Randy Shafer for not only taking and editing many of the photos, but for going the extra mile and teaching me how to correctly take and edit photos. He has truly been an invaluable asset to me and this book.
Ray Kuczera for his support and willingness to help. He was always there to remind me how important it is to share as much information as possible with as many people as possible.
Last but by no means the least, I thank my wife, Kathy, for persevering through this book while she was pregnant with our twin boys. It has proven to be quite a challenge for us. To my wonderful mother-in-law, Marilyn Kelley, who sacrificed months of her personal life to help out with the twins, so that I could finish this manuscript on time. There are not many in-laws who would make such a huge personal sacrifice. I am blessed to have both of you ladies in my life, supporting this endeavor and me. You never reminded me how difficult this was on all of us and, instead, offered support and continuous encouragement. This book has become a family affair that is not my book but rather our book. Thank you.
My first experience repairing axles was a front axle on a 1981 Buick Riviera. This was quickly followed by numerous Mustang 7.5- and 8.8-inch rear axles and then GM 10- and 12-bolt axles. There have been countless repairs of vehicles since then.
It seems that rear axles are a bit of a mystery even to the most seasoned mechanics. While we are comfortable with engines, brakes, and suspension, few are at home with rear axles. I’m not certain why this is the case—most axles only require minor maintenance to be trouble-free. Perhaps mechanics don’t get enough practice to stay sharp or maybe it stems from a bad experience of working on an axle. Either way, most shops do not like working on them.
My intention is to help you understand the theory and practice of rear axles. With this information, you can determine if you want to tackle your axle repair yourself. If you come to the conclusion that you would like someone else to perform the work, then at least you will understand what is involved and how to find a good shop. Rear axles are the only part of a car that utilizes a hypoid gear set. This requires some special training, tools, and skill to work on correctly. That being said, it is not beyond most competent mechanics and even the weekend repair person. It does call for attention to details and a large amount of patience.
In addition to providing some information to allow you to diagnose and repair your driveline, I also want to explain some of the terms that are used. I have found that many of the issues surrounding understanding rear axles can be traced back to simple lack of knowledge of the terms and what they mean. It can be very difficult to communicate with someone when you do not understand the language that they are using.
As you have modified your engine and transmission and probably had an axle failure, you probably have heard all types of recommendations. For example, “You need 4.10s with 35-tooth spline axles and a locker.” My hope is that this book helps you decide if 4.10s make sense based on your vehicle’s transmission ratios, tire size, and intended usage. Do you really need 35-tooth spline axle shafts? Make certain that the upgrades you make meet your end objectives and your aren’t just wasting your hard-earned time and money. Even if you do not do the work yourself, you need to be able to assess the shop that is giving you advice and service.
With the advent of the Internet, much free advice is available and often pushed. I caution you that, at times, the advice is worth what you paid for it.
I must also make an admission before going too far. I started out as a mechanic and while this is still my hobby, my formal schooling is as an engineer. As an engineer, I tend to explain theory and how it can be reinforced with math. The math in this book is there to help explain and reinforce the topics, but don’t panic if math is not your strong point. The topics and repairs discussed can be understood if you decide to skip the math. For example, in Chapter 6 I discuss hypoid gearing and manufacturing. Chapter 6 should be a reference for any future repairs regarding hypoids. My intent is to provide information that you won’t find in a normal shop manual while still covering the shop manual fundamentals. That being said, a manual on your own specific differential and driveline will be helpful, but I’ve gone to great lengths to pack as much information into the text and make sure that all of the facts are correct.
So when someone provides you with good advice like, “A Ford 9-inch is the best way to go for your car,” be a little leery and do your own research to get the best solution for your needs. This book will be your guide for determining whether that’s true or not. The key is to enjoy the work and, with the help of this book, achieve great results.
REAR AXLE FUNDAMENTALS
The automotive drivetrain may be the least understood part of a vehicle. After reading this book, you will have a better appreciation of the complexity, and simplicity, of the components that make up the rear axles, differentials, and driveshafts. With the information in this book, you will be able to select the best driveline equipment for your vehicle and application as well as be able to install it, set it up, and maintain it.
Let’s cover some of the fundamental items that pertain to just about all axle and differential repair work. If you are unfamiliar with what you are working on, it is alright to take pictures, to mark parts as they are taken off, and to mark the orientation of one part to another to make certain that they are put back together correctly, especially if there is going to be considerable time before you reassemble the parts. This is just good common sense to make the job easier. Also, it typically takes one and a half times as long to re-assemble than to disassemble. So, if it takes two hours to dismantle, it will typically take three hours to put back together. Be patient and take your time.
The driveshaft connects the output of the transmission to the rear axle. For most vehicles, the driveshaft is a single piece with a universal joint at each end. The universal joints allow the shaft connections to articulate at an angle. If you are standing at the back of the vehicle looking at the rear bumper, the driveshaft rotation is counter-clockwise. This may sound like a trivial detail, but this is an important aspect of driveline operation that I will discuss later in this chapter.
The axle housing assembly also interfaces with other vehicle systems, mainly the rear suspension and brakes. The rear suspension components, such as control arms, springs, and shocks, typically are attached to the axle housing. The rear brake components, including the calipers, parking brake cables, and hydraulic lines, also attach to the axle housing. Keep these other systems in mind as you work on, and make changes to, the axle so you make certain to not compromise their integrity.
This is a typical driveshaft. The transmission slip yoke is at the right side of the picture while the axle connection is at the left. This is the link between the transmission and the differential. The driveshaft delivers the engine’s torque to the rear axle.
The axle attachment connection is at the rear portion of the driveshaft. The axle has a single input from the driveshaft and two outputs, one to each wheel. The device that splits this torque is called a differential. A typical open differential normally balances the torque evenly between the two rear wheels. This balancing act can get disturbed when one tire is on ice and one is on concrete. The differential is trying to delicately balance torque, but if the torque is drastically reduced on one side, the balance