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Rear Ends

By Jeff Zurschmeide

Most classic cars are a front-engine, rear-drive configuration. For the first 70 years of mass-produced automobiles, the vast majority were delivered with this driveline design. Some, like Porsches and Volkswagens, used a rear or mid-engine design, and a few classics such as Saabs and the Oldsmobile Toronado used a front wheel drive layout. But for our purposes, it’s a safe bet that you’re looking at a rear drive axle with a differential in the middle. Even if your car is a different design, some of the specifics change, but the basic facts of restoration don’t change.

A rear axle may seem complicated, but it’s really not. However, it is a precision piece of machinery, and almost no amateur restorers have the right tools to set up a rear axle themselves. This means that you’re probably going to take your axle to a professional specialist for restoration.

The central fact of a rear axle is that it turns the rotating motion of the engine and transmission sideways to drive your rear wheels. This happens through a device called a ring and pinion gear seat. The pinion gear is coned-shaped and turns with your driveshaft, while the ring gear is mated to the pinion. The pinion turns the ring gear like a ferris wheel. By attaching the two axle shafts to the center of the ring gear, the longitudinal rotation of the engine is turned into tire rotation.

So far, so good, but your rear axle also includes a device called a differential. This assembly allows your drive wheels to turn at different speeds. For example, when you’re turning around a corner, your outer wheels travel a longer distance than the inner wheels. To corner smoothly, those wheels have to move at different speeds for a little while, then move at the same speed again when you drive straight down the road.

To simplify the design somewhat, the differential is another set of gears, typically mounted in the center of the ring gear. Each of the two axle shafts fits into the differential assembly, and a set of planetary “spider” gears allows the axles to move separately when needed, while still delivering power to both wheels when possible. There are variations on this design that limit the amount of difference in axle speeds or allow for easy replacement of the ring and pinion gears -- respectively known as “Limited Slip” (or “Posi-traction”) and “Quick Change” rear ends, but all of them perform the same basic function.

The important thing for the amateur restorer to understand is that there are a number of bearings and seals that must be replaced, and tolerance adjustments that must be set precisely for a differential and rear axle to operate correctly. Further, ring and pinion gears, differential gears, and axle shafts are subject to wear, and must be inspected and crack-checked during the restoration process. A worn differential, bearing, or ring and pinion combination can produce rattles, clunks, and most often a hideous howling noise. In extreme axle failures, your car’s rear wheels can even lock up while driving.

For this reason, you should send your rear axle and differential out to a professional for restoration. While it’s at the shop, it makes sense to have them replace the outer wheel bearings and seals at the same time they replace the bearings and seals in the differential and ring/pinion gear area.

You should also use the opportunity to clean and repaint the external parts. Rear axles are good candidates for powder coating because they operate in a dirty, hazardous environment and can use the extra protection.

HINT Hint...

Be aware that you can have steel parts dipped, but the central housings of some rear axles are made of aluminum, so check carefully before you send your axle out for dipping or media blasting. Before blasting any mechanical parts be sure there are no “pockets” where the abrasive may get lodged. Dipping is a much safer way to go for this reason. Also, if your axle has aluminum or other non-steel parts, you have clean it the old-fashioned way, or with detergent and hot water.

If your car is front wheel drive or rear-engined, your differential is almost always housed in the transmission case, and you can service it along with the transmission. If your car uses flexible “CV Joint” axle shafts, these are generally serviced by the same shop that handles the differential and final drive. And if you’re restoring a 4WD vehicle, you get to restore two separate drive axle assemblies, a transfer case, and three differentials!

The good news in all this is that once it has been restored, your drive axles and differential(s) are not likely to require further work. Most restored cars are driven lightly, and these parts are made to be tough. Use good gear oil and keep it topped up and you’ll never have to worry about this assembly again.

Dos and Don'ts
Thumb up  DO
  • Take your differential to a reputable shop for restoration
  • Listen for “howling” from your differential
  • Check your rear axle oil level at every oil change

Thumb down  DON’T
  • Don’t assume that an old differential or drive axle is OK
  • Don’t try to set ring and pinion lash yourself
  • Don’t drive a car without adequate oil in the drive axle
The Popular Restorations Feature Car
Author photo

1946 Packard: Restoring the Rear End



One of the advantages of buying a low-mileage classic is low wear on many of the parts. The Popular Restorations feature car only had 50,000 miles on it at the start of the restoration and as a result the differential was in relatively good shape. Still, the housing needed cleaning in a big way and the seals needed to be replaced.

