Engine Break-Ins: Follow the Service Data

We agonize over doing it right and we’re paranoid about doing something wrong. So when it comes to breaking in a new, rebuilt, or overhauled engine, we feel the pain. In some cases, you just shelled out close to $100,000 and twice that for a twin. There’s a lot at stake. On the other hand, breaking in a new engine or even new major components on an existing engine isn’t really rocket science because there’s a good chance the engine has already been run, and your job is to break it in correctly. Luckily there is plenty of technical data to assist, and in the end if you still aren’t sure how to do it, lobby the help of a qualified mechanic. Here’s a general guide to keep things on the rails so the engine lasts many hours.

It’s Already Been Run

First, even the NTSB recognizes that roughly 75% of the total normal wear of an engine occurs during the break-in period. But run-in comes before break-in. When it comes to an overhauled or rebuilt engine, before doing anything you should establish the difference between a run-in and a break-in. In reality, any reputable rebuilder will perform a run-in on an overhauled or rebuilt engine for up to a few hours (more if needed) in a test cell using a calibrated set of instruments with proper cooling, individual temperature probes for each cylinder, as well as other essential performance instrumentation.

This test stand run-in procedure is far and away the most desirable and dependable way to be sure the critical first moments of engine operation are done properly under controlled conditions. This initial operation is also the most crucial for mating parts as well as establishing proper oil consumption, making sure proper engine specs are met, and that there is no oil leaking out of the engine. When choosing an overhauler or engine shop, ask if it will provide you with documentation about how the run-in was conducted, including how much power the engine made. Good shops will provide this test data, and our opinion is since you will be paying big for a rebuild, find a shop that uses a test cell. The good news here is that the critical run-in of new mating parts (the rings to cylinder walls, as one example) will have already been well established.

After the engine is bolted on to the aircraft you will be performing the break-in portion, which will be the much less critical but includes the important task of being careful for the next 50 hours to be sure cylinders don’t get glazed and the engine is not run too hot or for prolonged periods at the same power setting. It’s worth mentioning that a test cell run-in is not something that’s needed for a single cylinder replacement, though you do need to use the proper protocols for breaking in cylinders if you expect them to have an effective service life.

We recall the owner of a Cessna P210 who couldn’t understand why the cylinders on his big turbocharged Continental wouldn’t last longer than a couple hundred hours. He never preheated the engine while parking it in a freezing cold hangar, would fire it up, do a taxiing run-up, and blast off without letting the oil warm up properly. Think that’s asking for trouble?

Follow the Service Literature

There’s a lot to learn from it, and service bulletins and advisories will offer break-in guidance, including which blend of oil and additives to use. In general, mineral oil is the most common and that might include specific weights. For some high-performance engines, ashless dispersant oil (for keeping internal contaminants suspended) is prescribed after a certain run time. No matter what engine you have, follow the oil recommendation to the letter, and that includes the prescribed oil for use after cylinder replacements as cylinder makers usually have their own recommendations for oils and break-in procedures.

When it comes to operating the engine, some of the engine makers have a service bulletin outlining the proper procedures for both replacement cylinders and the first engine run and break-in period. As one example, Continental Aerospace lists both the test cell procedures as well as using the aircraft in lieu of the test cell for run-in, plus detailed procedures can also be found in the appropriate overhaul manual for most all engine manufacturers. It’s worth a read.

While lots of engines have graphic engine monitors for minding the temps of each cylinder, understand that some older OEM systems might only have a single temperature probe. Our advice is the same as it ever was: If you’re investing in a new engine, an overhaul, or even new cylinders, consider installing an engine monitor with multiple temperature probes. Got a turbo? We think an engine monitor with TIT (turbine inlet temperature) is a must.

Ground-Running Precautions

For the majority of owners (except for homebuilders), the first ground run will be done by the engine installer, but this is a general guide for a first run to understand what you should and shouldn’t be doing (again, consult the service literature for the final procedure). Most techs will advise to keep ground running to a minimum. “In general, during the first ground runs you don’t want the head temperatures to exceed 400° F and oil temperature to exceed 200° F,” one mechanic told us. His advice also included to make sure all engine and cylinder baffling is properly installed and in good condition. Cowl the engine (particularly when it’s cold so the temps come up quickly), position the aircraft into the wind, and start the engine, making sure that the oil pressure rises to within the specified limits within 30 seconds. For typical Lycoming and Continentals, operate the engine at around 750 to 800 rpm for one minute, gradually increasing toward 1,000 rpm in three minutes. (Yes, Rotax models turn a lot faster.) Check the magneto circuit for grounding prior to a normal shutdown. Allow the engine to cool adequately and then make a visual inspection for any irregularities, including oil leaks, exhaust stains, and the security of wiring and hoses. Start the engine again and operate it at around 750 to 800 rpm, gradually increasing to 1,500 rpm over a period of four minutes.

