AOPA Pilot Magazine - January 2019 - 89

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A PROPERLY BROKEN-IN CYLINDER has smoothed-off peaks in the cylinder barrel; the valleys

between them provide tiny footholds for the oil. Running a freshly honed cylinder at low
power for a significant length of time can cause a buildup of carbonized oil, known as glazing.

enough surface roughness to make the barrel oil-wettable.
If you were to use a microscope to examine the crosshatch pattern on a freshly
honed cylinder barrel, you'd see that the
hone leaves a series of tiny peaks and valleys. The valleys (fissures) of the microfinish
cause the surface to be oil-wettable by providing tiny footholds for the oil to hang onto.
However, the peaks (ridges) are problematic: They increase friction and wear,
causing the barrels to run hot. The purpose
of breaking in a cylinder is simply to smooth
off those sharp little peaks to reduce friction
and wear, while leaving the sharp little valleys intact to provide oil-wettability.
It would be nice if this was done at the factory, and it is, to an extent: Some cylinders are
manufactured with a multi-step honing process that partially smooths the sharp peaks.
All factory engines, and many field-overhauled ones, go through a run-in period in an
engine test cell that starts the break-in process
(although this typically does not occur when
cylinders are replaced in the field).
Whether the engine is new, rebuilt,
overhauled, or simply fitted with one or
more replacement cylinders, the final breakin is always up to the pilot who flies the first
several post-maintenance flights.
BREAK-IN BASICS

During normal engine operation, the goal
is to lubricate the cylinder barrel with an
oil film sufficiently strong and thick enough
to prevent most metal-to-metal contact

between the compression rings and the
barrel walls. During break-in, however, the
goal is just the opposite: to breach the oil
film and cause sufficient metal-to-metal
contact to grind off the sharp peaks of the
crosshatch while leaving the valleys intact.
To accomplish this, we need to run
the engine hard for the first hour or two-
hard enough to create greater-than-normal
pressure on the compression rings. The
semi-trapezoidal design of the compression
rings uses combustion chamber pressure
to force the rings tightly against the barrel
walls. Break-in demands that this pressure
be maximized to provide enough force to
breach the oil film.
It's also important to make sure that
the oil film isn't too strong to be breached,
so the choice of break-in oil is important.
Traditionally, break-in has been done using
straight mineral oil that contains no synthetics, ashless dispersants, anti-wear/anti-scuff
additives, or viscosity index improvers. In
recent years, however, some manufacturers
and engine shops have been moving away
from using such primitive oil.
Synthetics have higher film strength
than petroleum-based oils, which is obviously bad for break-in. Anti-wear/anti-scuff
additives also should be avoided, since scuffing is precisely what we're trying to achieve
during break-in. However, I've seen no
persuasive evidence that ordinary ashless
dispersant (AD) oil is any less effective for
break-in, and it keeps the engine cleaner, so
I prefer to break in on AD oil.
www.aopa.org/pilot AOPA PILOT | 89

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AOPA Pilot Magazine - January 2019

Table of Contents for the Digital Edition of AOPA Pilot Magazine - January 2019