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Differential difference: eliminating rear end noise.


Axle bearings begin to lightly growl when the bearing roller surface erodes away. The growl increases in intensity as the bearing surface erodes enough to cause loose axle bearings. At that point, you can physically move the wheel up and down or side to side. Another indicator of axle bearing failure is change in growling noise as you make a wide sweeping turn. The wide sweeping turn unloads the failing bearing lessening the growl. When you are in a wide sweeping left turn the left bearing unloads and the right bearing load is increased. Vice-versa on the right side of the vehicle the right bearing unloads and the left is loaded


Should we drive our car after finding out that we have imminent axle bearing failure? It depends on what GM vehicle we're talking about. All solid-axle differential Camaros, Impalas, pickup trucks and 1953-1962 Corvettes have differential gear oil lubricated bearings. Solid-axle means the axle bearings, differential carrier, ring and pinion are in one common housing.

Differential gear lubricant bathes the axle bearings, keeping a constant flow of lubricant on the bearing rollers. When a solid-axle differential axle bearing fails, you have some time before severe damage occurs. Many of the GM solid axle assemblies use C-clip style axle retention. Hard to believe C-clips are the only means of holding the axle and wheel in place. The large C-clip fits onto a machined area at the inner end of the axle.

This information is important because the axle itself is the wear surface for the axle bearings. It is common to find worn axles on C-clip differentials, requiring the extra cost of axle replacement. There are modified bearing assemblies available that use another area of the axle to save the cost of axle replacement. The modified bearing assemblies work fine if the axle wear is minimal. The modified bearing kits have instructions on installation and their limitations. If you can use them, make sure the wear that your axles have will be covered under their guidelines.

GM also used axle bearings that are pressed onto the axle, which are retained in the housing by the axle bearing. Axle bearings are pressed onto the axles with great force, allowing a new wear surface every time the bearings are replaced. No matter what style bearing retention your GM axle uses, excessive wear will show up as too much wheel movement.

Rear independent suspension GM vehicles (Corvettes/Corvairs) have axle bearings similar to the front wheel bearings. These rear independent suspension vehicle axle bearings are referred to as spindle bearings. Corvettes and all Corvairs use inner and outer tapered roller spindle bearings. The significance is when these bearings fail from poor lubrication, major damage can occur. When these bearings start to growl, take care of them as quickly as possible. They can be tested in the same manner during a wide sweeping turn.

Differentials can also growl when bearings are failing. The difference is the bearing growl usually is noticed when you accelerate or decelerate. Ring and pinion problems are noted by a whine in the same manner. Sometimes the whine is more pronounced while accelerating or decelerating.


Why not directly connect the axles to the ring gear? During turns, the outer wheel spins more revolutions than the inner wheel. Differential gears inside the carrier connect the two axles while allowing the wheels to spin at different speeds. If we didn't have the varying wheel speeds, the rear tires would squeal around every turn. The downside is, if a tire loses traction no vehicle movement occurs. The pinion gears allow the wheel without traction to spin freely. The upside to the spinning wheel without traction is less chance of losing vehicle control. This type of differential carrier is considered an open differential.


Positraction or locker differentials limit wheel spin when traction loss occurs. Performance vehicles use positraction differentials to limit wheel spin during hard acceleration. True four-wheel drive vehicles use positraction to traverse through mud and slippery conditions. Open differentials can be converted to positraction if desired.

The majority of GM positraction differentials have clutch plates that equalize traction on slippery surfaces. The clutch plates have spring pressure applied to prevent unwanted wheel spin. All early differentials use sintered iron metallic grooved clutch plates that rub against each other during cornering. The metallic plates rubbing against each other cause wear and a metallic paste to form in the differential gear oil. Additives are used in the diff gear oil to allow the clutch plates to smoothly apply and release.

