well. Getting it out could be, well, er, let's just say it's fun to learn a new vocabulary.
With 170K, I'd imagine with more HP being thrown at it, the MINIMUM would be to rebuild what you have with new Timken bearings and an aftermarket clutch kit. You can find richmond gearset if you still need gears, so that's not a problem, but yeah, you'll need to probably run through the rear end while everything is out for peace of mind. At least THAT won't be your problem child when you're done.
Even the aftermarket clutch kit is better made than the original. Bruce Ponti had some made up several years ago and did have some left. Don't know if he's still got any. But check with him anyway. I got my set from him and it's super-nicely done.
Several years ago Bruce Ponti had a bunch of correct, square-eared posi-traction plates and frictions made for the 10 bolt corporate 8.5" GM rear end with carriers 410408/410409 (S- spring only) because the GM supply had all but dried up. They're believed to be a bit more of an upgrade from the...
gbodyforum.com
If you have the time, you should check out this video of a rear-end being setup and how to do it. Very informative. Video is about 2 hours, so you may have to absorb it in chunks depending on your time availability. Guy explains almost everything.
This video goes through the GM 10-bolt axle overhaul. I use the GM special tools, go through the measurements, do contact pattern analysis, and much more! ...
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