I started out trying to clean the housing with solvents but it that approach proved to be impractical. Besides being extremely heavy and unwieldy, the case was rusty with baked on grime and old paint. The grime was resistant to just about everything so I removed the axles and third member and sent the case out for powder coating. I cleaned the third member and painted it with POR-15 and prepared for reassembly.

The trouble with putting it back together myself was that the axle bearings are tapered and require very thin shims to set the end play. I could have just used the old shims and hoped for the best but instead I took everything to a rear end shop and had them assemble and adjust it for about $100.



Rear End Shop

By Jeff Zurschmeide

54 Maxwell Ct.
Santa Rosa, CA 95401
(707) 526-0200

We spoke to Robert at the Rear End Shop in Santa Rosa, California. This shop works on nothing but differentials and rear axles, and they have done thousands of classic axles back to the 20s.

PR: What steps do you go through to restore a classic differential and rear axle?

RE: First we disassemble it and inspect it and see what it needs to repair. Then we put a price together for the customer. At that time, we’ll present the customer with any options that are available, like changing the final drive ratio or upgrades like posi-traction. After the customer OKs the work, we order up the parts, do any required work, reassemble the axle, paint or powder coat it, and then it’s ready to go back in the vehicle.

PR: Can an amateur reasonably expect to rebuild a rear axle on their own?

RE: Not the average person, no. You need a master technician with special knowledge about how this stuff works. There are a lot of professional mechanics out there who can’t set up a rear end. A rear end is like making a watch.

PR: Many rear ends and differentials have specialty parts such as friction discs and springs. Are all those parts available for vintage cars or can they be fabricated?

RE: Any parts can be fabricated. That’s not a problem; it just costs more money. But about 90% of the time, everything’s available. Parts are available back to the 1930s or so, and from the 1950s on, there’s no problem at all. It depends on how popular the car was.

PR: If there’s no major trouble, how much should someone expect to spend for a restoration including new bearings, seals, and a setup?

RE: Most rear axles use 6 bearings, 3 seals, and all the cleaning and reassembly comes to around $400 to $600. If the ring and pinion gears are broken, that can add another $200 or so. If we’re restoring a posi-traction, that can add $200 to $600.

PR: Do you recommend any particular gear oil?

RE: We recommend a GL5 grade oil in 85-140 weight. That covers a broad range of temperatures and it’s better oil than 80-90. Amsoil is the best lubricant, but it’s a little more expensive than the others.

PR: Is there anything else to know?

RE: If you have a posi-traction rear end, most of the time you will need to reintroduce a friction modifier or you’ll end up with chatter. If you go to a Quick-Stop or Jiffy Change, sometimes they don’t use the right oil for these rear ends. If you just drain the oil and put in the right kind, the problem goes away.

Click on any item below for more details at Amazon.com

Jim Allen
Differentials: Identification, Restoration & Repair
Ring & Pinion Services, Paperback, 2006-01

This is one of the best books on differentials and axles. Written by Jim Allen and Randy Lyman of Randy’s Ring and Pinion, the 378 page text includes step-by-step repair procedures with photos and diagrams of differential operation, repair, set up, and modification.

Tom Birch
Manual Drive Trains and Axles (3rd Edition)
Prentice Hall, Paperback, 2001-05-22

Manual Drive Trains and Axles covers the traditional rear wheel drive, the modern front wheel drive, and four-wheel and all-wheel drive systems. This book is arranged so the major areas of the drive systems are described completely--the theory of basic operation as well as the methods used to diagnose, adjust, and repair them.

Thomas S. Birch, Chuck Rockwood
Manual Drivetrains and Axles (5th Edition)
Prentice Hall, Paperback, 2007-05-17
This book prepares its readers for ASE/NATEF certification and includes an exercise worktext, chapter quizzes and reviews. This edition includes more on hybrid vehicles, a revised chapter on 4wd service, and chapter summaries that reinforce key techniques.
External Links

2CarPros.com has a good overview of how differentials work.

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