Drawbacks of the OEM design
An OEM adjustable, Set 7 (A7) tapered wheel bearing is definitely strong but it is has a few drawbacks.
The bearing is flipped around backward, so the race is captive between it and the axle flange. This means the bearing must float in the housing end and requires an adjuster, thrust block or thrust pin. Of course, an adjuster, thrust block or thrust pin must pass through the center of the differential. A hole drilled in the cross shaft of a 4 pinion carrier creates a stress riser that is prone to breaking. A 2 pinion carrier allows the thrust block to pass around it unobstructed, but the 2 pinion design also limits the differential’s strength.
Axle spline engagement also suffers because the width of the thrust block (which must have room to slide side to side) protrudes into the splined area of the side gears. This limits spline engagement in differentials with a 2 piece cone or clutch-hub/side gear arrangement.
Beaded steel and foam gaskets don’t keep water from running into the housing end and into the non-sealed wheel bearing. Don’t forget to check your pickup’s A7 wheel bearings if you ever back a boat into the water.
Axle flange stand-out is not held constant unless you blueprint the axle lengths. Measure the axle flange stand-out on both sides of any stock 8 3/4″ rearend. Because of production tolerances, a single adjuster causes the axle flange to stick out farther on 1 side than the other.
The length of both axles, and the housing width is critically dependent on each other. Because everyone uses tape measurements to specify axle and housing lengths, making a set of axles with adjustable set 7 wheel bearings for a custom application is very hard. Axle flange standout varies greatly unless the axles are cut long, installed then blue printed to length.
The need for non-adjustable wheel bearings
Most of these problems can be avoided by installing non-adjustable sealed ball “Green” wheel bearings. The name comes from the Green Bearing Company which first produced them. The company has since been purchased by Bearing Technologies.
Unfortunately, the original Mopar Green wheel bearing design has 2 problems.
First generation (RP-400) Green bearings, still sold by Mopar Performance and others, are problematic because the crimped-on flange will not allow the bearing to wiggle around inside a housing that is not perfectly straight (none are).
In addition, the design causes the axle to be inserted DEEPER into the housing than necessary. This results in pre-loading against the differential thrust block and early bearing failure.
Second generation (MO-400) snap-ring style Green bearings are forgiving because they can move around inside the housing and they do not preload the differential thrust block in a stock application.
Most guys who have problems with Green bearings are running the RP-400 first generation version or incorrectly made aftermarket axles or housings or poorly designed rear disc brake kits, all of which cause pre-loading and premature bearing failure.
I have several customers running MO-400 snap-ring Green bearings in daily drivers. The design is no different than what came stock in millions of other vehicles, including ’60s era Mopar 7.25″ and Ford 9″ rears. (For example, see here) I do not stock, nor do I recommend the first generation RP-400 Green bearing with the crimped-on 5 hole retainer. I only carry the “loose fit, snap ring style” second generation MO-400 design.