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How to Build an 8-inch Ford Rearend for a 1959 Chevrolet Corvette

Eight Is Enough: How to build the perfect rearend for your C1 Corvette

Jun 16, 2015
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Engines get all the glory, but without a properly built differential, a Corvette is just relegated to noisy yard-art status. One of the biggest differences of early C1 Corvettes is their chassis. In fact, they are the only group of Corvettes to get live axles out back, instead of some IRS arrangement.

Since we upgraded our front suspension to IFS, we thought the rear of our 1959 Corvette deserved some upgrades. The plan solidified into some mono-leaf springs from Corvette Central along with a pair of adjustable shocks and a sway bar from QA1. The mono-leaf springs will drop the stance and knock 11 pounds off each side. This combination of parts should enhance the handling without being crazy expensive or difficult to pull off. A key aspect of this plan is swapping out the rear for a Ford 8-inch piece. Why an 8-inch over the more popular 9-inch version? Well, in this case size really did matter. The 8-inch will clear the spare tire well and won’t require modifications to fit. It’s also more than strong enough to handle the estimated 348hp 348 W-motor that will power the ’59. The 8-inch will also take less power to turn and will knock even more unsprung weight off the Corvette. Less important is that the 8-inch will look more like the original rear to anyone curious enough to peek under the car.

So with a plan in mind we hit up Currie Enterprises to see if they could stitch us together the perfect rearend for our 1959 Corvette project.

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01. The housing starts with 3-inch 0.188-inch-wall steel tubes that are cut to the proper length for whatever width housing Currie is building. In our case we want a flange-to-flange width of 59 inches for our ’59 Corvette. This is a bit narrower than stock, but perfect for the wheels we want to run. Normally, an 8-inch rear would have 2.5-inch diameter tubes, but the 3-inch tubes are stronger and make finding, and affixing, aftermarket suspension brackets easier since it’s more of an industry standard.

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02. Given the extra diameter of the 3-inch tubes, they needed to have the outside milled down on the lathe to fit in the 8-inch case. On the other end of the tube, the inside needed to be opened up to accommodate the larger “Torino style/big Ford” ends.

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03. The literal centerpiece of our build is this 8-inch Ford housing (PN CE-2005T) from Currie. This is a completely refurbished original part, not something cast overseas. The main benefit for us to run an 8-inch rear rather than the stronger 9-inch version has less to do with saving a couple hundred bucks, or a few pounds, than it does with fitment. On an early Corvette there isn’t a lot of room and the 8-inch clears items like the spare tire well.

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04. The axletubes are then inserted in the housing and measured to make sure they’re the perfect length. A steel guide rod and fixtures ensures everything is lined up perfectly straight. Measurements were made several times before the first part was tack welded in place. Before welding the axletubes to the housing, Currie bolted on a third-member casing. This provided structural rigidity to the housing to limit warping during the welding process.

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05. A jig, along with a level, was used to make sure the ends were properly oriented. In regards to the ends, Currie has several options. One is a billet Torino style, large bearing end that accepts easy-to-find Ford-style rear brakes. They also have a hybrid end that uses the same bearing but accepts GM rear drum or disc brakes. Since we will be using aftermarket Wilwood brakes, we opted for the Torino (or big Ford) end.

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06. After some time on the rotary jig we were left with these beautiful welds. The other end of the tube was similarly welded to both the outside and inside of the 8-inch housing.

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07. We also had Currie weld on leaf spring pads (PN CE-7000C, $24 each). You could do this yourself, but it’s better that any heavy welding is done prior to having the housing straightened and made true. This is also when a hole was drilled for a vent and when they add extras like fill and drain plugs.

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08. Welding warps differential housings, which can cause fluid leaks and premature bearing wear. That’s why, right before final cleanup, every housing is made true on a special hydraulic machine.

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09. Here’s one of the main reasons why a 9-inch rear is stronger than its 8-inch cousin. As you can see, the 9-inch ring-and-pinion on the left has a lot more steel than the 8-inch version on the right. Still, if you’re not making enough horsepower to break the 8-inch then the 9-inch is just extra, unnecessary rotating mass. For this build we chose a 3.55 ratio Motive ring-and-pinion from Currie (PN CE-4022).

