Frequently Asked Questions

No. Many people think that shim kits are simply an assortment of shims of different thicknesses. The reality is that shim kits take a lot of time, research & quality control to get right. Many of our shim kits are designed by our in-house engineering & design team, ensuring you’re getting the highest quality components available. We review all shim thicknesses and quantities to determine what is best for each application, verify that the correct steel is being used and verify that the heat treating process has been done properly. The end result is a kit that is not only complete, but won’t compress or warp after installation. We stock over 50 shim kits for a variety of applications, including our line of “Super Shims”. Super shims are an interlocking carrier shim kit that allows you to drive each side in as on complete unit. Standard carrier shim kits are difficult to install and easy to damage. We carry super shim kits for over 15 applications.

Yes. The carrier bearing caps are bored at the factory and are side specific. Mixing up the carrier bearing caps can be a major mistake in rebuilding a differential, especially if it is a design which uses side adjusters. A good practice is to take a punch and mark one of the carrier bearing caps along with the side of the housing it belongs to in order to prevent mistakes during reassembly.

No. Gear sets are lapped at the factory and are a matched set. Attempting to run two different gears together will result in gear noise and eventual failure.

No. Once the crush sleeve’s tension between the bearings is released it cannot hold the proper tension again. This is also true if a crush sleeve is over-crushed during installation. It must be discarded and replaced with a new one.

Here’s 4 Reasons To Change Your Gear Ratio.

If you have larger than stock tires on your vehicle, the benefits of gearing CANNOT BE MATCHED BY ANY OTHER MODIFICATION TO THE VEHICLE!

By regearing it is guaranteed that:

Even a simple change from a 31″ diameter tire to a 33″ diameter tire will affect the amount of available torque by 10%.

Modern day transmissions cost $600 or more to perform a basic overhaul. Driving with even a 10% larger tire will result in the transmission being under the same load as always being driven on an incline. The transmission may not suddenly fail, but the life will definitely be reduced by thousands of miles.

RPM alone does not affect fuel mileage. An undergeared vehicle (as the result of oversized tires) will result in a substantial loss of fuel mileage. Also, just as with the transmission, your engine life will greatly be reduced.

With kits in the $100+ range to calibrate speedometers after tire swaps, it seems pretty important to have an accurate speedometer, which can also be fixed with a simple gear change.

Don’t put a bandage on a serious problem. By installing the proper gear ratio, easily determined with our gear calculators, you can save thousands of dollars and days of vehicle downtime, plus have a little fun to boot.

Option (RPO) Codes are a standard coding system made up of a combination of 3 alphanumeric characters and refer to a specific option or modification to the vehicle.

Visit here to find out what your specific Option Code means:

Watch our video on GM RPO codes here: Decoding GM RPO Codes

It is nearly impossible to measure the preload on a carrier because it is in contact with the pinion at the time of assembly and therefore is receiving resistance from it as well. A carrier should have to be loaded in with some resistance, such as a few hits from a dead blow hammer. It should not simply load in by hand, and it should not take a huge amount of force to put into place.

Most people are reluctant to really crank down on the side adjusters in their differential. Attaining a good preload through the side adjusters can be critical in protecting your differential from the high torque loads that cause ring gear deflection. It is not uncommon to torque side adjusters to 150-200 ft/lbs when setting up a differential.

On a crush sleeve design differential it usually takes between three hundred (300) and four hundred (400) foot pounds of torque to crush the crush sleeve. Over the years I have used huge breaker bars and/or very strong air operated impact wrenches to crush the crush sleeve.

For a normal, daily driven, vehicles we advise that you change the differential oil every 50,000 miles. For vehicles that are hard worked, such as trucks that do a lot of heavy towing, we advise that you change the differential oil every 15,000 miles.

When testing your pattern on a used gear, it is often difficult or impossible to get a good pattern on the drive side of the gear. The reason for this is that through use, the drive sides of the gears get worn and won’t show you the pattern clearly. The solution is to check the pattern based on the coast side of the gear. In standard rotation front differentials, you will still want to check the drive side of the gear, since that is the side which gets the least amount of wear in those applications.

When a differential is traveling in a straight line, the spider gears remain motionless in the carrier. It is not until one tire turns faster or slower than the other that the spider gears rotate on the cross pin shaft. This most commonly happens when turning a corner. However, other situations cause the spider gears to spin much more rapidly, such as getting stuck in the mud or snow. When this happens, the spider gears can rotate on the cross pin shaft so quickly that it slings all the differential oil away from it, giving way to metal-on-metal wear. This causes the cross pin shaft and the gear to get so hot that they melt each other, sometimes to the point where they weld themselves together. Damage such as this can not only damage the spider gear set, but destroy the carrier and ring & pinion set.

