Unless you are a seasoned mechanic or a specialist differential rebuilder like Houston Rebuilt Axles, the thought of rebuilding a differential can be daunting. They may be less complicated than, say, a transmission. However, the infrequency with which people are required to rebuild differentials, and requiring the same transmission type ‘magic’ for setting specifications, make them a mystery and not something many people have done. We can say that anyone could rebuild a differential or a transmission, however, doing it correctly so it lasts its expected life is where many fall behind. Typically, non-specialist rebuilds fail long before their actual life expectancy, primarily due to improper (amateur) rebuilds.
Why do rebuilt differentials fail?
Well, the key is in the details, or more accurately, the specifications and how to properly set them. These include backlash, pinion depth and the lack of the specialist tools required. Furthermore, saving costs by not replacing some components that may look fine, will result in failures in the near future no matter how well your rear end or differential is assembled. That is why a warranty is a valuable feature of a rebuilt differential.
So for a reliable rebuilt differential, using an experienced specialist rebuilder is always better. However, if you are going to have a go, we would like to give you some guidance in rebuilding your GM 8.5” 10-bolt rear differential
In this article we will guide you in the process of rebuilding the GM 8.5” differential, however, the same method could be adopted for the GM 8.6” 10-bolt which is used in modern trucks. Being such a common axle on trucks today, chances are, if you have a chevy and are rebuilding your differential, it will be this one. If you are having rear axle problems check out our article on troubleshooting rear axle problems.
Common GM 8.5” differential failures
As with most axles, the most common failure are the wheel bearings. These are arguably not part of the differential itself but still require some disassembly to replace them. Being a consumable item, i.e. expected to wear out, it is just a matter of time before these give trouble.
Outer Bearing Failure
What kind of trouble? Well, their usual symptom is a howling/droning noise from the differential which changes with road speed, not with engine rpm. Also, if you turn one direction, it can intensify the noise by putting additional force on the bearing, or, when turning the other direction, will reduce the noise by relieving the pressure on the bearing. Despite this common failure, replacing wheel bearings is necessary to prevent metal shavings/filings from getting into the differential gearing which will cause more serious problems. Neglecting to replace bearings once symptoms arise can result in an unsafe vehicle.
Inner Bearing Failure
There are also bearings for the pinion gear and differential itself. When worn, they will cause a similar noise as the outer bearings but with less noise change when turning. Inner bearing noise changes respond more to acceleration/non-acceleration and braking. However, this can also be a symptom of worn out gearing so inspection is the only real way to be sure of the diagnosis.
Gearing Failures
In addition to the above mentioned noise, clunking and vibrations will indicate worn out
gearing, improper meshing of gears, broken gear teeth, damaged differential carrier and bent axles. Ignoring these symptoms of differential wear/damage, can result in catastrophic failure and increase the cost of rebuilding the differential.
Tips for rebuilding the GM 8.5” Differential
First step
Probably the most important tip for rebuilding the GM 8.5” differential or 8.6” differential, is to make sure you get the correct compatible parts for your axle. As an example, you may choose to upgrade your differential to that from an aftermarket company. This is a great idea, you could get a more durable, quieter, smoother differential than the factory system. You may even choose a new gear ratio for better acceleration. Instead of going for the standard 28 spline, you may go for the 30 spline. However, keep in mind this will require new axles also as the stock 28 spline will not be compatible with the upgraded differential. Compatibility is key and can require an increased budget.
Second step
Measure the backlash. Backlash is the amount of play in the ring gear before the pinion moves. Backlash is essential for prolonging the life of the differential and each differential has its own set of specifications for backlash. These can be found in the gear-set instructions and should be followed exactly. They will be given in a range of e.g. .006” and .010”. As this range of motion is small, measuring backlash will require a dial indicator with a magnetic base to attach to the axle housing. This facilitates locating the dial indicator on the gearing tooth. Backlash is essential to be set correctly before moving on as it affects many other aspects of the rebuilding process.
Third step
Once backlash has been set, you can now check the contact pattern. 
The contact pattern shows how the pinion and ring gear are contacting each other.In order to check how they mesh, a coloured grease is used on the teeth. When the differential is rotated as it would under driving conditions, the pattern of grease left behind shows how the components mesh and allows the rebuilder to adjust the gear contact correctly. Improperly aligned gear teeth can wear out a differential within just a couple of hundred miles.
Fourth step
When spending the time and money to rebuild a differential, don’t cut corners by not replacing seals. Seals are relatively cheap and will serve you well in retaining your differential fluid, prolonging the life of your differential and prevent the necessity for fluid top ups. Also, who wants differential fluid leaking everywhere they stop.
Hopefully the tips described here can help you through your rebuilding process to leave you with a reliable rebuilt differential that will serve you for years. If you decide the task is not something you want to take on, contact us at Houston Rebuilt Axles and we can do the rebuild for you. We provide you with a ready to go, warrantied rebuilt axle shipped right to your door.