Info You Need To Know Before Modifying The Rearend In Your Ride

Let’s face it, massive rear tires and deep-dish wheels not only look way cool, they also put the power to the pavement much better than many factory-original, skinny tires. Typically, fitting some big meats under the backside of your ride means a little work to make it all fit. One fat-tire enabler is narrowing the rearend housing, and there are some considerations to make if you want to make sure your rearend keeps its shape when the power is applied. Sometimes, you can find a stout housing from another vehicle that will fit, but in some instances, having one custom-made is the best solution.


If your car has already been blessed with a stout housing and simply needs a trim, cutting down the axles and rearend housing can be a cost-efficient way to fit those asphalt crayons out back. There are even times when one axle tube may already be shorter than the other, and you can get away with cutting only one side to achieve your desired width. Just be sure you adhere to all of the driveline alignment rules so you don’t have any unwanted vibrations or wear issues. We spoke with Jeff Anderson at Moser Engineering and asked him a few questions when getting your rearend in gear to fit wider rims and tires.

A cost-effective option for fitting bigger tires is to have your original housing chopped down and the axles re-splined. You need to ensure that the axle diameter does not get smaller than the splined area, or else there will be no material on which to cut splines. If the donor rear is wide enough, and the differential is off-center, you may get away with only cutting and re-splining one side.

Chevy Hardcore: What are some considerations when wanting to cut and re-spline axles?
Jeff Anderson: Cut and re-spline is an option that we offer for some that are on a budget and/or trying to use a pair of axles that otherwise would have been thrown away. It does require planning and checking of some basic dimensions to make sure it is possible before sending the axles to us. We always advise calling us with this basic information before sending them in, but here is what we will need you to check and have in-hand when you call.

First – You need to check and make sure you will be able to remove enough material to get past the old spline, which is typically 3 inches or more per side. The reason this is necessary is that you can’t re-spline over existing splines.

If all four bearings are not aligned, shortened axle and carrier bearing life is almost guaranteed! – Jeff Anderson, Moser

Second – Make sure the diameter of the shaft in the location of where the new splines will go is the same outside diameter as the spline diameter you are wanting to cut. If you plan to match the existing spline, then measure it with dial calipers and see if the shaft is the same diameter where the new spline will be cut. If it is a smaller diameter than the spline you are going to cut, then it can’t be re-splined.


If you go crazy cutting down the housing to fit massive tires, you still need to fit a complete rear suspension into the greatly-reduced space. There are many aftermarket suspension kits that allow for this.

CHC: What special tools are required to narrow a rearend?
JA: Narrowing a rearend isn’t difficult if you have some experience and some specialized tools. Most importantly is an alignment bar. This is a device that maintains the alignment at four different points in the housing. Basically, it aligns both ends of the housing so they are true with the machined areas in the center for the carrier bearings. It utilizes a straight and solid rod that goes thru all four points to maintain alignment during the tacking and welding process. If all four bearings are not aligned, shortened axle and carrier bearing life is almost guaranteed!

CHC: How do you ensure the rearend is strong enough without being excessively over-built?
JA: This is best to discuss with a professional. A lot of considerations come into play, including vehicle weight, horsepower, launch rpm, tire size and choice along with usage. You need to keep in mind that when installing larger, stickier tires, all that extra traction is going to put more strain on all of the components of your driveline – rearend included. Even if you are simply cutting down the original housing and axles, remember the horsepower they were designed to withstand and the tiny tires they originally used. If you are adding horsepower and traction, this must be considered to ensure that your rearend can handle the added performance.

Differential Differences

When it comes time to remove the one-wheel-peel from the rear of your ride, you have many options. But, your choices might be limited by your needs. Sure, it’s nice to have bragging rights by owning the biggest and baddest high-performance parts, but stepping away from the extreme edge might be the best choice for your ride. Everyone wants to know that when they stomp the tall-skinny pedal, both tires are going to grip equally, or go up in a haze of smoke simultaneously. Everything from a full-on drag race spool to a clutch-plate posi unit will give you that effect. But, unless you drive your car constantly like that (which if we’re honest, we don’t), it’s those times between the smoke shows that dictate which type of differential is best for you.

Clutch-plate or cone-style posi units work well, because they were designed with turning being as much a factor as gripping in a straight line. They were designed to act as a differential, but control one-wheel slippage, which helps when the going gets gone. They’re quiet (so long as you use the proper lubricant) and many have used them for years.