Do NOT use bearing grease on your carrier bearings or pinion bearings when setting up your differential. This could cause premature failure from the oil not having the ability to lurbicate the bearings properly. Use clean gear oil only to pre-lubricate your bearings during the install.

Set-up bearings are bearings which have had their inner diameters machined so they slide on and off a pinion shaft or carrier journal. The advantage to using set-up bearings is that you can quickly install or remove them with different amounts of shims to check both pinion depth and backlash without having to worry about the nice, new bearings you just purchased. Once the correct amount of shim(s) have been found, you simply remove the set-up bearing(s) and install the new bearings with the correct shim.

Jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posis and lockers and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading.

Differential Gear Ratio determines the number of times the drive shaft (or pinion) will rotate for each turn of the wheels (or ring gear). So if you have a 3.73:1 gear ratio the drive shaft turns 3.73 times for every turn of the wheel.

Gear ratio is calculated by dividing the number of teeth on the ring gear by the number of teeth on the pinion gear. The higher the number, the lower the ratio: a 5.29 gear has a lower ratio than a 4.10 gear. With a lower gear ratio the drive shaft (and thus the engine) turns more for each revolution of the wheel, delivering more power and torque to the wheel for any given speed. Lower ratios are generally desirable when going off-road. Higher ratios are better for freeway driving since they run at lower RPM’s and offer better fuel economy.

Changing tire size affects the final drive ratio. Switching from a 30″ tire to a 35″ tire changes the final drive ratio by about 17%. This may drop the engine out of its’ “power band” and result in poor performance and fuel economy. To restore performance you must change the gear ratio to compensate for the change in tire size. If you originally had 3.07 gears you need a ratio that is approximately 17% lower, such as 3.55. If you want to increase off road performance you might want a 4.10 or lower ratio.

A solid spacer replaces the crush sleeve in a differential. It is a solid piece of machined steel which is slightly shorter than a crush sleeve and comes with a variety of shims. The shims are stacked with the solid spacer to achieve different levels of preload on a pinion, the same way as a crush sleeve. The advantage of using a solid spacer & shim set up is that you don’t need to worry about over-tightening the pinion nut, as you do with a crush sleeve. Also, once the preload is set, you never need to worry about adjusting it or replacing anything, like if you have to replace a yoke. When setting preload, you add shims to decrease the amount of preload on the pinoin bearings, and remove shims to increase the amount of preload on the bearings.

A clunking sound that only occurs while turning is a result of broken or damaged spider gears. Spider gears do not move at all while traveling in a straight line. If this is the case then the spiders will need to be replaced and possibly the carrier as well. Be sure to inspect the ring and pinion to be sure broken debris did not damage it as well.

The best compound we have found for checking a pattern is a tan, “peanut butter” colored type. There have been other colors over the years, including a dark blue compound, however the dark color of the blue makes it difficult to get a clear view of the pattern against the dark color of the ring & pinion set. The tan color provides a good contrast of colors and is much easier to see.

All new gear sets require a break-in period to prevent damage from overheating. After driving the first 15 or 20 miles it is best to let the differential cool before proceeding. I recommend at least 500 miles before towing. I also recommend towing for very short distances (less than 15 miles) and letting the differential cool before continuing during the first 45 towing miles. This may seem unnecessary but I have seen many differentials damaged from being loaded before the gear set was broken in.
I also recommend changing the gear oil after the first 500 miles. This will remove any metal particles or phosphorus coating that has come from the new gear set.

Whirring noise only while decelerating at any or all speeds is most likely caused by bad pinion bearings or loose pinion bearing preload, and almost never by bad ring and pinion gears.

A howl or whine during acceleration over a small or large speed range is usually caused by worn ring and pinion gears or improper gear set up.

Rumbling or whirring at speeds over about 20 mph can be caused by worn carrier bearings. The noise may change while turning.

Regular clunking every few feet may indicate broken ring or pinion gears.

Banging or clunking only on corners can be caused by broken spider gears, lack of sufficient positraction lubrication, or worn positraction clutches.

Rumble while turning may indicate bad wheel bearings.

A steady vibration that increases with the vehicle’s speed can be caused by worn u-joints or an out of balance driveshaft.

Clunking only when starting to move or getting on and off the gas might be loose yokes, bad u-joints or worn transfer case or transmission parts.

It is most likely a result of the gear not being broken in properly or driven “too hard, too soon.” When this happens, the gear oil breaks down and the resulting temperature inside the differential causes the face of the gear to wear away exposing the softer metal underneath. When this happens the gear will wear down more and more until the gear completely fails.

In order to ensure long life, all moving parts of the driveshaft should be regularly lubricated. This includes the slip yoke and stub shaft assemblies, universal joints and the centering kits in the CV heads.