When the diff gear oil and additives are depleted, popping and groaning can occur during cornering. Unlike the bearing noises from the axle or differential, positraction clutch plates make loud noises during cornering. Neglecting differential maintenance is the number one cause of positraction clutch plate damage. As simple as it is, very few owners change the differential gear oil until they hear noises, very bad noises.

GM also used a locker differential in some their trucks to equalize traction. The mechanical locker option (code G80) used large springs to mechanically lock the diff under wheel spin conditions. These are not commonly used in early GM pickup trucks and we will not cover them.


Our differential project involves a 1969 Corvette convertible. The '69 had been stored in an inhospitable climate with no protection from the elements. To begin with, we changed all the fluids for our preliminary assessment of how well the vehicle drove. Once the temperatures were up on the initial road test, the differential started to pop during cornering. As we put a few more miles on the '69, the popping became very noticeable, causing concerns during cornering. We could feel the clutch plates grab and let go violently, making the rear of the car sway while we went around corners. Certainly there were other concerns but we had to do something about the differential.

After many years of working with Corvettes we knew that there were not many options. In some cases, multiple diff fluid services solve sticky clutch plates. Why not try at least one more fluid change? GM did not provide a drain in the 1963-1996 Corvettes, so you either add a drain or use a suction gun. Using a suction gun will remove a large portion of the used diff gear oil. The problem is the really dirty metallic laden gear oil is left on the bottom of the diff. Adding fresh gear oil to the nasty oil does little to help eliminate clutch noise.

The alternative was to use our diff drain installation kit we provide through our website to install a permanent drain. Once the drain is drilled and tapped we wash the diff case out with brake cleaner to start with no leftover gunk from the old stuff.

Unfortunately, our '69 did not respond to our multiple gear oil changes. We would have to get into the differential internals to make a permanent fix. The '69 Corvette had the original number matching differential so we decided to repair what we had. This is something to consider when you are repairing or servicing any vintage GM vehicle. GM stamped the last six digits of the vehicle identification number on major drivetrain components. To many aficionados having number matching driveline components is paramount and adds considerable value. On the other hand, if you have a highly modified vehicle number matching is not an issue.


We had a major concern with the differential carrier that the ring gear bolts onto. We have found many diff carriers with wear at the pinion gear support shaft area. The pinion gear shaft load works on the carrier, wearing the round hole egg-shaped. This is when severe popping and groaning occurs, which may explain why our '69 had the high noise level during cornering.

Our decision was to replace the original Eaton carrier with a new, loaded Eaton carrier. This would replace all the worn pinion gears, shafts and clutch plates inside a new carrier. Eaton made the original assembly and still offers new direct fit replacements for all their GM applications. This made perfect sense as our 41 year old Corvette deserved a new start.

One more thought: were there going to be any other changes in the '69's future? There would be a major change concerning the transmission. The original Muncie four-speed had to go. Knowing that, we should consider what ring and pinion ratio would make sense. We took a look at Keisler Engineering's website to determine what gearing we should use.

Their Speed Analyzer calculates transmission gear ratios for a particular set-up to optimize our ring and pinion gear selection. Corvette Central sent our complete bearing service kit with a 3.55:1 ratio ring and pinion. Our Eaton carrier from Summit Racing was on the way. The last piece of the puzzle is what gear oil to use. We chose Lucas 80-90 heavy duty gear oil with GM positraction additive. We know that the Eaton differential carrier lasted 40 years with minimal maintenance. Proper servicing should keep our new Eaton diff carrier assembly going for the rest of the '69's life with ease.


There are two things to consider: If your GM car has a solid axle, the differential carrier can be replaced without removing the differential. Corvettes with independent rear suspension must have the differential assembly removed for any internal work. The advantage on the independent suspension diff is that, once it is out of the car, it can be handled easier. The solid-axle diff is bulky all the time. There really is no advantage to having the diff removed on the solid-axle car; bring them the car if the plan is to have someone else do the work.