Helical-gear style differentials, like the TruTrac or Wavetrac use a worm gear in conjunction with a spur gear. The principal is that a spinning worm gear can turn a spur gear – but not the other way around. The worm gear’s friction with the differential housing prevents it from spinning while under load. Smooth, silent operation are key characteristics of these differentials.

Lockers are just that – locked – until the force to turn them overcomes the friction of their geared surfaces and springs. At that point, their teeth ratchet past each other until the turn is complete and both wheels are spinning at the same speed. While some savor the sound of solid lifter tappets and locker differentials, there can be some down sides to having a locker differential in the rain or snow.

Racing spools are for for racing, period. Big tires, wet roads, and a spool are not for the faint of heart. Just think back to your go-karting days, how did they turn with both wheels mounted solidly to the rear axle? The only difference now, are the speeds your traveling. We’re sure someone may chime in and tell us they’ve been doing it for years, but we don’t recommend it.

CHC: Pinion Angle: How important is it, and how do I know how much I need?
JA: For most basic performance applications, you are looking for 1-3 degrees difference from the tail shaft of the transmission to the driveshaft; and on the pinion to the driveshaft, you want the same 1-3 degrees. But, the two should be just opposite of one another. Keeping the transmission output shaft and pinion parallel to each other, on separate planes, helps eliminate vibration and universal joint wear.

As with so many other areas of the car, you always want to consider how you will be using the vehicle. Other applications may mandate 5-7 degrees, and as always, some specialized applications may require something completely different. Always do your research and talk to a driveline expert to help you determine what you need.

Moser can either cut down your housing and re-spline your axles (if possible), or they could supply you with a new set of stout axles. You could also order up a complete new rear assembly (left) or their Moser Muscle Pak (right).

CHC: Is it okay to use pro gears (racing) gears on the street? 
JA: Pro gears are determined by the material used (typically 9310 for pro, versus 8620 in a street gear) and as a general rule, NO. They should not be driven on the street. They will not last as long in a cruiser or daily driver. The softer material used in a pro gear finds its advantage in the fact it can absorb higher rpm launches without fracturing teeth on the ring or pinion gear. These are specialized gears that serve a purpose, and as such, are not suggested for street use by the manufacturers.

Posi unit (left), locker differential (center), helical-gear diff (right) like the WaveTrac or TruTrac.

CHC: Pros and cons of a factory-style posi, a helical-gear, or locker-style differential?
JA: Locker differentials are comprised of three ratchet gears (one attached to the carrier and one on each axle shaft), which are held together by a pair of springs. Locker differentials rest in the “locked” position and unlock temporarily when a large enough differentiating force is applied to it. They are known for “clicking” as the ratchet-gear teeth spin past each other when making a tight turn, and since they “unlock” at will, their operation can be tricky in low traction situations like rain or snow.

Most factory posi units utilize clutch/friction discs or a cone-style set-up. While they can be smooth if a friction modifier is used, they simply do not handle higher horsepower applications.

The OEM-style posi unit uses flat discs or cones that are pressed against the housing by springs. This ties the two axles together but also allows them to turn at different speeds when turning.

The best solution to quietly handle the horsepower is a helical-gear style differential like the Truetrac and Wavetrac. These units use friction developed between the case and the helical gears to limit power transfer to both axles, yet, allow for differential when turning. You will pay more for these types of units, but most are a plug-and-play unit. In fact, the Wavetrac units offer a lifetime warranty, even when used in a motorsports environment like drag racing. The helical gear units really are the best bang for the buck option for serious performance cars that are street driven.

The Bottom Line

Installing a custom-built rearend that fits both the tire/wheel combination you choose, and gets the horsepower to the pavement, is possible without breaking the bank. Being realistic about your expectations and doing your homework before the wrenches start flying are the best ways to ensure you only have to do this task once. Then, when it’s done, you’ll be able to enjoy all that additional traction whenever the mood strikes.

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About the author

Andy Bolig

Andy has been intrigued by mechanical things all of his life and enjoys tinkering with cars of all makes and ages. Finding value in style points, he can appreciate cars of all power and performance levels. Andy is an avid railfan and gets his “high” by flying radio-controlled model airplanes when time permits. He keeps his feet firmly grounded by working on his two street rods and his supercharged C4 Corvette. Whether planes, trains, motorcycles, or automobiles, Andy has immersed himself in a world driven by internal combustion.
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