If the engine is equipped with a controllable pitch propeller, cycle the propeller allowing only a 100-rpm drop. This is an important point for those with just a cylinder replacement as well. Cycling the prop deeply is both hard on the engine and in our view totally unnecessary. That said, do not skip the prop cycle or you may end up with poor prop control just when you need it the most at the first takeoff. It might be necessary to make adjustments to the idle mixture and rpm as required on carbureted engines and to the low unmetered fuel pressure, idle rpm, and mixture on fuel-injected engines. Run the engine up to full power for a period not to exceed 10 seconds, noting any discrepancies. Check the oil quantity. Re-cowl the engine in preparation for a test flight. It’s getting serious, now.

Flying It

Carefully choose your flight test area; it’s good practice to climb and stay within gliding range of the airport. Let the tower controller (if there is one) know what you’re doing. Ambient air and engine operation temperatures are of major concern during this test flight. Do a normal preflight run-up as prescribed in the aircraft flight manual or flight manual supplement if it’s an aftermarket engine upgrade/mod. Conduct a normal takeoff with full power and monitor the fuel flow, rpm, oil pressure, cylinder head temperatures, and oil temperatures. Reduce to climb power in accordance with the flight manual and maintain a shallow climb attitude to gain optimum airspeed and cooling. 

Use a rich mixture for all operations, except leaning it for field elevation where applicable, and lean to maintain smoothness during climb in accordance with airframe manufacturer’s operating instructions. Level flight cruise should be at 75% power with best power or richer mixture for the first hour of operation. The second hour power settings should alternate between 65% and 75% power with the appropriate best power mixture settings. Remember these power settings are critical, so if you live at a high-altitude airport and in the summer weather and you cannot maintain at least the power levels specified, then get the engine run-in at a test cell at a lower altitude or temperature.

Continental advises that if the engine fails to reach the rated, full-throttle rpm during ground operations, climb to a cruise altitude (more than 2,000 feet above field elevation) and verify the engine achieves full throttle, full rich rated rpm at cruise altitude, and operates within the limits established by the aircraft flight manual.

Engine controls or aircraft attitude should be adjusted as required to maintain engine instrumentation within specifics. The descent should be made at low cruise power settings, with careful monitoring of engine pressures and temperatures. Avoid long descents with cruise rpm and manifold pressure below 18 inches. If necessary, pull the prop back to decrease the rpm sufficiently to maintain manifold pressure. Any discrepancies detected during the test flight or any final adjustments necessary should now be made. The engine can be operated in normal service in accordance with the aircraft flight manual.

Hard Facts

Engine manufacturers go out of their way to offer useful guidance for engine break-in. Lycoming says that a new, rebuilt, or overhauled engine should receive the same initial start, warmup and preflight checks as any other engine. And as for the aircraft owner who would prefer to use low power settings for cruise during the break-in period, it really isn’t recommended. Lycoming reiterates that a good break-in requires that the piston rings expand sufficiently to seat with the cylinder walls. This seating of the ring with the cylinder wall will only occur when pressures inside the cylinder are great enough to cause expansion of the piston rings. Pressures in the cylinder only become great enough for a good break-in when power settings above 65% are used.

Full power for takeoff and climb during the break-in period is not harmful. It’s beneficial, though monitor the temps carefully to make sure overheating does not occur. We don’t know a seasoned mechanic who doesn’t agree that 65% (and preferably in the 70% to 75% of rated power) should be used to achieve a good engine break-in.

Last, Lycoming advises that if a preservative oil has been added, drain it at 25 hours of operation, while adhering to the manufacturer’s oil change intervals thereafter. Most mechanics advise to continue the break-in operation for 50 hours or until oil consumption stabilizes. Do it all right and with luck you’ll eliminate the possibility of cylinder wall glazing and other issues that could plague the engine for its service life.