Wednesday, May 11, 2011

Truetrac Mopar 8 3/4 Rear End Rebuild

The article featured on this page is from the June 2011 issue of Auto Enthusiast Magazine.


Bringing back the power potential in an 8¾-inch differential

Story Dave Verna

Mopar ring and pinion overhaul kits

Chrysler’s 8¾-inch rearend was in production from the late ’50s until 1974 in the passenger car line. It was used under literally millions of Mopars over the years.

In a street car, with its removable center section, it’s a tough rear to beat. Gear changes can be done in under an hour and you are back on the road. While not bulletproof with slicks and high horsepower, the rear is plenty strong for a weekend warrior that will see the track on occasion.

The rear we are working on has had a hard life under this 1970 Dodge Challenger. It’s still driven (and driven hard) by its original owner. The 383 four-speed car has seen over 114,000 miles with the original drivetrain. The engine and transmission were finally serviced at the 112,000 mile mark and the new-found power, combined with the 26x9-inch slicks, took the 489 cased, cone-type diff to the end of its line. The telltale signs start at the burnout. One wheel would not spin all the time and then the 1-2 shift would not get traction to both tires, causing the car to turn at the shift point. Not very good with concrete barriers on either side!

In 1970, the only sure grip you could get was the cone type. While it did hold up to 41 years of flat shifting and the occasional trip down the strip with slicks, it finally gave up on us. There are a few tricks to get a few more miles of use by machining off a few thousandths and shimming them to get the cone to engage again, but it’s just not the answer. The correct way to repair a cone-type sure grip differential is to replace the unit. We did some research and Eaton has one of the nicest diffs around for our application: the Detroit Truetrac. There is no maintenance on these, just traction! They don’t even require limited slip diff lube. The choice was easy after talking to Eaton, since the helical gear design eliminates the need for wearable parts and the unit has four pinions in the 8¾-inch setup compared to other housing designs. The effective design seemed like a home run in our book!


1 The entire 8¾-inch lineup: 742, 489 and 741. The 741, with its 13/8-inch pinion is the smallest of the bunch. Both pinion depth and preload are set with shims. The 742 has a 1¾-inch pinion and still maintains a straight stem like the 741 version. Both pinion depth and preload are set with shims. The 489 has a 17/8-inch pinion and tapered stem. Pinion depth is set with a shim but the preload was originally set with a crush sleeve.

2 The Detroit Truetrac differential by Eaton comes with four helical gears under the attached plate. If you still want to maintain the adjustable tapered axle bearings, they supply a spacer to put in-between the axle shafts to maintain an accurate axle end play.

3 The original crush sleeve and the new crush eliminator. The new collar eliminates the crush setup and replaces it with shims. The factory setup worked great on the assembly line; all you did was set the pinion depth and crank down on the pinion nut until the pinion had enough preload on it and away it went. This process will take longer, but will be stronger. This rear suffered from a pinion seal leak. The pinion wants to be pushed out of the case and is pushed against the crush sleeve its whole life. Over time, this can change the preload, and once that preload is gone, the seal will not do its job.

4 We split the original sure grip case and the cone on the passenger side showed no excessive signs of wear. The coiled area on the left creates friction on the cover to the right. The springs are what give away the cone- type differential. They press the cones into the case and supply traction to both sides.

5 This is the driver’s side cone. The wear on its face is the sign that the cone side has worn down so far that it basically bottoms out (shown). Once this happens, the show is over. As we mentioned before, you can cut the face and shim the back, but it will not last.

6 The pinion depth is set on all 8¾-inch rears with a shim under the bottom bearing. The stock depth was retained in this setup which was .031-inch.

7 The set it and forget it process with a crush collar is faster, but the eliminator will keep the preload accurate longer and will seal the pinion from leaks better in the long run. Neal is laying the shims out (a standard kit comes with .020-, .016-, .014-, .012- and .010-inch). The shims go on top of the crush sleeve and under the top bearing. This is what now makes up your preload on the bearings.

8 The first time we set it up there was no preload at all. Upon further review the crush collar inner diameter was interfering with it seating all the way on the pinion stem. A little cleanup and we were back in action!