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10. It’s hard to beat the street manners and handling performance offed by an Eaton Detroit Truetrac unit (PN CE-5018). Under normal conditions the Truetrac performs like a regular open differential and, when required, automatically transfers torque to the wheel with the highest traction. The Truetrac is capable of transferring up to 3.5 times more torque to the high-traction wheel. This torque transfer ratio, called the bias ratio, is accomplished by using parallel-axis planetary helical side-gears and pinions that mesh much like worm gears. The bias ratio is the result of pressure exerted by the side gears and pinions against the surface of the differential case. When the unit is loaded, the worm gears are forced away from each other and against the case. They develop locking torque by the light friction between the worm gears and the case. This light friction is multiplied by the ratio of the worm wheel. Since the Truetrac uses hardened gears against hardened gears, and not clutches or cones, there’s nothing to wear out or provide regular maintenance to. We then bolted the ring gear to the carrier. A good trick is to use slightly longer bolts to draw the ring onto the carrier since the actual bolts are a bit short for this task. This will bring the parts together close enough to allow the final bolts to reach. Also, we made sure to use a dab of red thread locker on each fastener during final assembly.

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11. We then pressed the bearing onto the pinion. This is an area where experience really pays off since doing it wrong can cause the bearing to fail and/or leak. The 8-inch yoke (PN CE-4224) and pinion support (PN CE-4033M) were both reconditioned, used parts. Currie secured the yoke with the supplied nut and torqued it to 125 ft-lb. Currie uses solid spacers, instead of the original style crush sleeves, so pulling the unit apart to adjust is easy.

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12. Here you can see a 9-inch gear case (left) next to the 8-inch version (PN CE-4026M) we’re running. As stated before, the cost difference isn’t much, but the 8-inch case takes up a lot less real estate and that’s critical in the tight confines of a C1 Corvette.

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13. Here you can see another one of the handy fixtures and jigs that Currie uses to make differential assembly easier. The assembly was spun to make sure the new gears cleared the case, which they did. We then started adjusting the backlash with our goal being 0.010-inch. To check how the gears were meshing together we used some marking paint. This is a critical step for quiet and trouble-free operation.

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14. The first check showed that we were low on the coast and high on the drive. After an adjustment we were rewarded with a good wide contact patch in the center of the teeth. Part art and part science, properly setting up a rear is one of the tougher things you can do on your car.

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15. Lastly, we torqued down the carrier cap bolts to 65 ft-lb and the pinion support to 45 ft-lb. Currie recommends non-synthetic, name-brand 85-140 weight gear oil. Since we’re running the Truetrac we won’t have to use any friction modifiers.

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16. After applying a bead of RTV, a gasket, and another bead of RTV we were able to mate the third member to the freshly cleaned housing.

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17. After measuring for axle length we then started installing the inner seals.

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18. As mentioned earlier, we’re running Wilwood rear brakes to match the front ones on our ’59 project Vette. The first parts to go on were the backing plate assemblies. These came fully assembled with the parking brake parts, caliper bracket, and cable bracket already in place.

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19. After doing some math, Currie cut down a set of axles (PN CE-0016) and gave them 28 splines to work with our TrueTrac differential.

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20. On the right is the axle retention plate supplied by Wilwood. The open-sided design makes installation much easier, but a drawback is that it doesn’t fully seat around the bearing. The part on the left is from Currie. It presses around the full diameter of the bearing to help lessen the chance of leaks and bearing movement. It’s also quite a bit thicker. The downside is that it has to be slid on the axle prior to pressing on the bearings.

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21. The axle assembly was then slid into place and secured using the access hole Currie cut into the flange.

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22. The slotted and drilled vented rotors from Wilwood came in their proprietary E-coat. The black coating will wear off where the pad sweeps and keep the rest rust free and looking good.

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23. Lastly, we installed the twin-piston Wilwood calipers (PN 120-6806-RD). They look great and will stop a whole better than the drums that used to reside under the back of our ’59.

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24. The finished rear should fit under our ’59 Corvette and will easily hold up to the power output of our small-block powertrain. We shipped the unit with three quarts of the recommended 85-140W 9-Plus gear oil (PN CE-9013).

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