Text Box: You have questions?
We have (most of the) answers.

How do I know my differential is going bad?


Well, the easiest way to tell that your differential is trying to tell you something is when your driveshaft turns and the vehicle doesn't move - that’s pretty much a given that you need rearend work NOW.


 Usually, though, your differential lets you know ahead of time by making odd noises. The most common noise is a steady whining or grinding that increases with vehicle speed - that usually indicates problems with the bearings or gears. A rhythmic banging or clunking around corners can also be indicative of impending failure.


Oil leaking from the seals is also something you want to take care of before  the fluid gets low enough to cause major problems.

Mellow yellow nine-inch Ford

Text Box:   Rearend
Text Box: Serving the San Francisco Bay Area since 1988

Differentials generally use tapered roller bearings such as these.


How long does it take to fix?


It depends greatly on what the year, make and model of your vehicle is, but we can usually rebuild the most common differentials in a day or two.


It’s even possible, in some cases, to rebuild your rearend while you wait, although you’ll need to make a special appointment so we can have a spot ready for you.


We stock thousands of parts in our on-site warehouse, but some less-common applications require additional time to order parts. If we don’t have the parts you need, we’ll find them as quickly as possible and keep you posted on the status of your car or differential.


In some cases, especially in the case of antique vehicles, we’ll need the customer to supply the parts due to their rarity - many owners of these vehicles have better resources and contacts than we do.


Jobs that require custom-made axles, powder-coating  or special machine work can take up to a couple of weeks due to the one-off nature of these sorts of processes. 




   What will it cost to fix my problem?


Ah, now comes the hard part. We can usually give you a rough estimate over the phone and we have some price guidelines here , but we generally have to look at the vehicle or differential to give you an exact quote.


Experience has shown us that we’re usually much cheaper than dealerships and tend to be cheaper than independent repair shops. Heck, we can often rebuild your differential with new parts for less that the cost of a junkyard unit and the labor to put it in. Our prices tend to be lower because we ONLY work on differentials and have become very efficient in doing so over the years, plus we buy parts in great volume.




Rearend Specialties offers a one-year parts and labor warranty in most cases—we stand behind our work. The exceptions?


   Commercial vehicles are limited to 25,000 miles.

   Trucks and cars with grossly oversize tires are not warranted against gear or axle breakage.

   Racing or high-performance vehicles with much more horsepower than the differential is designed for.

   Vehicles that have been overloaded, run with improper and/or incorrect amounts of lubrication, or otherwise abused.


For more information about our warranty policy, give us a call!



Racing axle shafts

Posi? Limited slip? LSD? Locker? Spool?

I’m confused.


Yeah, we don’t blame you a bit, all the terminology is confusing as heck. Basically, there’s five types of differential units in common usage today.


Open Carrier:


This is the simplest and most common differential unit found in cars and light trucks, it contains four gears (“spider gears”) which allow the rear wheels to turn at differing speeds so that cornering doesn’t cause the inside wheel to be “drug” along by the outside wheel. The drawbacks of this type of unit is that in low-traction conditions such as ice or mud, one wheel will spin frantically and the other won’t move at all.


Limited Slip Differential (also referred to as “posi” and LSD):


Limited-slip differentials use the same spider-gear arrangement as an open carrier, but add a clutch pack, worm-gear arrangement or tapered cone setup to feed some power to the wheel with the least traction, regardless of road conditions. These units are generally smooth and relatively unobtrusive, although some of the aftermarket and non-OEM units may make some noise on turns. The drawback of limited-slip units is that the clutches/gears/cones may not feed enough power to the slipping wheel to pull the vehicle out of wherever it’s stuck.


Ratchet-action lockers (usually called Detroit™ Lockers):


These units dispense with the spider-gear arrangement completely and just use a series of spring-loaded toothed rings to either fully-lock the differential together or totally unlock one wheel on turns. These “lockers” are extremely strong and will feed 100% of the engine’s power to either wheel for maximum traction. The drawback is that they are VERY noisy when engaging/disengaging and will not unlock under power-on conditions, making freeway onramps a white-knuckle experience. The also require a large amount of play internally, causing “clunks” when shifting the vehicle in or our of gear.


Electric/Air/Vacuum/OX™  lockers:


These types of locking differentials use a standard or limited-slip differential combined with a user-activated locking apparatus that fully locks both tires together when switched on.  These units have the advantage of combining silky-smooth operation on dry pavement with full-lockup when off-road or in slippery conditions. The Ox™ locker uses a cable-and-shift-fork setup to engage, while the Air-locker™ needs an on-board compressor to feed compressed air to the differential. The electric units are the simplest to install, with only a couple wires to plumb into the rearend, and vacuum-operated lockers are OEM only at this time. The downside to such diffs is high cost and complexity, but their operation is close to ideal.




Spools are simply a device that completely locks both wheels together 100% of the time, there’s no differential action at all. These units are not really suitable for the street, they’re primarily used for drag racing and certain off-road applications.  While cheap and very stout, spools cause a huge additional load on axles, so much so that aftermarket axles are highly recommended.


For more information and pricing, click here



It’s Grand Cherokee Day  at RS!

All set up and ready to go!


The springs indicate this is a limited-slip unit


Everyone tells me I should “change my gears” Why?


There are several reasons to change your vehicle’s gearing.


If your car or truck is fitted with a tire/wheel combination that’s much taller or shorter than stock, you may need to compensate for the difference in tire height by changing the gears.


When you modify your vehicle’s engine for more high-RPM performance, you’ll often need to change the gearing in order for your newfound power to be properly utilized.


If your older car or truck has excessively low gearing and cannot comfortably go down the freeway without the engine “screaming”, changing the differential gearing can help lower the motor’s RPM.


And if you tow a heavy trailer, sometimes changing the gears can extend engine and transmission life by reducing “lugging” and constant transmission shifting.


For more information and prices, click here

       Can I buy the parts from you and do the work myself?


Of course you can, our parts warehouse is open to the public. We do ask that you bring your old parts to us so we can match them up exactly, as there are many slight variations that can only be identified by visual inspection.


We do recommend that you’re sure you have the necessary tools that you will need before attempting to disassemble your diff, as a normal “shade tree” mechanic’s tool kit might not contain  some critical tools. A dial indicator, precision machinist’s calipers, torque wrench, drag gauge and an impact wrench capable of 400 ft/lbs of torque are all required to overhaul the typical differential, plus you’ll need access to a professional-type bearing separator and a hydraulic press.


If cost is a critical factor but you’re perhaps not confident in your ability to properly rebuild your differential, consider removing it from the vehicle and carry the unit in as a bench unit—the cost is far lower than driving or towing in the entire car or truck.


For more information and pricing, click here