One of the best mods you can do to a car is install a set of gears in the rear end. In fact, in my opinion, this should be your first mod; by far the best bang for the buck mod there is. What are gears? Glad you asked.
Gears multiply your engine’s torque to get the car up to speed easier. Without gears to multiply the engines torque, you will never be able to get your car to take off from a stoplight. Gears are numbered as a ratio, for example 3.27:1. This means that the drive gear (pinion gear) must rotate 3.27 times for the driven gear (ring gear) to rotate once. Let’s look at some math to better illustrate this. A stock Mustang GT weighs in at about 3500lbs, it’s engine has a horsepower rating of 225hp and a torque rating of say 300 lb./ft of torque. Without gears your engine only making 225hp and 300lb. /ft of torque will have to try to move a 3500lb mass. That simply ain’t gonna happen. Now lets see what gears can do for us. Your typical Mustang has a first gear ratio of about 3.3:1 (in the 96-up cars.) So by taking it’s torque of 300*3.3, you now have 990lb/ft of torque to help your car take off. That's still not enough to make for trouble free take-off’s. Now take that number and use rear-end gears of say 3.27:1 to multiply that onto the wheels you now have 3237lb/ft of torque to accelerate you. While this is a healthy number, its still less than your car weighs. Let’s see what happens when we install say 4.30:1 gears to the car with 3.27’s. 300*3.3*4.30=4257lb/ft of torque! That’s over 1000lb/ft of torque, 1020lb/ft to be exact that will help your car off the line. This means less bogging, and a car that is extremely fun to drive. By this example, you can see the advantage of a numerically higher gear ratio.
The Hard Part
When magazines talk about gear installations being difficult, they are only ½ correct, depending on your definition of difficult. The job is not hard, but it is rather time consuming and tool intensive. Improperly setup gears can whine, break, vibrate etc. Setting up the gears to the manufacturer’s specification can alleviate all those problems. The manufacturer will specify a certain pinion depth and backlash setting for each gear it sells. In order to check pinion depth, you need a pinion depth gauge. It’s a specialty tool only used on rear axle gears. You will not be able to find this at your local Sears outlet. Prices on this item run about 200 dollars and up. In order to setup your backlash properly you will need a dial indicator with a magnetic stand. These run in the $60 dollar range. Using these 2 tools properly will make or break your installation.
Now that I’ve got you thinking you cant do the install on your own, I’ll tell you the good news. If you use Ford Racing gears, you no longer need a pinion depth gauge. There is a system called the Master Housing Dimension that allows us to install gears without that hard to find tool (more on this later.)
There are quite a bit of “non-regular” tools needed to complete the install.
Bearing separator and puller, or hydraulic press
These are pretty much a requirement along with a set of ½” and 3/8” sockets, and combination wrenches. Most of these items can be had cheaply, and you will no doubt be using them again in the future. If you don’t care to buy them, you can rent them at any good rental or parts store for a few bucks.
Since this installation is for a late model Mustang with an 8.8” rear end, I will teach you how to install it without pinion depth gauge. How is this possible? We will use Ford Motorsport gears that can be setup using the Master Housing Dimension technique that is used by Ford in the assembly line. So follow along, look at the pictures and soon you will be able to do your own gear installation and be the envy of all your friends.
Pinion Gear- The pinion gear is the drive gear. This attaches to your driveshaft and transmits the power to the ring gear.
Disassembly begins by first getting the car up on jack-stands as high as you possibly can go. You will be working underneath the vehicle and using a jack of any type to support the car is extremely dangerous. With that said, lets get to work.
1. Remove the wheels
2. Remove the top half of the brake calipers with a 13mm wrench. (See figure 1) Loosen them one at a time, but don’t remove until both bolts are loose. After the bolts come out, the top simply lifts off leaving the pads on the rotor. (See figure 2) Just let these parts hang.
3. Remove the lower half of the brake caliper with a 15mm wrench. (See figure 3)
4. Place an oil drain pan underneath the rear axle pumpkin. It needs to be able to hold at least 3 quarts of fluid.
5. Remove the bolts on the rear cover of the differential. (See figure 5) Loosen, but don’t remove the top most bolt. This will keep the cover in place while you break the seal on the bottom of the cover to drain the oil. If you do not do this, the fluid can gush all out and get gear oil all over the place. After all the oil has drained, remove the top bolt and rear cover.
6. Rotate the differential until you see the differential shaft locking bolt. (See figure 6) Use an 8mm-combination wrench to remove this.
