The rear differential on a C5 Corvette gets a lot of talk. While exceptional factory limited slip diffs, many of these units are approaching 25 years old. Many have been neglected and still contain the original OEM GM fluid, which has likely broken down a bit. Add in some hammering on them... these differentials can and do wear out! When asked we asked GridLife's most winning C5 Corvette driver Matan Rosenburg if there were any C5-specific quirks or recurring issues that had tested his patience, his simple reply was on the rear differential being "a bit weak".
So the obvious thing to do with your differential is make sure the fluids are fresh and current. We did a straight forward article on how to easily change your differential fluid. That's all good, but... how do you know your diff is tired?
Well besides the obvious methods of telling if it's bad like "it doesn't turn anymore" or "it screeches like a thousand fingers on a chalkboard"... there is one easy way to tell if the differential clutches are good, and that's by checking breakaway torque.
Required Tools
- 33mm Socket (We recommend Impact so it can be utilized on an impact gun for axle nut removal)
- Any adapters necessary to work with your regular old ratchet. 3/8" to 1/2" adapter likely.
- A digital torque adapter that can measure peak torque, or a digital torque wrench that can measure peak.
- A ratchet (We're sure you've got one if you're reading this, but just incase we love these)
- A car jack, wheel chocks, and ideally a jack stand for safety's sake.
So the concept is pretty simple. Jack the car up on one side, leaving one rear wheel on the ground. Be sure to chock and support the car so the car doesn't go anywhere and you are safe about the process, though you will not be going under the car as part of this process.
You'll also need access to the rear axle nut, so be sure to remove your wheels center cap. You'll be turning the rear axle nut with a 33mm socket, and more than likely need a few adapters. Above is what we used to do the job.
Now, assemble your ratchet, digital torque adapter, and socket that more than likely has a few adapters in the mix. Make sure your digital torque adapter is set to peak, and don't worry if you did something silly like us and wind up with a NM specific torque adapter, the conversions are easy.
Start to turn the wheel that is in the air via your ratchet and socket on the axle nut. Slowly and smoothly apply increasing force to the ratchet, you don't want to introduce a spike. Eventually, the wheel will turn freely. That initial breakaway force though is what you want to measure. Perform this 3 times in each direction (forwards, and backwards). Note the average in each direction and you'll have likely removed any outliers where perhaps you "spiked" the input with a forceful push of the ratchet.
It is also worth noting just what you feel the breakaway buildup and release is like. Did you experience a smooth breakaway or does the breakaway point suddenly give way immediately? Is the ramp up force required to breakaway smooth up to the breakaway torque, or are there notable steps of increasing force on the way?
What Values Are You Looking For
| Good Value | Fine Value | Bad Value | |
| Breakaway torque as measured as peak | ~90–150+ ft-lb (≈ 120–200+ N·m) | ~35–90 ft-lb (≈ 40–120 N·m) (Worn, but working. Lower reading, more worn) | < ~35 ft-lb (≈ < ~40 N·m) (Internal components not doing their job and/or damaged) |
Hopefully when you perform all of this, you'll get a nice clean breakaway with smooth action at a number you are looking for. If that's the case, congratulations your diff clutch packs are fine, give them the lube they deserve and keep on going!
Please note that while it is worthwhile to feel for how the breakaway was... we didn't cite any "good to bad" levels for them as other factors can contribute to this. A warped rotor for example can introduce intermittent additional resistance that then breaks free, and we'd hate for somebody with a bad $100 rotor to go purchase a several thousand dollar overhauled differential!
If you are in the unfortunate circumstance where your clutch diff packs have given out though... it's likely time to look into a new differential. RPM Transmissions specializes in overhauled C5 Corvette drivetrain components, and their Differentials are available in various stages (and prices) to meet your needs.
