Few powerplant-related subjects are more controversial than the best procedure for breaking-in a new or freshly-overhauled engine. Ask a dozen A&Ps and you'll get a dozen different recommendations. John Frank explains the principles behind proper break-in, and provides a proven step-by-step technique for achieving good ring seating every time.
June 16, 1996
With a new, remanufactured, major overhauled or top overhauled
engine utilizing proper break-in procedures is critical to avoiding
high oil consumption and its related problems. The main purpose
of break-in is to seat the compression rings to the cylinder walls.
Let me explain what ring seating is all about.
While a new steel cylinder may look like a smooth surface inside,
it really isn't. A stone hone has been used to give the surface
microscopic grooves...peaks and valleys so to speak. Each tiny
groove acts as the oil reservoir holding oil up to the top level
of the groove where it then spreads over the peak surface. The
piston ring must travel up and down over this grooved surface,
and must "hydroplane" on the oil film retained by the
grooves. Otherwise, the ring would make metal-to-metal contact
with the cylinder wall and the the cylinder would quickly wear
the ring will only ride on this film of oil if there is sufficient
surface area to support the ring on the oil. When the cylinders
are freshly honed the peaks are sharp with little surface area.
Our goal when seating the rings on new steel cylinders is to flatten
out these peaks to give more surface area to support the rings,
while leaving the bottom of the groove intact to hold enough oil
to keep the surface of the cylinder wet with oil. See illustration
(taken from Sky Ranch Engineering Manual
by John Schwaner).
Conventional chrome-plated cylinders, generally referred to as
"channel chrome", are not honed because they already
have tiny channels (or cracks) in the chrome surface created during
the electroplating process. However, the same flattening of the
peaks must be accomplished. Because chrome is much harder than
steel, this seating process can take substantially longer with
channel chrome cylinders than with steel ones.
Ceramic-impregnated cylinders such as Cermicrome, Nu-Chrome and
CermiNil work a bit differently. The cylinders have a porous surface
that retains oil. Only a brief period of time is required during
break-in for the piston ring to smooth the surface area to provide
sufficient area for the ring to be supported on a film of oil.
These cylinders break in very quickly.
You sometimes hear about cylinders becoming "glazed' if break-in
isn't done properly. When glazing occurs, oil oxidizes in the
tiny grooves or channels on the cylinder walls, causing the grooves
to become plugged with varnish. If the grooves get plugged, they
can't do their job of maintaining a consistent oil film on the
cylinder walls. The result is usually high oil consumption and
Things you should know
As the owner or pilot, you need to do a couple of things to assist
this ring seating. First, you should use straight mineral oil
during the initial break-in period, because it has less lubricity
than normal ashless dispersant oil and therefore provides increased
friction to aid in this seating. Second, you should operate the
engine at high manifold pressure during the initial break-in period,
in order to push the rings out against the walls as hard as possible
to aid in the seating.
Try to keep ground runs to an absolute minimum. This is most important
with engines that have not been run in a test cell, and will be
run for the first time on the aircraft. All factory new and factory
remanufactured engines will have been run in the factory test
cell for 30 minutes to 2 hours. Some large overhaul shops also
give their newly overhauled engines a test-cell run before shipment,
but most shops don't. So be sure to ask if your engine was run
and for how long.
All ground running should be done with all cowlings and baffles
in place. A decowled engine receives very little colling air,
so running without the cowling could damage the new cylinders.
Preparing for the first run
Fill the engine oil sump to rated capacity with straight mineral
oil, preferably 40 weight. We find 40 weight is better than 50
weight as the lighter oil will flow a little faster and carry
off heat a little better. Dissipating heat is a major concern
during break-in. You should use 50 weight oil if the ambient temperature
will be above 80F. However, hot weather isn't ideal for break-in.
Remove a spark plug from each cylinder, preferably a bottom plug.
Hook the aircraft up to an APU and crank the engine with the starter
motor for a period of one minute. This will allow the engine's
oil pump to distribute some oil throughout the oil galleys of
the engine. If the engine is equipped with a turbocharger, remove
the oil discharge line from the turbo and make sure there is oil
flowing out of the turbocharger.
If the aircraft is equipped with an electric boost pump use it
to pressurize the fuel system to look for leaks. Run the pump
on "high" or "emergency" speed with full throttle
and mixture at idle cutoff.
First run (30 seconds to 1 minute)
Keep this run to minimum time necessary to complete task.
Start the engine and run at 1000 RPM or less for approximately
30 seconds to one minute. Inmediately after startup, make sure
that oil pressure starts rising and goes to the upper part of
the green arc. If it stops in low green or lower, shutdown immediately
and determine source of problem.
Check that idle RPM is approximately correct (usually about 600
RPM at minimum throttle), that both mags work, and that idle manifold
pressure is in the vicinity of 12 inches (if the engine is equipped
with a manifold pressure gauge). Check idle mixture at shut down:
as you slowly pull the mixture control, you should get a slight
RPM rise before the engine quits.
After shutdown, check for oil leaks and make adjustments to anything
that is grossly in error. Let the engine cool down completely.
Second run (1 to 2 minutes)
Keep this run to minimum time necessary to complete task.
Start engine and allow to warm until oil temperature needle comes
off of peg. Do normal but brief run-up, checking mag drop. However,
do not cycle the prop at all.
If the engine is equipped with an electric boost pump, make sure
that it will boost pressure, even to the point of starting to
flood the engine. If the aircraft has a two-speed boost pump controlled
by a throttle switch, it may not be possible to get high boost
at idle throttle position, but even low boost should bring up
the pressurea bit.
