How you operate an overhauled engine, or an engine that has had one or more cylinders replaced, will have a large influence on how quickly and how completely the piston rings will be “broken in”. The first 10 hours of operation of an engine that has had new piston rings fitted are the most important to ensure the correct “break-in” of the piston rings.

Typically most engine overhaul shops will have run and tested your engine for 1 to 2 hours in a dedicated test facility to verify that the engine is operating satisfactorily before the engine is installed in the airframe.

The main objectives of the test run are:-

  • To verify that the engine is producing its rated horsepower.

  • To identify and correct any oil, fuel or air leaks.

  • To adjust the initial oil pressure, idle speed and idle mixture settings.

  • To verify that the fuel system is correctly calibrated.

  • To verify the ignition systems is operating correctly.

  • To provide a safe operating environment where the operation of the engine can be monitored, and if necessary, the engine shut down.

  • And, lastly, to provide the initial “break-in” of the piston rings.

This initial test run starts to break-in the piston rings but it may well take another 50 hours of engine operation to complete the break-in process.

As it is necessary for some wear to occur between the piston rings and the cylinder bores, the type of engine oil used for the first 10 hours of engine operation is very important. In general, oils that contain friction modifiers or anti-wear additives should not be used while the rings are being broken in.

Most aircraft engine manufacturers require that a straight type mineral based oil is used for the first 25 – 50 hours of operation.

The common straight mineral based oils available in Australia are:- AeroShell 100, or Phillips 66 Type M.

By “break-in” of the piston rings, we are referring to the wear that occurs to both the face of the piston ring and the wall of the cylinder bore to create an effective seal of the piston rings in the cylinder bore.

When the piston rings have been correctly broken in, they provide a seal that does not allow the combustion gases to escape into the crankcase section of the engine, or allow oil to enter the combustion chamber.

Combustion gases escaping past the piston rings is commonly known as “blow-by”. Blow-by can cause:-

  • Higher oil temperatures.

  • High oil consumption.

  • Early oxidation and break down of the engine oil lubricating properties.

  • High crankcase pressures which may in turn force oil out of the engine breather.

Understanding what happens during the engine break-in process allows us to comprehend how the engine needs to be operated to ensure reliable break-in of the piston rings.

The following information applies directly to steel cylinder bores, but the same principles with some qualifications also apply to other types of cylinder bores such as channel chrome or Cerminil.

Most compression rings used in aircraft piston engines are a semi-wedge design, with a tapered portion on the ring face that wears against the cylinder wall. Combustion pressures act on the semi-wedge profile of the ring, forcing the tapered face of the ring against the cylinder wall.

A seal is formed between the ring and the cylinder wall when these parts have worn to conform to each other’s shape.

For the required amount of wear to occur between the piston rings and the cylinder wall, the piston rings must be expanded against the cylinder wall with sufficient force.

It is the combustion pressures created during the power stroke, that force the piston rings against the cylinder wall. Generally the combustion pressures in the cylinder only become great enough for reliable ring break-in when power settings above 65% are used.

When a cylinder is made or overhauled, the cylinder wall is honed with abrasive stones. The honing process, roughens up the cylinder wall and produces a series of minute peaks and valleys in the surface. It is these peaks and valleys that are commonly referred to as the “cross-hatch” pattern on the cylinder wall.

During the break-in process, these peaks are worn off the cylinder walls by the piston rings.

One particular problem we want to avoid while breaking in the piston rings is a condition known as “glazing”.

When a cylinder is said to be glazed, it means that oxidised oil has been deposited as a varnish layer in the valleys all the way up to the peaks of the hone pattern.

This varnish layer of oxidised oil causes two problems.

Firstly, it stops any further wear and break-in of the piston rings. As a result the rings may not conform completely to the cylinder wall leading to blow-by.

Secondly, the varnish layer is too smooth and can cause the piston rings to hydroplane over oil that is on the cylinder wall. That is, excessive amounts of oil build up in front and under of the ring face. The resulting hydraulic pressure of the oil on the ring face is enough for the ring face to lift off the cylinder wall. This allows oil to pass under the ring and into the combustion chamber resulting in excessive oil consumption and fouled spark plugs. 

There are a number of views as to how glazing can occur. The most common view is that when the engine is operated at low power settings, the rings are not pushed hard enough against the cylinder wall, leaving a very thin film of oil between the ring face and the cylinder wall. This oil film is of sufficient thickness that it stops any wear occurring between the face of the piston ring and the cylinder bore. This results in the surfaces of the piston ring and the cylinder bore not fully conforming to each other. In addition to the necessary wear of the surfaces not occurring, the oil film is also oxidised by the high temperatures in the combustion chamber creating the varnish layer and the glaze effect.

Another contributing factor to glazing is also thought to be excessive heat. During operation at very high power settings, if the heat generated from the frictional contact between the piston rings and cylinder wall is allowed to build up, then the high cylinder wall temperatures can oxidise oil that has been squeezed into the valleys of the hone pattern. This oxidised oil builds up as a layer of varnish on the cylinder wall and causes the cylinder bores to become glazed. Keeping the engine cool during high power operation stops glazing occurring by this process.

Once a cylinder becomes glazed the only effective way to correct the problem is to remove the cylinder, re-hone the cylinder bore to remove the glaze, replace the piston rings and start the break-in procedure again.

In summary, to promote the reliable break-in:-

  • Follow the manufacturer’s recommendations regarding the type and grade oil to be used for break-in.

  • Perform the start, warm-up and pre-flight checks as you would for any other engine, but avoid any prolonged operation at low power settings.

  • During break-in try to keep cylinder heads cool and oil temperatures in the normal operating range.

  • Use full power for take off and climb, but carefully monitor engine temperatures. Use cowl flaps and generous mixture settings to keep the engine cool.

  • Step climb the aircraft and use cowl flaps in cruise if necessary to keep the engine cool.

  • In cruise maintain power settings of between 65% and 75%.

  • For normally aspirated engines it will be necessary to cruise at lower altitudes to obtain the required cruise power settings. Density altitudes should be kept below 8000 ft.

  • Avoid long descents with low manifold pressures.

  • Monitor your oil consumption.

  • Change the oil and oil filter in accordance with the manufacturer’s requirements.

For further information please refer to the latest editions of :

– For LYCOMING ENGINES Service Instruction 1014 and SB480

For CONTINENTAL ENGINES Service Bulletin SIL99-2