Removal and installation of the diff requires typical hand tools. You can expect some corroded fasteners that require a breaker bar or heat to get them moving. The smart plan is to start by soaking all the fasteners with rust penetrant weeks before the work is planned. This makes sense, even if you decide on taking the car in for someone else to do the repairs.

Internal diff work requires special tools for quiet results. If you decide to keep the original ring and pinion that was quiet, incorrect assembly can cause a whiny rear end. We always check ring gear to pinion backlash before disassembly any time the original ring and pinion are to be reused. A simple bearing replacement job can go horribly wrong if the original backlash specs are not used.

Bearings can have different stack heights, changing pinion depth and resulting in different contact patterns. In most cases, the diff will be noisy from then on if the pinion depth is changed. This is why it is so important to check ring gear to pinion backlash before disassembly. We use a magnetic-base dial indicator to measure the backlash. There are no dial indicator rental outlets that we're aware of. You can find them at swap meets, Harbor Freight and other auto tool retailers.

Once you get by the dial indicator, bearing removal and installation tools make the job easier. You can use brute force to remove the bearings, just beware of their potential danger. The bearing races are very hard, shattering upon impact in most cases when hammers and chisels are used for removal. The metallic shards can pierce your skin easily when they fly off at high speed. Bearings can be installed with modified pieces of iron pipe. We use a specific removal tool to safely remove the bearings for reinstallation after a shim change.

Replacing bearings every time you need to make a shim change can be costly. This brings up setting the pinion depth. Specific tools are used to measure pinion depth that can save a lot of time. If you don't have access to these tools, you will have to rely solely on the ring gear to pinion contact pattern. We always use the contact pattern as the final test for proper pinion depth. The tools allow you to hit the mark much quicker without multiple disassembly procedures.

While we are in the pinion gear area we need to cover pinion bearing preload. Pinion bearing preload is used to assure that the pinion stays on center during the tremendous loads that are placed on it. When new bearings are installed the pinion bearings are tightened until there are 20 inch-pounds of drag required to rotate the pinion. This drag quickly loosens up and assures the pinion's center line is maintained. This is accomplished with a crush sleeve that sits between the inner and outer bearing inner races. As the pinion flange nut is tightened, the crush sleeve collapses inward until the bearings are seated. Careful wrench control is required as the sleeve collapses to apply the 20 inch-pound rotating drag on the bearings. If you surpass the prescribed rotating drag, you start all over by replacing the crush sleeve and going through the motions of crushing the sleeve again. When we are checking for proper ring gear and pinion pattern, we leave the crush sleeve out and tighten the pinion flange so we have drag on the pinion.

This helps provide a load between the ring and pinion gear when checking the pattern. We usually tighten the pinion flange to apply 40 inch-pounds of rotating force for the best pattern check. This will not hurt the bearings during assembly but it would, if we used the higher rotating torque for driving purposes.

Have you ever had numerous pinion seals replaced only to find them leaking in a short while? In most cases it is because of bearing or crush sleeve issues. Once the pinion flange is removed, it is very difficult to tighten the bearing's preload properly. Either the flange is loose or too tight, causing the pinion to go off-center, damaging the seals. The proper way to replace the pinion seal is to replace the bearings and crush sleeve. Once proper rotating torque is set, the pinion seal should not leak again.

Now that we have an idea of the skills and tools required, we can think about the task this way. If you are changing just the carrier and/or bearings, the task will be much easier than compounding it with a ring and pinion change. You would check the ring gear to pinion backlash and the contact pattern before disassembly. Once those numbers are recorded the work proceeds. As long as the backlash and pattern stay the same, the diff should be as quiet as it was before you started. We'll walk you through the Eaton carrier replacement soon. Our '69 Corvette will be another step closer to a full chassis restoration so we can confidently push it through some tight curves!

1 This is a typical 1963-1979 Corvette differential removed for restoration. The differential crossmember must be removed with the diff.


