Engineered Innovative New Stuff That
Project “Super Wagon”
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
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.
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
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
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
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
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
Well, now the rear is ready for disc brakes and
suspension to finish the installation on to the frame, in
the next article.