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Herd WebMaster
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Posted - 05/04/2007 :  07:35:54 AM  Show Profile  Visit Herd WebMaster's Homepage  Reply with Quote
GM 8.5" Axle Warning
By Scott Mueller.

I have discovered something that is very important for the Impala SS (or for that matter any GM B-body or D-body) owner!! It pertains to the rear axle, specifically proper lubrication of the outer rear axle bearings.

If you look at the rear differential cover, you will notice two small indentations on either side directly in line with the axle tubes. These indentations are to direct the flow of oil through two holes in the differential case. The oil then flows down the axle tubes to lubricate the axles and the outer axle bearings. Note that these holes actually have two purposes, one is to be used by a case spreader to spread the case during factory assembly (but this is not required during service), but the second and more important function of the holes is for additional lubricant to feed the outer axle bearings, especially under severe service. In order to allow oil to pass through these holes, the differential cover gasket is also supposed to have two corresponding holes in it, which line up with the holes in the case as well as the indentations to allow oil to flow down the axle tubes.

Starting in the 1990 model year, GM has been improperly installing differential cover gaskets THAT DO NOT HAVE THE LUBRICATING HOLES!!! If you do not change this gasket to one that has the proper holes, your outer axle bearings, as well as the axles themselves, will be destroyed by around 70K miles. The damaged (under lubricated) bearings will wear grooves in the axle shafts, requiring that they also be replaced. If this is left unchecked, the metal bits from the damaged outer bearings and axles will also damage the inner bearings, pinion bearings, as well as cause the spider gears to wear into the differential carrier.

I have a friend who has worked for a fairly large local municipality servicing all of their fleet vehicles (80+ Caprice Police cars) for the last several years. They observed that the correct gaskets with the holes were used until about 1985. Then GM (or more accurately Saginaw, now called American Axle) switched to silicone sealer instead of a gasket. As long as the silicone did not fill up the holes, there were no problems.

Sometime during 1990, GM went back to using a gasket, but unfortunately it was one that did not incorporate the oil holes. This is where the problems started. In the fleet my friend works on, they started going through axle jobs left and right. They keep excellent records and found that the axles would trash at about 50,000 miles, which was 14,000 miles out of the warranty. What was noticed with every rear end failure was bad axle shafts. The bearings wore 3/16" groove in the axle shaft. At the least just the axles and bearings would be bad, however in many cases the material removed from the axles and bearings had also ruined the case, spider gears, ring and pinion, and all of the other bearings. In some cases they literally had to replace everything but the housing.

By going through their records, they tried to find the possible cause for the numerous axle problems they were having. Since there was no problem the year before they started to look for a design change. You should know by now what they found, the missing gasket holes! They then determined that the correct gasket (Fel-Pro gasket p/n RDS 55028-1 which does have the proper lubricating holes) was still available through sources other than GM, and installing it should correct the problem.

They then immediately started a new maintenance program on the rear axle gasket and fluid changes. The first change was done at 15,000 miles with the proper gasket. This allowed the axles to make it to about 80,000 miles before the same type of excessive wear occurred. Then they altered the maintenance program further and went to a 3,000 mile gasket and oil change the first time, then every 15,000 miles thereafter. Now their squad cars go 120,000 miles without any axle problems. They don't have any data after that, as that is when the cars normally are decommissioned.

In case you are wondering, yes they have made GM aware of this problem, but GM currently has done nothing about it, and even new '96 vehicles still come with the improper gaskets. To check this further, I purchased a new factory replacement gasket over the counter and it also lacks the lubricating holes!

I was amazed to find this problem on my '94 Impala when changing the rear axle fluid, and can tell that there was more wear than should be present at 7K miles (the first factory recommended axle service is 7.5K miles). I advise that anybody with a '90 or up GM B-chassis (Caprice, Impala, or Roadmaster) or D-chassis (Fleetwood) undertake to IMMEDIATELY replace the factory axle cover gasket with the Fel-Pro RDS 55028-1 gasket. If you do not, then you will very likely be performing major service on the rear axle by 70K miles. Again, I must emphasize that this problem is still present on the '96 models as well.

We were hypothesizing as to why GM does not do something about this problem, and can only come up with the fact that all will seem fine until the vehicle is well out of the warranty period. GM seems unmotivated to correct the situation, as the local municipality has communicated this problem to several of the Police and Fleet people at GM. Now that these cars are going out of production, it is likely this type of problem is not a high priority.

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