With the throttle pulled back to idle check for correct idle speed
— 600 RPM for most engines but consult your manual in advance
to be sure. Slowly pull the mixture out to shut down the engine,
there should be about a 25 to 50 RPM rise if the mixture is set
correctly. A greater rise indicates to rich an idle mixture, a
lower rise or no rise at all indicates to lean an idle mixture.
Shut down and check for leaks, make any indicated adjustments.
Let the engine cool down completely.
First flight (30 minutes)
Pick a time when you will be able to taxi right to runway and
take off. If necessary, make prior arrangements with tower. Start
engine, taxi out, do a normal runup but do not cycle prop. If
everything appears okay—oil pressure high in the green and oil
temperature off of peg—initiate takeoff on longest runway available.
On carbureted aircraft without an engine-driven fuel pump, watch
for any indications of mixture problems which may cause a rough-running
engine. On aircraft with engine driven fuel pumps (including all
fuel injected engines), monitor fuel pressure or fuel flow closely,
If too high (way beyond red-line), reduce to red-line with mixture
control. If too low (two gallons-per-hour short of red-line or
less), abort the takeoff and determine the reason.
Closely monitor RPM. If it doesn't get within 100 RPM of red line
and there is sufficient runway available, abort the takeoff. There
could be a problem here if the tach calibration is off to the
low side, which is where most mechanical tachs are. Some have
suggested doing a tach check on the second ground run with a digital
tach checker such as the Cardinal tach checker. However, I prefer
to avoid getting RPM up in the 2000+ range during the ground runs.
If the aircraft is equipped with a multiple probe EGT and you
are is able to monitor EGT in addition to the above-mentioned
items, abort the takeoff if any single EGT exceeds 1500 degrees.
Also abort the takeoff if anything sounds, smells, or feels unusual,
even if you can't quite "put your finger on it." You
should be "spring-loaded" to abort this takeoff, continuing
only if everything seems very close to "just right".
This is a good rule for all takeoffs, but especially
the first takeoff on a new engine!
After takeoff, make a shallow climb and maintain the highest climb
airspeed with which you are comfortable. Once you get to a safe
altitude, you should make your climb very flat—around 200 to
300 feet-per minute. The goal is to keep as much cooling air flowing
over the engine as possible. Circle above airport for 30 minutes
(just to be on the safe side). For a normally-aspirated engine,
do not get much above pattern altitude so that power output remains
On fixed pitch propeller aircraft, keep the RPM at the top of
the green. On controllable pitch aircraft keep MP at the top of
the green or higher and high RPM as well. If you have cowl flaps,
keep them wide open. Use maximum rated continuous power if that
can be done without over-temping the engine; otherwise, reduce
power only to the extent necessary to keep cylinder head temperature
and oil temperature in the green. Use full rich mixture to help
keep CHTs down.
After 30 minutes make normal landing, carrying as much power as
possible during approach. Taxi as quickly as prudent to parking
and shut down immediately.
Holding this first flight to 30 minutes over the airport just
above pattern altitude is a concession to safety. The first flight
would be better extending for a couple of hours, but I have been
surprised too many times by problems to stay up very long or get
very far away from the airport.
Un-cowl and closely inspect the engine for any signs of problems,
leaks, cracks, etc. Pay close attention to those things that might
have come loose such as clamps and fittings. I have been amazed
at how many things can get loosened up after the engine starts
providing some vibration. Make adjustments dictated by flight
test results. Let cool down completely.
Second flight (1 1/2 to 2 hours)
This is a fun one! Take off normally. Stay low and carry as much
power as possible, especially MP at very top of green or higher.
Use rich mixture to keep CHTs in line. Staying at low altitude
is important if the engine is normally-aspirated because this
allows for the greatest MP. We have seen problems with break-in
at high altitude airports.
For me this flight is great fun because it means I get to whip
up and down a gorgeous section of the California coast at 500
AWL (above water level) with the airplane balls-to-the-wall...an
expression, by the way, that originally came from having the ball
ends on the early throttles all the way forward towards the firewall.
After two hours, I return to the airport. If there are no problems
found and nothing that needs adjustment, I turn the aircraft over
to its owner. I instruct the owner to fly the airplane "hard"
for the next eight hours, keeping the MP as high as possible and
(if normally-aspirated) avoiding any high-altitude flights, preferably
staying below 5,000 feet.
Finishing the job
How can you tell when an engine is broken in? In the old days,
it used to be when the oil consumption stabilized, which is still
a good indicator. With today's sophisticated probe-per-cylinder
engine analyzers I can often see individual cylinders seat. When
the CHT on any cylinder drops about 50 degrees in the space of
a few minutes with no change in engine operating conditions, that
cylinder has seated. I almost always see Cerrnicrome seat within
an hour. Standard nitrided steel cylinders take three to four
hours, and channel chrome a couple hours more.
At five hours, change the oil and filter, or clean the screen.
Refill the sump with fresh mineral oil. At ten hours, drain the
oil again, change the filter or clean the screen, and refill with
whatever ashless dispersant oil you are going to use. I recommend
Aeroshell 100W unless operating circumstances dictate a multi-weight
oil. Give the engine a thorough going-over. Put a torque wrench
on every exposed nut and bolt and check torque. I am amazed at
how many loose bolts we find. If anything is dramatically loose,
do whatever is necessary to check the bolts around the one with
Some recommend going on break-in procedure and oil for up to 25
hours. My experience is that if the engine isn't broken in at
10 hours, it just isn't going to happen. The only possible exception
is channel chrome jugs which might take slightly longer.
If the engine hasn't broken in after 10 hours, you either have
to put up with the high oil consumption, or pull the cylinders,
break the glaze with a hone, check the rings for damage and correct
material (personally I would install new rings), reinstall the
cylinders, and start all over from scratch.
But if you do the right thing during those first critical 2 or
3 hours of break-in, you'll get good ring seating and low oil
consumption every time.