Rear End Swap

Q: Rear End Swap

A: Classical Pontiac a good reference sheet in their restoration section on putting a second generation diff into a first generation. There are some mods to be made, but it is possible, and I think worthwhile. I am planning on replacing my 256 open diff with a 373 posi unit from a 74 f body. I am counting on the info from that site to be accurate, and the change to be not too difficult

A: I did this swap this last summer, and in reading Geebgens report, posted both on his own website:

http://www.angelfire.com/on/geebjen/rearswap.html

(geebjen is a member of this list and has an excellent site with tons of useful info)

…and on the Classical Pontiac site:

http://www.classicalpontiac.com/

(click the restoration link)

…I found that it is pretty much spot on with the description of the rear swap. The only thing I could add to this would be that I found that I needed to keep the parking brake cables and the approriate bracket from the 1st gen. rear end. The 2nd gen cables wouldn’t work.

Pricing varies from area to area. I paid $75.00 to get the spring perches done. I didn’t realize until too late that they had used a generic perch that required 2 ubolts per axle, compared to the single ubolt that is stock. The original perch has a pocket or recessed area for the spring to seat, while the replacement perch is flat.

Another gotchya was that they welded the perches about 1/2 to 1″ too far apart. I had to muscle the springs into place, and this placed the shock mount in contact with my passenger side wheel rim.

My rear wheels are 8″ wide, w/ 4.74″ back spacing.

I also paid $110 to have my drive shaft shortened and balanced.

Check the ratio on any second gen. rear you think about buying. While 3.23 and 3.42 ratios are fairly common, you can still run into Posi rears with 2.73 and taller gears. Not optimal ratios.

A: I made this swap last winter and found out that replacing the perches was not necessary. The first gen. perches are slightly angled versus the second gen. which is straight. The leaf springs are further apart in one end than the other on our cars. Still the sec gen. perch is wide enough to accommodate the angled springs. Therefore I just grind the perches off the sec gen. rear end and moved them closer together. I did not buy any new perches and I did not remove the old perches from the original rear end. Only thing to remember is the sec gen. perches are about 10 mm higher which LOWERS your car by the same amount. ( oh sorry, 3/8″ )

How to get the correct position: I took lots and lots of measurements of the rear ends and also the distance between the leaf springs under the car. I then drilled two holes with the correct distance in a wood plank and bolted the front spring brackets to it. When I had this assembly (springs, plank, perches) clamped to the rear end with the U-bolts I could measure that everything was correct and weld the perches.

The only parameter I couldn’t measure and had to use my eye was the angle of the yoke to the horizontal plane. The only way to do this in a better way, that I can think of, is to have the rear end mounted in the car with the prop shaft attached and weld the perches in place when the cars weight is on the springs.

Regarding prop shaft length. Don’t go wasting your money to get it shortened before you have searched your junk yard. You will be surprised how many different lengths there are. I managed to find one that bolted right in.

A: Niklas, your input on the 2nd gen. perches is useful. I’d read in another write-up that the user had not accounted for the perch angle and had a minor problem due to that. If I had it to do over again, I’d probably stick with either the 1st or 2nd gen perches.

Regarding prop shaft length. Don’t go wasting your money to get it shortened before you have searched your junk yard. You will be surprised how many different lengths there are. I managed to find one that bolted right in.

Interesting, though I might add that a junk yard prop. shaft is an unkown factor. After hunting for the right length, yoke and u-joint types, I might still end up with a unit that is either bent (causing severe vibration) or with bad u-joints that could fail shortly after installation. As my car has some modifications that enhance it’s power output, I didn’t want to go too cheap on this critical link. I’ve had 1st hand experience in drive shaft failures, and want to avoid them whenever possible.

The 1st time was when the rear joint in my 1969 Chevelle gave way while driving down Colfax Ave in 1976 in Denver. While resulting in a lot of noise and a loss of power to the rear wheels, I lucked out and coasted to the side of the road. The second time was when I was following some friends at Lake Tahoe in California in 1980. They had piled into thier freshly painted 1968 Camaro Ralley Sport. The front u-joint on the Camaro failed at about 45 mph. The drive shaft dropped to the ground and immediately pole vaulted the Camaro’s rear into the air. The rear axle assembly tore loose from the leaf springs and was thrust up into the fuel tank, spilling gas everywhere.

The occupants got lucky in that the car didn’t end up on it’s roof, and further lucky in that aside from some bad bruises, no one was seriously hurt. That wasn’t the case for the car. Sadly, the wounds were mortal and the car ended up in the junk heap. We salvaged what parts weren’t bent up, and that was the end of that car.

From my perspective, the money I spent on the cutting, balanc- ing, and new joints was a good investment. But I do see your point when trying to keep the budget under control.

Any proposed updates, changes, pictures, and/or corrections, please use our comment section below (may need to click on permalink to access comments feature). Information is subject to change and offered as is without any warranties or guarantees. Please review our Term's Of Use for more information.