Rearend Rehab: Why Pay Someone Else To Change Gears?

When it comes to the rearend of your car or truck, have you ever thought about changing the gears? Of course you have, you’re a hot-rodder. But like many, you probably have a few reservations about starting the task. Don’t be embarrassed, you’re not alone. Hot rodders have been changing rearend gearing for years, scratch that, they have been paying someone to change rearend gears. But why? Why pay someone else to do a job you can certainly do at home?

Since many are reluctant to tackle this task, we thought we would try and help alleviate some of those fears by reaching out to Jeff Anderson at Moser Engineering, and have him give some insight into this “black magic” of automotive upgrades.

It’s not that the task is physically tough to accomplish. Sure, it takes a few specialized tools, but they can usually be purchased inexpensively. You might even have a friend that will loan them to you.

The basic concern that many have, comes down to a lack of confidence. Installing a set of gears does require precision, and the math involved can be daunting. If not installed properly, a couple things can happen: The gears will be noisy while driving the car, and/or wear prematurely and need replaced again.

Setting your reservations aside, you’ll learn that if you take your time, you too can swap rearend gears and realize great results. To help alleviate the concerns that many people have about installing rearend gears, we decided to put together this little outline to demystify anything that might seem like a black magic of sorts. This is not an install article, but rather, just a guide to help you decide if taking on this task is something for you.

We started by asking Jeff, what was the one piece of advice he could give someone before they start tearing their car’s rearend apart if they are thinking about handling a gear swap themselves.

“I would say have someone help you who has done it before… At least as an over the shoulder instructor or guide. Some parts of the process are more than can be discerned off YouTube videos, and being your first time, you want to make sure you are following the proper steps. You really need to understand the interaction of the pinion to the ring gear, the preload, shims, and everything else that is critical to setting up a ring-and-pinion,” he said.

Okay, maybe that didn’t instill much confidence in you, but it really is good advice. If you’ve never worked on your car’s rearend, it is a setting-sensitive piece of the automotive puzzle. But then again, so is rebuilding just about any aspect of a car, and you learned most of the other puzzle pieces, so, you got this. It also relays the fact that this is something that does require patience, a solid knowledge of how a rearend works, and finally, the proper tools.

What’s What?

Like we said, this is not going to be an installation article, but rather, an informative piece that gives you some insight into some of the terminology and conditions that will need to be understood. For instance, backlash is not something you encounter when you incorrectly answer your wife’s question, “Do these jeans make my…” Trust me, automotive backlash is a good thing – the other one, not so much.


This describes the clearance between the ring gear and the pinion gear, and is usually measured in thousandths-of-an-inch by using a dial indicator. For instance, US Gear recommends .008- to .012-inch for street use and .006- to .008-inch for competition use. Backlash between the ring-and-pinion gear is set after the pinion gear is properly installed. Backlash is adjusted by adding shims to either side of the differential side (carrier) bearings. The side bearing preload is also adjusted when setting the backlash. There is no specific number to achieve when preloading the carrier bearings per se, but when you have backlash set, the differential should not spin freely enough that it coasts when you spin it and then let go. Also, you do not want it too tight. Compare it to loading or slightly tightening a front wheel bearing.

rearend rebuild

Backlash is the crucial setting of the ring gear contact of the pinion gear. If there is too much backlash, the ring gear needs to move closer to the pinion gear. If there is not enough, the ring gear needs to move away from the pinion gear.

Just like the side bearings, the pinion bearings are adjusted by adding and removing shims of varying thickness. After the pinion bearing preloads have been set, this is the time that backlash is set. You will be checking the gear mesh pattern with dye or marking compound. Shoe polish is not recommended – especially for the novice, as it does not make the wear pattern as visible.

What happens if backlash is too tight or too loose: Too tight will generate excessive heat and quickly wear out the gear. If it is too loose, this will allow the gears to beat on each other, and you will eventually fracture or break teeth off the gears. Either way, the rearend will be noisy.

