Posi Rear End and Shocks - 1954 Pontiac
My parts car is a 1976 Camaro and it has a 10 bolt 3.08:1 posi rear end. The really nice thing about this is that the rear end is almost a bolt in replacement for the original 54 rear. The main difference is the Camaro uses 2 1/2" wide leaf springs and the 54 uses 1 3/4" wide springs. The other problem is the camaro differential mounting plates have stud mounts for the shock versus through hole style required for the 54. The Camaro has fore and aft mounting for outboard located shocks. The 54 has forward inboard mounted shocks.
I want to lower the car 1 1/2" and require a lowering block. Combine the two requirements and I have come up with a design for an axle mounting plate that places the 54 style shock mount in the correct location and accepts my new design lowering block. This allows any 76 era Firebird or Camaro rear axle to be bolted on to the original springs and shocks. No welding or frame modifying required.
The lowering block mates the 1 3/4" spring to the 2 1/2" mounting pads on the Camaro differential. It also allows a wheel well centering range for the axle of 3" in 3/4" steps. This gives you the opportunity to center the axle in the wheel well up to 1 1/2" forward or backward from the axle center. The 54 axle mounting plates are offset by 1 1/2" (maybe 1 3/4") due to the original torque tube drivetrain. For some reason they had the plate mounting hole offset.
I will be mounting the axle in the next couple weeks and one of the details I will be checking is the pinion angle of the differential. It should be 3 degrees. I don't know if the pinion angle has been built into the Camaro diff's mounting plates or if I'll have to machine a 3 deg slope into the the lowering blocks. I'll just put it together and find out.
The way I understand it is that you are setting up the universal joint angles on the driveshaft. The rear-end pinion shaft should be pointing up at 3 degrees or less (measure from the flat on the top of the pinion). The tranny output shaft should point down at 3 degrees or less (measure at the engine). This means that the angle that the engine sits at is critical too!
The two angles should be equal to cancel vibration and not share a common centerline (don't point directly at each other). This is when the car is at normal weight and resting on the tires. The whole point is to minimize the vibration caused by engine power running through the universal joints. As smooth rotating power exits the tranny, it is converted to pulsating power through the driveshaft's forward u-joint, this pulsating power is converted back to smooth rotating power through the driveshaft's rear u-joint. If the angle is the same then the pulses (which we feel as vibration) cancel.
Here's a blueprint of the lowering block that I used. I machined it out of a solid 2.5"x2.5"x6" block of steel. I figure that you can make it easier out of 3 pieces: a 1.5"x1.812"x6" block and two side plates of 0.25" that are bolted together. It can be steel or aluminum. The drilled and tapped holes are for hex cap head bolts (5/16" NC18) for lining up the diff mounts. For some reason, the drawing package that I use didn't allow drawing hidden line circles, so I marked the two hidden circles with text and arrows.
The blueprint for the driver's side shock mount is basically a steel plate with a 5/8" x 10" bolt welded on to it. Note the drawings are not to scale. Mirror the driver's side for the passenger side. When I installed the shock mount, I found that I couldn't measure two dimensions properly (duh!) and screwed them up. Had to file the four u-bolt holes to fit - aaagh! The dimensions shown on the drawings have been corrected so that if I ever had to do it over again (fat chance), I could do it right the first time :-)
This is a hobby for me and there is a few things that I have to do before I can install the differential. First I'm cleaning up the leaf springs, painting them and installing button sliders. Interestingly, I've found that standard multileaf trailer springs are 1 3/4" wide also. You can purchase them in different lengths and weights from 1000 to 3500 lbs ratings. The nice part is that they go for about $25 per side. Could be a cheap source for leaf springs!.
The differential is going to be cleaned up and painted next. All of this takes time which is something I always seem to be short of...
Here's the rusty rear end right after being pulled from the Camaro
A little wirebrushing and paint - looks new!
Front spring bushing and bolts, notice the aluminum painted springs - oooh!
Lowering block and custom shock mount
Rear shackles and bushings
The front spring bushing costs about $21 Cdn each and was bought through the local spring manufacturer. The rear shackle bushings are about $2 each and are just a generic part. You need to know the spring width (1 3/4"), inner diameter and outer diameter. I bought 1 3/4" diameter sliders and drilled holes in the springs to mount them. There were already rectangular sliders mounted that had worn out several decades ago. I had to glue the sliders in place to keep them from falling out when I put the springs back together. All it took was a little muscle and a few C clamps - not the difficulty that I had envisionned.
