Proper Engine Break-In
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.
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 out.
However 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 blow-by.
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 high.
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 loose torque.
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.