Universal Joints: Using a grease gun, pump grease into the universal joint until fresh grease can be seen coming out of all four seals. If all four seals do not purge fresh grease, move the driveline to free up the bearings caps and try again until successful.

Slip Yoke & Stub Shaft Assemblies: Coat both the slip yoke and stub shaft thoroughly prior to assembly, and then fully collapse the driveline. Apply grease via the grease fitting until grease begins to come out of the welch plug vent hole. After grease appears in the vent hole, cover it and continue to grease until it begins to show grease in the seal. During re-lubrication it may not be possible to fully collapse the driveshaft. Use the same greasing guidelines as used during initial assembly but be careful not to overfill the driveshaft. Overfilling the driveshaft with grease may cause the welch plug to pop out during use.

Centering Kits: Using a needle nose grease gun, pump grease into the flush mount zerk fitting on the centering kits until fresh grease appears at the ball seal or purge hole. Failure to maintain grease in centering kits can cause failure very rapidly. Worn centering kits will squeak, indicating that greasing is necessary immediately.

How often a driveshaft should be lubricated is determined by how the vehicle is used. We recommend the following lubrication guidelines:
Vehicle Usage Lubrication Schedule Approximate Mileage
City Bi-months 6,500
Highway Monthly 12,000
Off Highway Every three months 6,500
Off Road Monthly 2,500

Yes. It is not uncommon to gain up to a third of a rotation of “slop” in your driveline after a locker has been installed. This is perfectly normal, and is all part of how the locker operates.

Yukon recommends the use of high grade GL-5 gear oil and Yukon LSD (Limited Slip Differential) additive for long life and smooth operation. Part number OILADD. The use of synthetic oil is allowed but may result in aggressive or noisy operation.

A limited slip, or positraction, differential typically uses some form of clutches that bind up the differential, providing traction to both tires. The clutches will slip to some extent to allow tires to turn at different speeds on corners. Some limited slip differentials are more aggressive than others, and some can be set up or “pre-loaded” more or less aggressively. Limited slip units require a special gear oil additive and may chatter when turning. Clutch packs may also wear with time and require replacement.

Open – Torque takes the path of least resistance and will go most easily to the tire with the least amount of grip. (If a tire slips the torque to every other tire always decreases) The rule is that the available traction torque is 2x the amount of traction torque (drive torque) available to the wheel with the least traction.

Limited Slip or GM Posi – Torque biasing differential, can move a certain amount of torque from the low traction side to the high traction side. This is done by applying a measured amount of braking force to the low traction side and moves it to the high traction side. The more braking applied (traction torque) via clutches, the more torque can be distributed to the other tires. These clutches will slip to some extent to allow tires to turn at different speeds on corners. The amount of torque transfer available is called its locking factor or bias ratio. Ex. 4:1 bias ratio, low traction tire can support 100 lbs-ft of traction torque, the limited slip can deliver 400lbs-ft to the high traction side. Types: Plate Clutch, Cone Clutch and Gear Type, each work slightly differently with different pros and cons but have the same result.

Locker – True lockers deliver 100% of the input torque to either wheel or both wheels, while being able to differentiate in a turn. Types: Automatic Locker – is speed sensing. On-demand – is driver controlled.

Put the transmission in neutral and jack up both tires. Turn one tire. If the other tire spins the opposite direction then you have an open differential, and if it spins the same direction then you do have a traction device.

The ID tag location varies by manufacturer.

New Venture Round tag on the back of the unit.

Borg Warner Tin tag located on the case half bolts.

Magna Sticker that can be found in a few different locations on unit.

First, find an area that allows you to drive your vehicle in a straight line approx. 100 feet.

Mark all 4 tires at the bottom equally, then either roll or drive the vehicle in a straight line watching one tire counting 10 revolutions, stop and set the hand brake. Inspect the marks on the other three tires. On most torque on demand or AWD vehicles the marks should be under 1 ½” to 2” at the most. If the marks are greater than 2” you should check tire wear or air pressure to see what is causing the difference. If you were to operate any vehicle with AWD, Auto 4WD or engaged in 4WD for any length of time with tires past specs it will greatly shorten the life of the driveline.

Normally the flow of power goes from the transmission to the rear wheels through the driveshaft. However, when you shift into 4WD, that power has to be split with the front wheels, that happens in the transfer case. When you shift into 4WD, gears are engaged to power a chain drive that runs from a gear driving the rear driveshaft to power another set of gears behind the front driveshaft. Once engaged, this driveshaft delivers power to the front differential and out to the front wheels.

Transfer cases use a combination of Drive, Housing and Shift types.

Drive types:

Gear driven transfer cases use a set of gears to send power to the front and rear axle. While gear driven transfer cases are more durable, they are also louder and less practical for smaller vehicles because of their weight.

