Why Install an 8.5" rear?

Here is a pic of a 1965-70 GM 8.2" rear axle.

Here is a pic of a 1971-up GM 8.5" rear axle, notice the two corners cast into the housing indicated by the white arrows.

 

So why is it important to install the 8.5" rear axle in your car that has an 8.2" rear? It is a matter of cost! Since the 8.2" rear axles were only made from 1965-70 for GM A-bodies, there is very little aftermarket support for them. Also, some axle bearings for 8.2" rears have been discontinued and/or are extremely expensive. The advantage of the 8.5" rear is that you have a wide variety of new gears to buy, new posi units are available and bearings/axles are much cheaper. Another benefit is that GM basically used the same center housing up to 1990. After 1990 GM went to 31 spline axles and a slightly different design. So, from 1971-1990, you have a good chance of finding posi units in salvage yards. Read the posi article on the site which tells how to identify and remove the posi units. When installing an 8.5" rear in a car that originally had an 8.2" rear, you will need to shorten the driveshaft about 1" This would also be a good time to replace the u-joints and have the driveshaft rebalanced. You can not interchange parts between the 8.2" and 8.5" housings except for brake parts.

Stock 8.2, type "O" 67-70 Olds, or 12 bolt driveshaft will need to be shortened 7/8-1 1/8". The "nose" on the 8.5 housing, along with the pinion yoke are both longer & require the shorter shaft.

64 model A-bodys used their own specific upper rear control arms & swapping in anything other than original '64 rearend will require use of '65-67 upper control arms.

64 through 66 A-bodys used a different rear spring, than '67 & later. The 64-66's had a bolt-on "ear" that clamped the rear spring down. One can trim the spring guides of the later rear & weld small square plate on then use the clamp on ears, OR one can go with later model springs.

Width issues... Width changed DURING the 66 model year on BOP 8.2 rearends. Have ran across this in parting many 66 Tempests & LeMans & have been trying to track down exactly when this happened out of all the BOP A-body plants...for many years. Ck'ed our records & in so far custom building & selling now 317 early 8.5 A-body r/e's, the main tight fit issues I've run into were when installing 8.5 A-body rears under a pair of 64 Chevelles & a 65 Elco. The wheelhouses under the '64's & 65's are the smallest of all A-bodys & customers in two cases were running 275-60R15's on aftermarket 8.5" (9.5" overall) wide wheels. Correct offset wheels were a must for such tall wide tires & even then, the tire-wheel combo is getting cose to the 1/4 panel lip. Never have run across this problem on a 66 or later, but on one of the 64's, we had to actually jack the Chevelle up with the shocks unhooked, bolt the wheel & tires on, then carefully let the body & chassis down & then reattach the shock mounts. Would not have had to go to this trouble with a 255-60R15 & a different wheel