7. Push the shaft forward about an inch and then rotate the differential around 180°. Remove the shaft completely. (See figure 7)
8. If your car has ABS, then you will need to remove the ABS sensor on the back of the brake mounting plate. This uses an E-8 female Torx socket. They can be stubborn, but spray em with some WD-40 and gently pry them out. (See figure 8)
9. Push the axles inward, toward the differential. This will expose the C-clips, which retain the axles in the housing. Remove both of these clips with a screwdriver. They should slide right out, if you are having trouble, try moving the axle in and out slightly to get them to clear the S-spring and remove them.
10. Now you can remove both axles. They simply pull out. Remove them gently and be sure not to damage the seals on the housing ends when you remove them.
11. Mark the main bearing caps left and right, I simply put them on one side of the floor. Left goes on left of car, right on right. Remove the four bolts for bearing caps. (See figure 10)
12. Using a pry bar to brace the differential, (See figure 12) turn the driveshaft. The ring gear will ride up on the pinion gear and pop loose. BE CAREFUL! Do this part slowly as a differential assembly falling on a foot or hand can hurt you pretty badly. Turn slowly, once the diff starts to come out, gently pull it and rest it on the bottom of the housing. Be sure to separate the bearing races and the shims from left to right. Remove the differential from the housing.
13. Mark the driveshaft flange and the pinion flange so you can assemble it the same way. Remove the driveshaft from the pinion flange with a 12 point 12mm wrench. Use a pry bar or long screwdriver to brace the driveshaft at the transmission side. Remove the four bolts and slide the shaft out of the transmission.
14. Using a 1-1/16” socket and either an impact gun or a long breaker bar, remove the pinion nut. (See figure 13)
15. With a rubber mallet or brass punch, hammer the pinion gear. It will need to go in about ½” to ¾” to get past the pinion flange. The flange will fall off and a few more whacks should free up the pinion. Remove it from the housing and set aside. (See figure 14)
16. With a brass or steel punch, drive out the pinion-bearing race if you plan on using a new bearing. If you plan on re-using the old bearing, skip this step.
17. The last step is to spray down the inside of the housing with some good brake cleaner. Scrape the bottom of the housing out and get the inside as clean as you can. You don’t want any metal flakes or dirt to harm your new gears or bearings. Please be careful of metal flakes, the first time I did this I had about 4 or 5 metal splinters stuck in my hands. Spraying it down and scraping it out with a rag will make things a lot easier on you.
Calculating Pinion Shim Thickness with the MHD technique.
Up until this point, everything can be accomplished with a pretty basic set of hand tools and a hammer. This section here will show you how to calculate pinion shim thickness when using FRPP/SVO gears. This is the most critical step and requires the use of a Bearing separator, a puller or hydraulic press to remove the bearing, and a set of calipers to measure both the pinion head and the shims. We will use the bearing separator and the puller to remove the pinion bearing and pinion shim from the pinion gear to transfer onto the new set of gears. We will also measure the pinion shim and pinion gear for this calculation.
The Master Housing Dimension (MHD) is the distance between the axle centerline and the back of the pinion head. (See figure 15) This equals Pinion Head Thickness + Pinion Depth. Since your factory gears were installed using the MHD method from the factory, you know that your pinion depth was properly setup with the correct pinion shim. Since we are changing the pinion, we must now calculate the correct pinion shim thickness. For an example, if Pinion Head Thickness was 1.781 and the shim was .019” thick, doing the math: (4.415-1.781=pinion depth of 2.634) The .019” shim was used to reach that pinion depth of 2.634. If our new pinion head is now 1.770” thick, doing the math: (4.415-1.770=2.645 pinion depth). Since the difference in pinion thickness is .011 less on the new gear, we must ADD .011 to the existing .019 shim to get the correct pinion depth. This leaves us with a .030 shim for the new thinner pinion gear. If the new gear was 1.792, .011” thicker than the old gear, we would subtract .011 from .019 and use a .008” thick pinion shim. Using this method, I have double checked the MHD with my pinion depth checker and found it within .001” of my calculations on both installs. This is the easiest way to do it, and will save a lot of time in installing, measuring, disassembling, and changing shims; not to mention re-installing and measuring all over again.
Now that I have explained the method, I will go through the list on installing a new gear set.
Pinion Gear Installation
The first thing we will do is install the pinion shim and pinion bearing onto the pinion. Be sure you have calculated the proper shim thickness before you install the bearing. To get the bearing on, we will use a trick Scott Brock, a.k.a. CASPER taught me. Use boiling water to expand the bearing and it will literally fall right over the pinion gear. No press needed. Simply put the bearing in the boiling water for about 3 minutes to get it nice and hot, grab it with a pair of pliers and slide it onto the pinion gear. Be sure to use gloves or a rag when doing this, you don’t want to get burned here.