9 With the collar not interfering with the preload anymore the .014-inch shim was the right number for us. The required torque to spin the pinion is 20 to 25 inch-pounds for the new bearing. You want to have this all set up and then install the seal. The last thing we had to do was replace the locknut, since they really are a one-time use item.

10 Don’t forget to stamp the caps and the housing to put them back in the proper location. You can wind up in a mess real fast by swapping them.

11 The instructions are clear: you need to make sure there are no burrs or anything on the surface on the back of the ring gear. We got ours nice and flat with a piece of glass and some 400-grit paper.

12 Neal took our old ring gear bolts and ground the heads off and added a few slots, so he could locate the ring gear on the Truetrac unit. The ring gear is heated to 150 degrees for one hour and will grow just enough to slide right on. This is the correct way to install a set of gears. With his oven mitts in hand, Chef Neal proceeds to use the old bolts as guides and slides the ring gear home; with a nice clunk things are set and he waits for it to cool down.

13 With the ring gear bolts installed, the bearing can now be pressed on. The bolts holding the ring gear on are left-handed threads, so be sure to keep that in mind.

14 The Truetrac unit can now be placed in the housing. The caps are loose so the adjusters can be threaded in without too much hassle. The adjusters will eventually set your backlash. For now, everything is loose and will be assembled first to get things close.

15 The caps will be torqued after the backlash is close and checked. It might take several times to get things just right but the time invested is worth a quiet operating rear that will give you miles of trouble-free driving!

16 In order to check the backlash you need to set up a dial indicator and check at least three spots along the ring gear. The ring and pinion are a matched set. They were cut, heat treated, lapped and tested together, so having an incorrect backlash setting will cause lubrication issues as well as a short bearing life. An engine stand can be attached to the 8¾-inch housing for this procedure. Be sure to check the backlash several times around the ring gear. There should be no more then .002-inch variation from all measurements. Remember when we heated the ring gear? That pays off as we had +/- .001-inch variation.

17 &18 Your last step is to run the pattern. The supplied marking compound is applied to the ring and the pinion runs through it, creating the pattern you see here. The teeth should create a pattern centered in the teeth from peak to valley of the ring gear. It should be an even, wide pattern that is centered in the gear mesh. This is where all your adjustments will show if you are within specifications.

Installing a Gear Vendors Overdrive In Your Muscle Car

The article featured on this page is from the June 2011 issue of Auto Enthusiast Magazine.

Installing a Gear Vendors Overdrive in a First-Gen F-Body

Story Larry Weiner

We live in a day and age of rapidly escalating fuel costs, to say nothing of the constant outcry from environmentalists about the carbon footprint, global warming and air pollution caused by vehicles powered by internal combustion engines.

It’s no secret that the vintage and muscle cars are often held up as examples of what is damaging the environment. The ever-tightening emissions standards and mandatory smog testing of older vehicles clearly illustrates the direction where this is headed.

Most of the data used by government bureaucrats and environmentalists to blame our collector cars for harming the environment is flawed for a wide variety of reasons. Unlike regular daily drivers, the annual mileage of most classics is nominal, often no more than 500 to 1,000 miles per year. In addition, these vehicles receive a high degree of maintenance and are generally driven very carefully. Regardless of the efforts of influential organizations such as SEMA and Goodguys, it’s safe to say that the pressure on the collector car hobby is not going to ease up.

With that thought in mind, there is something we can do that will dramatically improve the performance of our vehicles while simultaneously further discrediting many of the arguments against them. A proven driveline component is readily available to significantly enhance fuel economy and reduce greenhouse gas emissions by 25 percent while increasing vehicle performance. It’s called an overdrive.

We take it for granted today in new vehicles, as most manufactured since the early 1980s have been equipped with transmissions that include an overdrive. The majority of classic cars and virtually none of the mighty muscle cars from the golden era were equipped with an overdrive. The result is that these vehicles, especially those equipped with high numerical final drive ratios, deliver marginal fuel economy because they operate at elevated engine rpm, especially when driven at highway speeds. Installing an overdrive is the practical solution to this problem and the good news is that it’s easy to do.

