REAR DIFFERENTIAL MODS

This article will focus on the rear differential. My wagon has an “open” 10-bolt rear. “open” means it is not “posi”. Only one tire drives the vehicle. Posi is short for limited slip positive traction. That is when both tires drive the vehicle but with limited slip. Different from full posi which has the left and right axles connected via hard connection. Limited slip is connected via clutches. This allows limited slip posi rears to be more daily driver friendly by giving good traction yet not chirping around corners. 12-bolt refers to the number of bolts on the ring gear not the bolts on the cover. Even though they are the same number of bolts. I wanted to go with a 12 bolt because the axles are thicker then 10 bolt rears. So with that info, I found a 12-bolt posi to put in the wagon.
The identification number stamped on the rear axle tube is CHC 217 1E E51.CH = 2.73 gear ratio, C = made in Buffalo, 217 = assembly date 217th day of the year (August 11th), 1 + first shift, E = Eaton posi, E51 = cast date May 5 1971. These numbers tell me that unless someone has changed it, that it is a 2.73:1 gear ratio posi. I checked it to see if it still was. First, you turn the yoke to see if both axles turn the same direction at the same time. If they don’t, then either the clutches are worn out or it is ad “open” rear end. Next mark the yoke and an axle hub. Turn the axles one revolution and count the number of times the mark on the yoke passes its starting point. Mine turned 2.73 times for 1 turn of the axle. I searched for a 2.73 ratio because I am taking my drive train and vehicle’s intended usage into consideration. I am building a low rpm 468 fuel injected big block for the street with a daily driving application. I wanted a low rpm, high torque, pump gas motor powered through a TH400 automatic transmission, 2.73 12-bolt posi and about 27.6” tall tires. I wanted 16” rims to give a little better road response so 295-50-16 happens to be 27.6” tall. When you plug that into the equation ( RPM = MPH X Gear Ratio X 336 / Tire Diameter) you get (2160 RPM = 65MPH X 2.73 X 336 / 27.6) this allows me to drive highway speeds of 65 and only turn the motor 2160 RPM. After determining that this rear was a good start for me, I began to disassemble it in a particular order as follows. Place rear on jack stands with a rag between the stand and the rear. Press out upper control arm bushings, remove brake lines and wheel cylinders.
This rear was from an el Camino. You can tell because el Camino and Wagon have thicker rear coil springs so to prevent the spring from reaching its “solid height” (that’s the point in which the coils touch each other), GM put spacer brackets or bumper risers under the rubber bumpers that hit the frame for the spring bottoms out. Remove them for repainting. The other feature this rear had which was specific to el Camino, is air shock stone guards. Remove and por-15 paint those. Now that everything is removed but the yoke, cover plate, vent tube and axles, keeping the inside sealed, do a thorough degreasing and cleaning. I used several cans of carburetor cleaner. It works great! Squirt down every inch from the top down, then I removed the cover plate and vent tube and flush out the thick oily residue with carb cleaner. I ran a tap through all the tapped holes to be sure they were nice and clean for the new hardware.
Upon inspection, the clutches and gears looked great, no excessive play in bearings and such. There should be between .006-.010 inches of backlash. Use a dial indicator to check. To get the drum brake backing plates off you can just air chisel them off or remove the axles. It is much easier to remove the axles. These axles were in fine shape, but I wanted to replace them with new stronger axles since my wagon was having more then stock power and a very heavy vehicle with suspension upgrades. The axles will be worked a little harder and I wanted a little insurance, not to mention harder steel axles will last longer on the bearing surfaces.
I am putting disc brakes on the rear and one added benefit is if the axles did brake the calipers would hold the rotors and wheels on the car. To pull the axles, first you remove the bolt that holds the big pin between the axles. Then, drop out the pin, push the axles in slightly and remove the 2 c-clips in the ends of the axles. Then, pull the axles out. I had to rotate the posi carrier to get through the window to access the axle clips. Before I turned the carrier, I put a wire through where the pin went to keep the gears and spherical washers from falling out while turning it. Use the axles as a pry bar to pop the seals out of the end of the axle tubes. I borrowed a bearing puller with a slide hammer to pull the outer bearings out. Take out the 4 preloading springs and 2 plates in the middle. A limited slip differential needs a traction difference between wheels to work. Note “traction difference” not “lack of traction”. Posi will not work when one wheel is on a very low friction surface like ice and snow. The engineers had to do something to back up the claim. The result was preloading springs, which make the posi have limited slip action on low/no friction surfaces. So if you can handle not needing high performance in snow and ice, you can trash the preloading springs and plates. All the preloading springs do is cause premature posi clutch wear and higher rear axle oil temperatures (Muscle Car Review 11/87).