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Pinion depth is the distance from the axle center line to the end face of the pinion gear (checking distance).

Pinion Depth

Since we mentioned the pinion gear in the previous paragraphs, we should give a little explanation about that piece of the puzzle. The pinion gear is the input gear of the rearend to which the driveshaft connects. This gear delivers the power from the engine to the ring gear, which then send it to the wheels. This gear requires a specific depth be achieved within the rearend housing when installed, and is set with the use of shims. The shims control how the pinion contacts the ring gear in relation to the axle center. Pinion depth is made deeper or shallower by adding or removing these shims.

When it comes to setting pinion depth, Jeff had this to say, “Having the right tools will save you time and eliminate headaches. I would suggest renting one from a local parts store if that is an option. It is cheap, and will save you a ton of headaches and worry about getting it right.” None of our local auto parts stores carried the proper tool, but we found a simple pinion depth-setting tool from Ratech that is available through Summit Racing and it only cost $25.00. While this is not what many would call a “precision” tool, it is a relatively inexpensive option for the hobbyist, and it will get you very close.

Crush Sleeve

The crush sleeve is basically a soft bushing or spacer that is used to hold the inner and outer pinion bearings apart and offer some resistance when tightening the pinion nut. When installed, the pinion nut is torqued to spec, compressing the “crush sleeve” until proper pinion bearing-preload is achieved. One thing to keep in mind when working with a crush sleeve, is that it is a one time use item. If you over tighten the pinion when setting the bearing preload, you will have to replace the sleeve – it’s not reusable.

rebuild rearend

While the crush sleeve (left is the factory method for filling the space between the pinion bearing and setting bearing preload, a solid spacer is an alternative.

We also asked Jeff if there were any tricks or an easy way to deal with a crush sleeve.

“There are not really any tricks when it comes to setting up a gear. Shortcuts are not advised. One thing to remember when you get to higher horsepower applications, you may need to use solid shims instead of a crush sleeve, as high horsepower power can cause the crush sleeve to compress under extreme loads, which could change bearing preload,” he said.

Having the right tools will save you time and eliminate headaches. – Jeff Anderson

The solid shims (or spacer) that Jeff mentioned is not collapsible like a crush sleeve, and a spacer can be reused if need be. For instance, if you have a leaking pinion seal and need to replace it, if using a crush sleeve, you will also need to replace the sleeve. If using a solid spacer, it is reusable.


When someone mentions the drive or coast area of the gear, this refers to the “face” of the ring gear’s teeth. The drive side of the tooth on the ring gear is what contacts the pinion gear when the vehicle is accelerating. When someone mentions the coast side of the gear, this is the backside of the tooth on the ring gear that is contacted by the pinion gear when the vehicle is decelerating. When setting up gears, the set up specifications will mention this.

Wear Pattern

This is a visual interpretation of gear mesh that lets you know if you have the gears set up correctly. A “pattern” refers to the visible tooth-contact area on the ring gear that is exposed when using a set up dye to see how the ring gear and the pinion gear mesh. This is where the face and coast side of a gear tooth comes into effect.

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Proper wear pattern is determined by the pinion depth. A proper wear pattern shows the wear centered on the gear teeth.


The pinion bearings and the differential’s carrier bearings must run a little “snug”, and have a certain amount of resistance (preload) they need to overcome. This is similar to the way a wheel bearing should be loaded during assembly. The bearings require this “snugness”, in order to eliminate any play within the bearing. This “preloading” is measured by checking the torque, usually with a “beam style” torque wrench that is required to turn the bearings.

The Gear

When it comes to the describing the ring gear, there is the toe of the gear, which is the portion of the tooth surface that is nearest the center of the ring gear. The heel of the tooth is the area nearest the outer edge of the gear. The top land of a gear tooth is the surface at the very the top of each tooth.