Chain driven transfer cases use a chain in place of a gear set. Though most chain driven cases only drive one axle, there are case systems designed to drive both axles with a chain. Chains are lighter and quieter, but weaker than gear sets.

Housing types:

Married transfer case housings are bolted to the transmission, often between the output shaft and the main driveshaft. Some married transfer cases share housing with the transmission.

Independent housings are installed separately from the transmission casing and are connected to the transmission output shaft with another driveshaft.

Shift types:

Manual Shift On-the-Fly (MSOF) transfer cases are controlled with a lever on the driver’s side floor of most vehicles. These transfer cases have two automatic sealed front axle hubs or two manual front axle hub selectors. High 4WD settings can be engaged at low speeds, but low 4WD settings must be engaged when the vehicle is stationary and the transmission is in neutral.

Electronic Shift On-the-Fly (ESOF) transfer cases have a dash-mounted selector, usually a switch or set of buttons. These cases have sealed automatic locking front axle hubs and a transfer case motor. High and low 4WD is engaged in the same ways as a MSOF transfer case.

First, let’s consider a rebuilt transmission. In most cases, a vehicle experiencing transmission problems is brought to an automotive repair shop. There, the transmission is removed and worked on by a mechanic who completely disassembles the transmission all the way down to its component nuts and bolts.

The mechanic inspects and cleans each and every transmission component and determines which parts are in working order, excessively worn, or just plain damaged beyond repair. Parts deemed worn or damaged are replaced with new or “used” components, and the unit is then reassembled. Once completed, the transmission is reinstalled and the vehicle is ready to hit the road. In this scenario, the transmission has been “rebuilt” – repairs have been made, bad parts have been replaced, but the unit has received few, if any, upgraded components.

A remanufactured transmission is basically the same as a rebuilt transmission, but the work has been performed by a dedicated transmission repair facility or shipped to the original transmission manufacturing facility. By opting for a transmission remanufacture, the customer is assured the transmission is being repaired by absolute experts on that particular transmission and has been remanufactured to incorporate all design and product upgrades that weren’t available when it was originally manufactured. This example also applies to transfer cases and differentials.

The terms 4WD and AWD are often used interchangeably, but they don’t refer to the same type of power train.

4WD refers to a system where your vehicle runs with two-wheel drive under normal road conditions. If you drive in a difficult situation, such as ice, snow and mud, you can engage the transfer case to activate 4WD. The power is now transmitted to the road by all four wheels, which helps negotiate extreme conditions.

If your vehicle is AWD, it is one of two types: full time AWD or automatic AWD.

Full time AWD vehicles have a differential inside the transfer case which normally gives the same amount of power to all four wheels. When slipping occurs, a coupling device or clutch unites the front and rear drive shafts to keep torque flowing to the axle with traction.

The automatic AWD may or may not have a differential in the transfer case. In normal driving conditions, it only sends power to two wheels, just as any other two-wheel drive car. As long as all four wheels turn at the same speed, the control system is inactive. When it senses a loss of traction in a powered wheel, it connects the inactive drive shaft to the powered drive shaft so all four tires get some traction.

If you shut off the engine and the transmission falls into gear, it’s a clutch issue and not a transmission problem. The clutch needs to be addressed.

For vehicles made after 1995 it’s recommended to look in your owner’s manual for part number and description. If using non-OE oil, make sure the fluid you use states that it is a direct replacement for the OE part number. If the fluid meets or exceeds OE specs but does not cross to the OE number, do not use and look for one that does.

High stress driving will repeatedly increase the temperature of your transmission, eventually breaking down the stability of the gear oil in your car. If you regularly tow, drive in low traction conditions, or drive in mountainous regions, you will likely need to change your gear oil every 30,000 miles, or potentially after even less.

In normal driving conditions, most cars can hit up to 80,000 miles before needing a gear oil change. However, this number is a maximum, and most manufacturers recommend changing your gear oil between 50,000 and 60,000 miles.

Other conditions that require a gear oil change can develop under unusual circumstances. For example, if your transfer case, differential component, or transmission has experienced submersion in water or another foreign fluid, your gear oil should be replaced. Contaminated oil can seriously damage your gearbox, transfer case, or differentials with inadequate lubrication.

You should also replace your gear oil if your transfer case or differentials recently had a leak repaired. A leak could signal contamination or low levels of gear oil. Most mechanics will replace your gear oil after repairing a leaky component, but this is sometimes forgotten during home repairs.

Lastly, if you or your mechanic finds dirty gear oil during a checkup, it should always be replaced. Old or dirty oil leads to destabilized viscosity and poor lubrication, which will prematurely damage your gear box, transfer case, and differentials.

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