This story will illustrate how simple it is to make your vintage Chevy a modern highway cruiser thanks to a Gear Vendors overdrive. This is the most popular overdrive conversion for factory transmissions, with over 100 applications. The Gear Vendors unit is manufactured right here in the United States, just like your classic Chevy.

Our 1967 Camaro SS350 is equipped with its original Muncie M20 four-speed manual transmission and 12-bolt differential with a 3.73 gear set. Since fourth gear in the transmission is a 1:1 ratio, with the 3.73 gear, every time we reached 55 miles per hour, the Camaro felt like it was ready to be shifted to the next higher gear. The problem was that we were already in fourth, and there weren’t any more gears left.

Although the vehicle only weighs 2,940 pounds, which is light by today’s standards, the high gearing resulted in a busy engine and mediocre fuel economy. Here in Southern California, the 18 wheelers run faster than the double nickel in the slow lane. Because we like life in the fast lane, and certainly wanted better fuel economy, to say nothing of doing our part to help save the environment, we decided to install a Gear Vendors overdrive in the Camaro. Gear Vendors offers a 22 percent overdrive, turning our 3.73 gear into a 2.91 final drive. This lowered the rpm at 70 mph from 3,700 to 2,886, making our Camaro SS far more freeway friendly. Goodbye semi trailer trucks, hello fast lane.

Living life in the fast lane will not be limited to the freeway. While the Muncie M20 is a robust design and was a state-of-the-art transmission in the 1960s, today a four-speed is something of an anachronism. The Gear Vendors overdrive brings this legendary muscle car transmission into the 21st century. It not only provides the Camaro with an overdrive, but it also enables you to split the gears in the Muncie, and take full advantage of the benefits of both the transmission and the overdrive.

If you really want to maximize the versatility of the combination, get ready to shock your friends and competitors by starting in first gear like a conventional manual. Upshift using the overdrive to first over, a lightning shift that will make your vehicle act and sound just like an automatic. Then, leaving it in overdrive, shift normally to second and third. During the shift to fourth, press the button on the Gear Vendor shift knob and you’ll get the regular fourth gear ratio and then follow that up with fourth over by pressing the button again. The result is a fully functional six-speed transmission that retains the tactile feel and precision you expect from a Muncie, complemented by the advantages of the Gear Vendors overdrive.

Follow along with us as the master technicians at Gear Vendors install one of their under/overdrive units on the 1967 Camaro and turn it into a muscle car with a newfound ability to compress time and space while achieving fuel economy that will rival that of many small import cars.


1 These parts, every part for installation, are included in Gear Vendors kit.

2 The original Muncie M20 transmission with a vintage 1960s Hurst Competition Plus Shifter instead of the Muncie shifter that would have been installed on the Camaro at the factory. This Hurst shifter has a unique mounting bracket not used for many years and is a good indicator that it has been on the Camaro since it was nearly new.

3 The stock driveshaft is measured by a Gear Vendors tech prior to removal.

4 Remove the nuts and lock washers that secure the two straps on the universal joint at the differential, then remove the driveshaft.

5 Unscrew the speedometer cable retaining nut from the transmission and remove cable.

6 Remove the two bolts from shifter where shift arm is attached. Next, remove factory reverse switch from bracket on side of transmission

7 Position the transmission jack under the transmission. Remove the four bolts that secure the transmission brace to the front subframe. Then, remove the two bolts that secure the rear transmission mount to the transmission brace.

8 Raise the transmission slightly by using transmission jack to take the weight off of the transmission brace. Then remove the transmission brace by sliding it at an angle and pull it free of subframe.

9 Remove four bolts at the front of transmission where it is mounted to the bellhousing.

10 Place a jackstand under the oil pan to support the engine. Use a block of wood to distribute the weight and prevent damage to the oil pan.

11 Carefully pull the transmission straight back from the bellhousing to avoid damaging the clutch.

12 The Muncie M20 is taken to the bench ready for removal of stock tailhousing.

13 The shifter must be removed.

14 The next step is removal of roll pin (drive out with punch) that retains reverse shift shaft. With the roll pin removed, grasp reverse shift shaft with a wrench and pull outward.