2 Make sure to check and record the ring gear-to-pinion backlash. Rotate the ring gear and check the backlash in two or three other locations to see if the ring gear is running true. If you find variances, use the average of all your checking spots.


3 We mark the diff carrier caps to make sure we put them back where they should be. The caps are line-bored into the raw casting. This means that the caps are unique from one diff to the other and side to side.


4 Used bearings usually allow you to remove the carrier without much trouble. New bearings require a slight load on them during assembly. As we pull out the carrier, the bearings and shims on each side are put in their respective place on each side. This way we can record the shim thickness and put the shims back where they came from.


5 If you plan on using the original pinion gear again, carefully tap it out with a brass drift. Don't be surprised if the pinion flange and outer pinion bearing have a tight fit on the pinion, requiring a lot of force to remove the pinion gear.


6 Removing the inner and outer pinion races can be difficult. Work the drift punch back and forth while tapping the race out. Keep an eye on progress. It is easy to get the race cocked and damage the race bore.


7 Our Mac Tools bearing removal tool easily removes the side bearings. Of course, a punch and hammer can be used but beware of the possibility of shattering the inner bearing cone. Those slivers of hardened steel hurt and can go deeply into your fingers.


8 Installing new bearing races takes patience. They must go in straight to avoid bearing or bearing bore damage. The bearing race must also be seated fully in the bore for proper pinion bearing depth.


9 Always torque the ring gear to the carrier according to the manufacturer's specs, using new bolts if provided. Tighten and torque the bolts in a criss-cross pattern to assure proper seating of the ring gear on the carrier.


10 Here is where you need to be careful. The carrier bearings sit slightly below the bearing mount area. The socket we are using will slip over the carrier at the bearing area allowing us to fully seat the bearing.


11 What pinion depth shim do we need? We start with a .028 shim if we don't have measuring tools to set depth. This is where our bearing removal tool is priceless. We can change the shim without bearing damage, if required.


12 White grease paint is used to check pinion pattern and position. This is an ideal pattern on the drive side of the ring gear. Too bad the pattern is incorrect on the coast side. Our contact pattern is too close on the inside. We need to take at least .004 of shim out and try it again.


13 Once the pattern is established we install the crush sleeve that sets pinion bearing pre-load. Crush sleeves are used one time only, if you go too far they must be replaced. You should never back off the pinion nut to loosen the pinion gear.


14 Now we can finally install the pinion seal with the crush sleeve in place. We have a holding tool and long bar for our ratchet to be able to crush the sleeve. The six-foot holding bar and wrench extension still require a lot of force to get the job done.


15 The carrier is installed for the final time using the hardware kit supplied shims. The replacement shims can be tapped into place with some force. This sets carrier bearing preload and must be done this way. If the shims slip in easily, there will be ring and pinion gear damage.


16 This is a typical 1963-1982 Corvette axle (spindle) bearing. The bearings are similar to a front wheel bearing assembly with inner and outer seals.


17 Here we have a 1992 Chevrolet 1500 series pickup truck axle pulled out of the diff housing. If you look closely, there is wear on the axle shaft that required axle replacement or a modified bearing assembly.


18 The truck axle seal was leaking from bearing wear, allowing the axle to move around. All too often the seal would be replaced without a concern of bearing or axle condition.


19 A slide hammer is used to pull the seal and bearing out as an assembly. There are no retaining rings keeping the bearing or seal in place, just tension. Watch your hand when you start slapping the slide hammer to remove the bearing. We've got more than one nasty bruise from pinching a finger with the slide hammer.




Eaton Positraction carrier assembly


Ring and pinion bearing and seal kit


Transmission Wizard

Speed Analyzer



(800) 230-3030


(800) 345-4122


(888) 609-0094

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Title Annotation:CORVETTE & CHEVY
Author:Petris, Chris
Geographic Code:1USA
Date:Jul 1, 2011
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