Time For A Change

Every gear has a required set of measurable parameters that need to be established during installation. These must be followed so that the gears don’t make noise when driving, and so they last as long as possible. When you are rebuilding and need to make pinion gear-depth changes, it is general practice to use shims measuring within the range of .002-inch to .004-inch at a time, until the correct wear pattern is achieved.

When a change in backlash is required, shims approximately 1 1⁄2 times the amount of change required are typically needed to bring the gears into spec. For example, if you need to change the backlash by .004-inch, shim(s) measuring .006-inch is a starting point. The actual amount of change obtained will vary depending upon the gear ratio and gear size.

A loose or high backlash (too much), is corrected by shimming the differential and moving it closer to the pinion gear. A low or tight backlash (not enough), is corrected by moving the differential away from the pinion. Correcting these conditions is easily done by moving shims from one side of the differential to the other.

rearend rebuild

This pinion-depth tool is relatively inexpensive, and is a good option for the home enthusiast as opposed to the otherwise expansive precision tool.

That Doesn’t Sound Right

Sometimes, your rearend will make an unwanted noise. Okay, we’ll wait for you to stop giggling… Anyway, in regards to your car, this can happen before or immediately after a rebuild. If the rearend in your car is making an unwanted sound, understanding what causes it should be the first step in diagnosis.

Humming Noise

If you hear a humming sound coming from the rear of your ride, that is not typically caused by anyting going bad, but rather, an incorrectly installed ring-and pinion-gear. Typically, the noise is created when there is too much backlash. Incorrect installation of the ring-and-pinion gear prevents proper tooth contact and can cause rapid tooth wear and gear failure. This humming noise converts into growling noise when the wear increases. A humming noise can also be due to worn out bearings, which need to be replaced.

rearend rebuild

Noise When Accelerating/Decelerating

A noise that gets louder while your vehicle is accelerating, probably means that there is heavy contact of the pinion-gear teeth on the outer ends of the ring-gear teeth (pinion gear not set deep enough). Conversely, if the noise gets louder when the vehicle is coasting or decelerating, it probably means there is not enough tooth contact (too much backlash).

“Sometimes it can be hard to distinguish between differential noise, pinion or carrier bearing noise, axle bearing noise, and gear noise. A ring-and-pinion can have noise related to the bearings that hold the alignment to one another. As the bearings wear out, the gear engagement, alignment or depth can change and cause these issues. Sometimes it can be a pinion bearing causing the noise because of a bad driveshaft angle or load. Of course, sometimes the gears just wear out,” Jeff informed us.

Knocking Noise

A knocking or clicking sound may be due to the splines on the axle shafts being worn out, in which case the defective shaft has to be replaced. Another cause could be the result of a chipped tooth on one or more of the gears in the differential. If that is the case, they’ll need to be replaced.

Noise When Turning

If there is a noise coming from the vehicle’s rearend while making a turn, this sound is usually due to trouble in the axle shaft, or a problem inside the differential itself. More often than not, a bad axle bearing is the reason for the noise that occurs when cornering. When the noise comes from the right side of the car during a right turn, it usually means that the outside axle bearing is worn. If that is the case, if you turn in the opposite direction it will usually reduce or even eliminate the noise if it is a bad axle bearing.

rearend rebuild

installing the shims to adjust pinion depth does require a shop press, as the bearing is a press fit to the pinion shaft. Setting pinion depth usually means removing and reinstalling the bearing multiple times to get the proper depth.

If the cause of the noise is coming from deep within the rearend at the differential, this can be caused by either tight-meshing gears, or too much backlash on these gears. Another cause could be that the differential-case bearings have gave up the ghost. These can also cause noise when turning.

While we will concede that swapping the rearend gears of your car is not something to be taken lightly, hopefully, this overview about rearend gear set up is enough to give you the encouragement you need to tackle this job yourself.

Article Sources

About the author

Randy Bolig

Randy Bolig has been working on cars and has been involved in the hobby ever since he bought his first car when he was only 14 years old. His passion for performance got him noticed by many locals, and he began helping them modify their vehicles.
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