15 Remove four bolts that attach the tailhousing to transmission main case.

16 Gently tap the tailshaft with a soft-faced mallet to separate tailhousing from transmission main case.

17 With the tailhousing removed, clean the surface in preparation of installing new gasket.

18 Here’s a comparison of the original Muncie tailhousing with Gear Vendors one-piece and two-piece adapters. The one-piece housing is the taller of the two GV adapters. The two-piece adapter is used in 1963 thru 1981 Corvettes to accommodate the transmission crossmember that cannot be removed, while the one-piece is used in GM F-Bodies. Note that while the one- and two-piece GV adapters are longer than the original Muncie tailhousing, the tunnel of a 1967 – 1969 GM F-Body accommodates the length of the longer adapter and Gear Vendors unit with no clearancing required.

19 The reverse arm in the Muncie tailhousing must be removed and reused. Here’s a view of the reverse arm, detent spring and ball removed from Muncie tailhousing.

20 On the Gear Vendors adapter, there’s a hole for the detent spring and ball. Install it, then slide the reverse arm over the detent spring and ball. This is the reverse arm fully installed.

21 Install the Gear Vendors adapter, then reinstall the roll pin retainer for reverse gear arm.

22 Reinstall the original bolts in Gear Vendors adapter and torque to 20 lbs-ft.

23 Install rubber O-ring seal prior to bolting on second section of Gear Vendors adapter, then put on the second section. Use a small amount of Loctite on bolts used to attach the second section.

24 The Gear Vendors single piece rear adapter is installed on Muncie M20 transmission for F-Body application.

25 Gear Vendors’ splined coupling is used to connect the transmission output shaft with the Gear Vendors Overdrive unit.

26 Install the coupling and check tolerance to face of adapter with gasket installed.

27 Using a straight edge, determine the number of shims necessary to equal the distance between the top of the coupling and the face of the gasket on the GV adapter. In this case, four shims were used. We placed the shims inside the coupling recess.

28 Install the coupling on the transmission output shaft.

29 Check clearance to confirm that the coupling and face of the gasket on the adapter match up.

30 After the shifter is installed on the transmission and the shift arms are adjusted, install the transmission in vehicle, and replace the transmission mount and crossmember.

31 Note the ample space around the Gear Vendors transmission adapter when it’s installed in the vehicle.

32 Install the gasket on the Gear Vendors overdrive unit. Temporarily install Gear Vendors overdrive unit onto the transmission adapter so that measurements can be taken for a new driveshaft.

33 Measure for the new driveshaft. Be sure to allow 5/8 of an inch clearance from base of the driveshaft yoke to back of Gear Vendors overdrive unit to prevent bottoming out.

34 Remove the Gear Vendors Overdrive from the vehicle and install the speedometer drive and gear. Use white lithium grease on rubber seal. After drive is installed, secure with retaining bolt. Note use of 90-degree speedometer drive to clear the transmission tunnel.

35 Install Gear Vendor overdrive unit on transmission adapter and tighten all nuts.

36 Note the difference in length between the original driveshaft and the new unit.

37 Install the Gear Vendors computer inside vehicle. In the case of the Camaro, the computer was mounted just beneath the heater core, under the carpeting. Connect wiring to computer per instructions.

38 Gear Vendors signal lights are mounted on a small plate at bottom of dash using an existing hole and screw. The green light indicates unit is engaged and red light indicates unit is on.

39 The speedometer cable extension is connected to speedo drive on the Gear Vendors overdrive unit. Note the wires connected to GV solenoid.

40 Connecting original speedometer cable to GV extension cable.

41 The new driveshaft is installed in the vehicle. Note the 5/8-inch travel for the yoke on the new driveshaft as per GV instructions.

42 Fill Gear Vendors Overdrive unit with 28 ounces of oil. Note the ample space around the Gear Vendor unit when installed. No modifications to the floor or tunnel of any kind were required for the installation of the unit in the Camaro.