2. If you have used a new bearing, now is the time to install the new bearing race. Tap it in with a good brass punch or better yet a big socket. Most won't have a socket this big so a punch will do. Make sure you seat it evenly, moving around the bearing with light taps to get it seated.
3. Install crush collar onto pinion gear. (See figure 16)
4. Install pinion gear in housing, you may need to tap it in with a hammer and punch. Install flange and pinion nut loosely. (See figure 17)
5. This is where you will need an inch/pound torque wrench and either an impact or a long breaker bar. Tighten the pinion nut down until you can no longer move the pinion gear back and forth. (See figure 18) When moving it, you will hear a clicking noise from the play, tighten until the noise stops. Using your inch/pound torque wrench to check for pre-load, tighten the nut down until you have a drag on the pinion and it reads 16-29 in/lbs. for a new bearing, or 8-14 in/lbs. for a used bearing. (See figure 19) Be careful, once the collar starts to crush and you get a reading on the wrench, it will take VERY LITTLE to get it in the right range. Too much and you will exceed the limit. If you exceed the limit, you MUST install a new crush collar and start again. I suggest tightening in 1/16 of a turn intervals between checking. So you should tighten with a breaker bar, check pre-load with torque wrench, tighten with breaker bar, etc.
Ring gear and carrier installation
We will install the ring gear similarly to the pinion bearing, heat it up with boiling water and then draw it in with 5 of the bolts. This to me was the hardest part just because of the weight and needing to align the bolt holes up properly. To remove the old ring gear, use a press, a dead blow hammer or a brass punch to tap it out--moving around the gear to get it out evenly.
1. Using some 300 grit or so sandpaper, sand down both the back of the ring gear, and the mounting surface on the differential it sits on. This will remove any high spots, which could throw off run-out and cause whining.
2. Heat up the ring gear with the boiling water. Line up the bolt holes and slide the gear as far down as it will go. Install 5 bolts in a star pattern. Tighten them down in a star pattern like you do for installing a tire. Do this evenly until the ring gear is fully seated. Remove the 5 bolts and put loc-tite on the bolts and re-install. Torque ring gear bolts to 70-85 foot pounds in stages. (See figure 20 and 21)
3. Pickup the old shims and bearing races, remember you marked these earlier as to what side they came from. Install the races onto the bearings and put the shims in place. Now slide the differential into place. It should be a little tight, where you would need a rubber mallet to get them in.
4. Install main bearing caps and torque to 70-85 LB/ft of torque.
5. Setup dial indicator to measure backlash. The spec is .008-.015, I recommend shooting for .010-.012 See picture (See figure 22)
6. If you don’t come in correctly and have too much lash, you need to move some shims from the right side over to the left. Whatever you take from the right, you install onto the left. If you have too little lash, the opposite is true, take from left, and add to right. Do this until you get the backlash within spec.
7. Now setup the dial indicator to check the run-out of the ring gear. You want to setup the dial indicator to read the back of the ring gear, the part that sits on the differential. You should have no more than .004” of run-out.
If you have checked everything twice and all your measurements are in spec, you are finished with the gears. See it really wasn’t that hard at all. Now re-install the axles, c-clips, pin and bolt, brakes, diff cover, ABS sensors, driveshaft, and wheels. Fill the housing with gear oil and your new set of gears will serve you well. I have just a few finishing notes I will give you now.
1. Use loc-tite on the pin-retaining bolt.
2. Use loc-tite on the ABS sensor bolts
3. Instead of using a gasket on the diff cover, use RTV gasket maker. Its easier and works much better. Be sure to clean the area with brake cleaner or carb cleaner to get a good seal.
4. Gear break-in procedure should be as follows for a streetcar. Drive at normal street speeds for 10 miles; let it cool for 30 minutes. Do this 3 times and then let the car sit overnight. Check for whining or vibrations while doing this, there should be none if you followed the shimming procedure and setup the backlash within spec.
That’s all there is to it. It really isn’t that hard, if you have any questions or would like me to clarify anything e-mail me at firstname.lastname@example.org and I will be happy to help. I must give you this warning in advance though, once everyone knows you can do gears properly, you will be showered with phone calls and beer and pizza by friends and strangers alike. You will be king stud in your circle of mustang friends as you are now a “Experienced Professional” and chicks just dig that. ?
James Adkins, AKA IslandVobra