Now with the rear as striped down as far as I wanted, it was time to do some suspension mods. This is done only to the passenger side of the rear. It relocates the pivot point of the lower control arm 1.5" apart from its original mounting point. Note that it is not 1.5" down, but "apart". This is because you do not want to change the position of the rear, so you have to leave the control arm mounted to the frame and let it swing on it's radius until the new bolt hole is 1.5" apart, then weld 3/16" ears on.
Grind smooth welds, then I welded in 2 braces to help support the new ear since it was now extended pretty low and unsupported. This is all that is necessary to avoid wheel hop and get excellent traction by moving the center of mass during launch back further over the rear wheels. Same principle as used by South Side Machine bars and no hop bars. Only one side is necessary to counter the engines torque translated thru the chassis and suspension. This has been track proven and tested. Feel free to use this if you want, if you have doubts you can always move the arm back to its original mounting position.
I Degreased and painted the aluminum cover with silver paint to prevent it from oxidizing and also to make it easier to keep clean. Test fit the gasket to the rear. Cut the gasket material out to match the holes in the rear where the oil returns to the main bearings. I like the red gasket spray. Apply spray to one side of the gasket, let it dry. Flip and spray the other side. Dump in 1 quart of 90-weight gear lube and place gasket on rear. Place cover on, add a thread-locking compound to the 12 cover bolts and torque on. I used new ½-20 x ¾” long stainless steel bolts and lock washers too. Press new polyurethane upper control arm bushings in. they come with polyurethane washers. The driver’s side needs a flat spot ground in it to clear the rear housing. (Don’t forget to do that before you press the bushing in all the way).
Now that all the welding is done, it’s time for the painting process. I like the performance and results of por-15 products. First use “Marine Clean” spray to degrease the surfaces. Waite 15 minuets then fill a squirt bottle with water and spray off the Marine Clean. Then spray down with “Metal Ready”. This neutralizes the rust in the tiny pits and gives a surface for the Por-15 paint to stick. After 15 minuets of Medal Ready, spray the rear off with water again. Let dry then brush on Por-15 paint. The brush marks will self-level. Swab out any excess in holes or threads with Q-tips. After a day to dry give a second coat. Wait another day, now its time for reassembling the rear. I bought new axles, bearings & seals from Moser (www.moserengineering.com).I ordered them with 5 on 5” lug pattern and ½-20 x 2” screw in studs (about ¼” longer then stock to accommodate the new disc brake rotors), and 6-1/8” diameter flanges. The rest was the same as the originals. Unfortunately, the axle flanges are slightly thicker and it complicates my rear disc conversion but I’ll talk about that more in depth later. L wanted the 5 on 5” lug pattern to match up with the pattern on the front wheels. I will describe in detail later also but the front rotors will have the 5 on 5” lug pattern. Tap in with a wooden dowel, the new bearings and seals. Use grease on the seals before you put them in. insert axles, put c-clips back, pull axles back out to seat the c-clips, rotate the posi carrier, remove the wire, insert the big pin and pin screw. I like to get rid of the lock washer and use thread-locking compound. Scrape any paint off the gasket surface for the rear cover. I bought a finned aluminum Mr. Gasket rear cover at a swap meet. If your putting a non-stock cover on, check for clearance of the ring gear. Place the cover on the rear without a gasket. Rotate the axles to see if the gear hits the cover. Mine did, so I put grease on the gear where I thought it was hitting, put the cover back on and the grease left a mark where it was hitting. Take the cover back off and grind a relief in the oil drain guide ridge until no grease appears when the process is repeated. I wanted easy access to draining and filling the rear with fluid so I put a tapped hole as low as I could on the cover and another at the same level as the fill plug in the rear housing.
Paint the bumper riser brackets and stone guards with the Por-15 process described earlier and install on rear with new stainless steel hardware. Add bumpers, I reused mine, silicone spray protects the rubber. Don’t forget to add GM posi lube additive and top off the gear fluid once the car is level again. I am not ready to do that yet.
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