Expert answer provided by Norm Parker, General Motors
The short answer to this question is a resounding, “false.”
But there is confusing information flying around that may
be the source of these rumors. Let’s start with the highlevel
goal of breaking-in or wearing-in an axle. There are
two big wear mechanisms in an axle—the ring and pinion
gear mesh (or hypoid mesh) and the bearings. There is
some wear that occurs in the differential gears, but due to
the intermittent usage, it is very slow by comparison. Providing
the system is functioning correctly, most of the wear
occurring in an axle will happen within the first 500 miles
and then slowing down until around 2,000 miles where it
eventually tapers off to into steady state. The overarching
goal of the break-in period is to have the same functional
axle that you started with.
The bearing wear mechanism is fairly straightforward.
All of the tapered roller bearings in an axle will wear to
some extent between the bottoms of the rollers and the
large rib that supports the heavy axial (horizontal) loads.
Up until a few years ago, this surface started life around
2-3?m Ra (~100? in.). Over the course of a couple of thousand
miles, this would smooth out to about 0.15?m. During
this period there was higher friction, which means
higher heat and debris particles in the oil. Today, nearly
all modern bearing companies provide honed ribs to the
OEs, which are nearly standard product now. These are often
marketed as ‘non-wearing’ or ‘no-break-in’ bearings.
The honed rib surface is 0.1- 0.2?m Ra, which greatly reduces
the initial break-in wear (although it’s not zero, as
was the original intent). Honing takes care of surface finish
but does not take care of part-to-part variation. There will
always be a less-than-perfect fit between a new roller and
a new rib. These parts must wear in together to achieve the
perfect fit. We have found that no matter how smooth the
mating parts are, there is still wear due to mating part tolerances.
Just a note of caution for rebuilt or repaired axles:
There are many aftermarket suppliers that will use cheaper
bearings without honed ribs or, in some cases, have old inventory
with non-honed ribs. Without having any sure way
of knowing, you should err on the side of caution and assume
that a rebuilt axle, or bearings ordered for a rebuild,
do not have honed ribs.

The hypoid mesh is the other source of considerable
wear and heat during the break-in period. Just as with the
bearings, we are dealing with a surface finish that has to
be polished down along with the mating gear teeth that
need to run together for a period of time before there is
true full contact. Unlike the bearing, there is both rolling
and sliding friction in the hypoid. From a physical perspective
this means that we can shear some of the surface
finish off while some material can be rolled back onto the
surface. You can loosely correlate this into elastic vs. plastic
deformation of the local surface asperities. Higher force
and lower lubrication film will force more shearing, while
lower loads and a thicker film will allow more rolling. With
rolling comes some surface work hardening which further
adds to our benefit.
In order to help with the break-in period of the hypoid,
most axles have some level of manganese phosphate
coating applied to the ring and/or pinion. This is a chemically
applied, hard porous coating that is intended to wear
off during the first two thousand miles or so. There are a
couple of benefits; the porosity allows the surface to hold
more oil than a bare steel surface, which helps with lubrication
during the higher-friction break-in period. Secondly,
as the hard coating wears down, the surface asperities and imperfections of the underlying gear are slowly introduced
into the system, allowing the wear to occur much
more slowly than two hardened steel parts mating for the
first time. Phosphate coating isn’t terribly expensive, but it
isn’t free either, so companies like to experiment with only
applying phosphate to one surface vs. both. There is no industry-wide
consensus on a best practice.
Similar to bearing superfinishing or honing, gear manufacturers
use a process referred to as isotropic super-finish
(ISF). This is exactly what it sounds like—non-directional
honing. The principal behind the benefit is just the same
as with the bearings. The traditional gear finish around
0.40?m Ra with ISF can be reduced to 0.05?m Ra. This
results in much lower friction during the break-in period
which produces less wear particulate running through the
oil. This benefits all of the other components in the system
as well. ISF is mostly used in aerospace at the current time,
but is starting to gain momentum in the automotive industry
as efficiency and performance demands continue to increase.
This process certainly shortens the break-in period,
but does not eliminate it.