I was going to purchase new shackles as the original ones were rusted. The local spring shop said that if the shackle bolts were going into steel sleeves than it would be a problem but since they are going into rubber bushings, the rust disfingeration wouldn't make any difference. So in they went.
How's the fit?
The completed assembly bolted in perfectly. The Camaro rear end has the exact same mounting dimensions as the 54. Here's two vehicles 22 years apart and the widths are identical - talk about luck!
Here's the fit with the lowering blocks centered axle and spring
Axle set back 1 1/2" from spring
The lowering blocks were adjusted to set the axle back towards the rear by 1 1/2" to center (front/rear) the wheels in the wheel wells. The springs haven't had any load on them since rebuilding and I expect that they will flatten out once the engine is in and the vehicle goes for a ride. Should drop in height also.
I am running wide (9" tread) P255-60R15 wheels on stock Camaro mags on the rear. They just fit in the wheel wells. There is 3/4" clearance between the Fender lip and the tire on the outside. On the inside there is 1 3/4" clearance between the tire and the wheel well/frame. A mag with a negative offset of 1/2" less than the stock Camaro mags would center the tire perfectly at 1 1/4" inside and outside. Ideally, there should be 1 1/2" clearance on both sides but 1 1/4" should do.
Shocks
I am using Monroe gas shocks on the front and rear. Monroe makes gas shocks to fit the original 54 Chev/Pontiac. They don't appear in their catalogs unless as a special order. Here's the numbers:
- Front shock pair: Monroe part # LE10001
- Rear shock (individual): Monroe part # 33044
Front Shock
Delco 536F 2G8/8L/L.25 Front 5520054 12B3
Rear Shock
Delco 581V 4F6/0V/B1 Rear 5518019 12A3
You can also purchase shocks from J.C.Whitney catalog #634G-01 P.184 Front Shocks Part # 55HE3661P and rear shocks part #55HE3662Y. These are generic part numbers for "hard to find" shock absorbers and they will ask you all the pertinent questions about your vehicle.
Another source which you should be aware of if you have one of the forgotten years of Chevs and Pontiacs (49-54) is the National Chevy Association which deals specifically with 49-54 Chevs. They have online catalogs and links to almost everything that you would need to restore one of these vehicles.
C Clip Information
I have 2 rear axles: a spare 2.73 non-posi and a 3:08 posi unit installed on the car. I felt that the posi unit had excessive end play on the axles while the non-posi unit had little or no end play. The end play is the play felt when you pull and push on the axle shaft.
The 10 bolt rear end uses C clips to stop the axles from falling out and the pinion shaft stops the axles from moving in. Since I had to take the axles out to install rear disk brakes on the posi unit, I decided to take the spare non-posi unit apart, make some measurements and use the best (least worn) parts on the posi unit to reduce/eliminate the end play.
Pinion shaft on right, C clip in center and axle showing spline and C clip groove
I measured the play between the pinion shaft and the axles on both diffs. Then I measured the wear on the C clips (thickness), the width of the C clip grooves on the axles, the pinion shaft diameter wear (shiny spots on the photo) and distance on the axle shafts between the end and the C clip groove. Here's the results:
Pinion to shaft C clip groove C Clip Axle end Pinion shaft Non-posi 0.010" 0.179" 0.150" 0.250" 0.795" 0.015" 0.178" 0.150" 0.249" Posi unit 0.045" 0.178" 0.147" 0.245" 0.790" 0.060" 0.178" 0.148" 0.225"
The pinion to shaft measurements are what I was feeling as end play. The non-posi's 0.010" and 0.015" play feels quite acceptable and what I would call tight. The posi unit's 0.045" and 0.060" play feels loose and as if something is wrong.
The play is the result of wear from the end of the axles against the pinion shaft. On the posi unit there is 0.015" of wear on the end of the axle and 0.025" on the other side. Add up the minor wear on the pinion shaft, C clip and groove and you get the "excessive" end play feel. I swapped the posi and non-posi parts and ended up with a much tighter feeling rear end ;-)
By the way, I couldn't find any specifications on axle movement end play any where. It's like a topic that no one wants to discuss. Anyways, hope this info clears up some things.