43 A view of the completed installation.

44 It is almost impossible to detect the installation in the interior. The shift knob with the button and the small plate under the dashboard with two indicator lights are the only clues that the Camaro is now equipped with overdrive. Note that the shifter is still in its original factory location and that it fits perfectly in the console.

Check out YearOne.com to find your Gear Vendors Overdrive!

Replacing Your Front Wiring Harness

The article featured on this page is from the June 2011 issue of Auto Enthusiast Magazine.


Replacing a GTO front lighting harness

Story Jim McGowan

I replaced the original engine wiring harness in my ’70 GTO Judge with an exact duplicate and was very pleased with the results. Now it’s time to replace the front lighting harness with a new one as well.

The best part about these harnesses is they fit perfectly and use all the same connectors as the originals from GM. No cutting, soldering or splicing is required. Both of these harnesses are removed and replaced almost exactly, depending on your options, for the 1968 through 1972 GTO, LeMans or Tempest. There is a difference between Ram Air and non-Ram Air harnesses, so specify your engine and options when ordering.

The lighting harness is a little more difficult to install, as it must be snaked through some tight places. Obviously, it would be much easier with the urethane nose off the car, but that is really a pain. The front end on this car is lined up too nicely to fool with it. So, I will install the harness with the nose in place.

The degree of difficulty is not that great if you simply take your time. I took about an hour and figured the best way to approach the removal and replacement. By fitting the components on the new harness through those tight places to make sure they would clear, a lot of wasted time and headaches were avoided.

Disconnect and remove the battery; you’ll need that area empty to work with the wiring. To access the front work area, you will have to remove the filler panel at the top, the front grilles, the headlights and turn signal lights. All this took about an hour. No special tools are necessary and small hands are a plus. I started by unplugging all the wire connectors from the bulbs and side marker lights.

There are also two ground wires under each front fender at the core support. These are a little tough to access but you can do it! With all the connectors free, I pulled the wiring through an area at the bottom of the nose-mounting bracket. The three-prong headlight connector has to be worked through but the rest simply come right out. Then disconnect the horns. The harness in this car was lying at the bottom of the radiator instead of in its original holders at the top of the core support. After a repaint years ago, the body man neglected to put the harness back in the clips while replacing the nose. Rather than fight all that wiring through the second nose support and up through the core support, I cut it just before it passed through the support. Then it comes out through the grille opening.

After disconnecting the second set of ground wires under the driver’s side fender, removing a plastic plate in the core support and unplugging all the connectors, the rest of the harness can be pulled up from under the fender and through a rectangular hole in the support. The photos show this access hole.

Now to the bulkhead plug. Loosen the 3/8-inch bolt securing the large wiring plug to the firewall and pull the plug out. In this case, the engine wiring plug is new, and the lighting plug is original. The lighting plug slides out of the larger engine plug. Disconnect the wiring from the horn relay and unplug the wire at the brake distribution block on the frame. Now you can remove what’s left of the old lighting harness.


Ready to Re-wire

I started the install at the same place I started the removal, at the passenger side of the nose. I snaked the new wiring and connectors through the nose support bracket from the empty grille hole, reached through the directional light hole to get them, and let them hang out the light sockets. Then I placed the new wiring into the factory holding clips working toward the center support. The two ground wires need to be pulled up through the hole on the core support but there is no way to reach that high through the turn signal light opening. I used a test wire with alligator clips at each end to do this. I fed the wire through the access hole from the engine side, and connected the clip to the ground wires ends. Then, I carefully pulled the wires up and through the hole to be reconnected. This worked well.

Moving toward the driver’s side, I first wrangled the bulkhead plug, still wrapped in plastic, through the space at the nose support bracket. This took a little doing, but it went through after a few tries. Then the rest of the connectors were pushed/pulled through. Using a little body English, I reached under the fender while pushing the bulkhead plug up with my arm in the directional light hole. This worked just fine and I’m only on cut number five. Ya gotta bleed a little! Pull the plug and ground wiring into the engine compartment and set it aside for the moment.

