How to Find and Remove 8.5" GM Posi Carriers

By George Nenadovich

I recently had some luck finding 8.5 GM posi carriers at the local u-pull-it yards and thought some members would be interested in some helpful tips on how to locate and then remove the carriers and gears.

The first step is to locate the cars with posi (limited slip) units. This may sound simple but very few cars have posi units. The early cars, 1968-72 A-bodies, with posi units are usually found in the musclecars so finding one of those is not going to happen. The next best chance is to check the 1973-87 full size cars. Okay, you do not want to check all the cars so you have to limit the selection to cars that will typically have the units. It is no fun getting under the cars to check for posi units. The two best choices are the 1972-81 Trans Ams/Z28s and full size station wagons such as the Caprice, Cutlass, Electra and Catalina. Most of the Trans Ams/Z28s have the units. Most of the time you will see a small metal tag attached to one of the cover screws that states "USE LIMITED SLIP DIFF. LUBE ONLY."

The next step is to determine the ratio which is accomplished by removing 10 1/2" hex head screws holding the cover in place. Then, rotate the yoke to find the GM part number stamped on the ring gear. Immediately after the part number, the ratio will be stamped like the following 15:41, 40:13, 42:13 which translates to 2.73, 3.08, 3.23 respectively. See the picture below for a 2.73 ratio:

Directly after the ratio is the date which is hard to see in the photo, it is 12 82. This came out of a 1983 Caprice wagon

Lower ratios are not desirable so do not waste your time on them. As for the station wagons, look at the rear axle cover for the tag. Not all wagons will have the tag so also check for the factory tow package which consists of the following: heavy duty radiator (four core), oil cooler, tranny cooler and heavy duty hitch. The posi unit is part of the tow package. The 3 series carrier will have a casting numer of 410408 on the right side (pass. side) with four large "8"s also on the carrier. Please see photo below for the casting number(between the two 8s) and two of the large "8"s .

The 2 series carrier has cast numbers 410409 and three large "9"s on the carrier.

The next step is the removal of the unit and possibly the gears. Let's assume we have found a 1985 Caprice station wagon with posi 3.08 gear ratio which is quite common. This is an excellent ratio for a performance street Buick so we decide to get the gears and the carrier.

The first step is to remove the driveshaft which is held in place by four cap screws with 7/16" hex heads holding two straps to the yoke. After removing the driveshaft, the yoke will have to be removed. The yoke is held in place by a 1.25" hex nut which is applied with a great amount of torque. The easiest way to break the nut loose is to find a couple of space-saver spares to wedge under the back of the rear tires to keep them from rotating. Now you can apply a 1/2" ratchet with 1.25" socket to the nut to break it loose. If you can not break the nut loose, use a three foot section of pipe to provide additional leverage. I always carry a section of galvanized fence pipe which works great! Once the nut is loosened, you can now remove the wedges and rotate the tires until you see the 5/16" hex head spider gear shaft retaining screw clear the rear axle housing. Remove the 5/16" hex head screw and gently slide out the shaft from the posi carrier. See the photo below:

Now the fun part begins! The axles slide into the carrier s side gears and are held in place by C-clips. In order to remove the C-clips press each tire/axle towards the center of the car. This will allow you to see the C-clips on the end of the axle shaft and gain access to them. There is an S-shaped spring in the middle which restricts your working area. See the photo below:

The small section at the top and bottom of the opening contains the posi clutches which you see here as a stack of 4 plates each for the top and the bottom.

On earlier cars 1973-80?, there are four coil springs and two plates which make it very easy to gain access to the C-clips. The trick now is to use a medium sized flat-blade screwdriver to poke, spin, or rotate the clips off the shaft. Sometimes it helps to have someone hold the axle shaft pressed towards the center of the car as the axles can move out of position. Once the C-clips are removed, slide each axle shaft out about six inches.

The last step is to remove the carrier bearing main caps which are retained by four 5/8" hex head cap screws. Once the screws and caps are removed, the carrier will come out. Most of the time, the best technique is to use a rag and grab the ring gear and pull the carrier out of position. Use two hands as the carrier is quite heavy and can fall to the ground (sand/dirt) which is not desirable. I use a rag since the gear lube will make your hands slippery and prevents any dirt from contaminating the carrier. If it does not come out with a hard pull, use a large screwdriver or tire iron to gently pry loose the carrier. After you have the carrier out, reinstall the spider gear shaft and 5/16" screw. Lastly, remove the pinion nut and slide out the pinion. That is all there is to it! It should take only 20 minutes to remove everything!