Now connect all the bulbs, etc., on both sides to the harness and reinstall the headlight bulbs with the retaining rings, directional lights, etc. Then reconnect the two ground wires under the driver’s side fender and move back to the firewall plug. Install the lighting plug into the engine harness plug and reattach to the firewall. Don’t overtighten the 3/8-inch bolt or you might break the plug case; just seat it to the firewall snugly. Then connect the two wires to the horn relay. They will only go on the correct terminals, so no worries. Reconnect the wire to the brake fluid distribution block. Now you can reinstall the battery and connect the cables.


Here’s where I ran into trouble

All the lights and horn worked perfectly except the directional signals. NO light there at all. I checked power at the socket and it was fine. So I compared the original socket to the new socket and found the problem. The original sockets have two connector spades in each that must be removed and transferred to the new sockets. They simply pull out and push in. After I did this, both directional lights fired up and everything worked. The removal and reinstallation took about four hours working slowly as to not amputate any body parts. The new harnesses look great and fit perfectly.

Since a picture saves a thousand words, check the photo sequence and you’ll find a few tips that will make the job easier.


1 This is the new headlight harness. The bulkhead plug is packed with electrical grease so it comes covered in a plastic bag. Leave the bag on the harness until you’re ready to connect to the firewall.

2 Remove the front radiator filler panel to access the grille retaining bolts. The grilles must be removed to move the harness parts across the front of the car and fit into the original retaining clips. There are four 3/8-inch screws holding each front grille on the ’70 model GTO. You’ll find two at the bottom and two at the top.

3 This is looking at the driver’s side urethane nose support. Notice at the bottom you can see how the wire passes through the curved opening. This is where the large firewall plug and other connectors will pass during the harness installation.

4 Under the driver’s side front fender at the core support is this block off plate, with a slit in one side that the harness passes through. Carefully remove it, as it might be brittle. This one was still very pliable. You can see the large hole in the core support with the old harness still in place.

5 This is the access area in the passenger side of the nose-mounting bracket. I’m pulling the connectors through that hole without any problem. There are a lot of sharp edges on all these metal parts, so beware.

6 Here’s the original harness after I cut it to facilitate removal. I never throw out wiring like this, as all the connectors could be used in a pinch. Bag it and stash it.

7 The new three-prong headlight plug can be cantankerous. It will fit through the opening with a little patience. You can reach through the turn signal lamp hole to work it from both sides.

8 The firewall plug is now detached and the lighting harness plug removed from the engine harness plug. This requires a 3/8-inch small socket for removal.

9 In order to snake the two passenger side ground wires through the hole in the core support I used a wire with two alligator clips. Push it through from the engine side and reach in and attach it to the ground wire connectors. Then pull up through the hole.

10 Here is the new harness in the original factory retaining clip. There is plenty of slack available in the harness so you don’t have to pull it tight. If any clips are missing, finding the correct clips and using them will ensure a reliable harness.

11 By reaching through the turn signal lamp hole, you can feed the firewall plug up to the hole in the core support and grab it yourself or have an assistant pull it through. The contact grease is messy; that’s why the bag was left in place. The block off plate and ground wiring can now be put back in place. I cleaned the metal where the ground wires mount for a better contact surface.

12 This is the complete firewall plug connector with both the engine and front lighting harness together. It can now be pushed back onto the firewall connector and snugly secured with the retaining bolt. A pair of thin rubber gloves will keep the grease off your hands.

13 Here we see the harness running along the inner fender well and all the wires attached to the horn relay and down to the brake fluid block. The installation is finished.

14 Since you’ve already got all the bright work off the front end, it’s a great time to polish and detail the items. Also clean any bugs, etc. from the front of the radiator.

15 Here’s a tip. Wrap a piece of tape around the head of the headlight trim ring screws and they will be a lot easier to reinstall.

16 I installed the battery and tried all the front lights before reinstalling the trim. Everything worked fine except the directional signals. There is power to the harness, so what’s the problem? The original harness plug is on the right. Notice the spade connectors in the original. They must be removed and inserted into the new connector. Once I did that the directional lights functioned normally. This is another reason for not throwing out the old wiring.