Whenever an engine's piston rings are replaced whether in part or in entirety it is necessary to break in the engine. Piston rings are replaced at a complete engine overhaul or repair, top overhaul or single cylinder overhaul or repair.
When we refer to engine or cylinder break in, we are talking about the physical mating of the engine's piston rings to it's corresponding cylinder wall. That is, we want to physically wear the new piston rings into the cylinder wall until a compatible seal between the two is achieved.
Proper engine break in will produce an engine that achieves maximum power output with the least amount of oil consumption due to the fact that the piston rings have seated properly to the cylinder wall. When the piston rings are broken in or seated, they do not allow combustion gases to escape the combustion chamber past the piston rings into the crankcase section of the engine. This lack of "blow-by" keeps your engine running cleaner and cooler by preventing hot combustion gases and by-products from entering the crankcase section of the engine. Excessive "blow-by" will cause the crankcase section of the engine to become pressurized and contaminated with combustion gases, which in turn will force normal oil vapors out of the engine's breather, causing the engine to consume excessive amounts of oil. In addition to sealing combustion gases in the combustion chamber, piston rings must also manage the amount of oil present on the cylinder walls for lubrication. If the rings do not seat properly, they cannot perform this function and will allow excessive amounts of oil to accumulate on the cylinder wall surfaces. This oil is burned each and every time the cylinder fires. The burning of this oil, coupled with "blow-by" induced engine breathing, are reasons that an engine that hasn't been broken in will consume more than its share of oil.
When a cylinder is overhauled or repaired the surface of it's walls are honed with abrasive stones to produce a rough surface that will help wear the piston rings in. This roughing up of the surface is known as "cross-hatching". A cylinder wall that has been properly "cross hatched" has a series of minute peaks and valleys cut into its surface. The face or portion of the piston ring that interfaces with the cross hatched cylinder wall is tapered to allow only a small portion of the ring to contact the honed cylinder wall. When the engine is operated, the tapered portion of the face of the piston ring rubs against the coarse surface of the cylinder wall causing wear on both objects. At the point where the top of the peaks produced by the honing operation become smooth and the tapered portion of the piston ring wears flat break in has occurred.
When the engine is operating, a force known as Break Mean Effective Pressure or B.M.E.P is generated within the combustion chamber. B.M.E.P. is the resultant force produced from the controlled burning of the fuel air mixture that the engine runs on. The higher the power setting the engine is running at, the higher the B.M.E.P. is and conversely as the power setting is lowered the B.M.E.P. becomes less.
B.M.E.P is an important part of the break in process. When the engine is running, B.M.E.P. is present in the cylinder behind the piston rings and it's force pushes the piston ring outward against the coarse honed cylinder wall. The higher the B.M.E.P, the harder the piston ring is pushed against the wall. The surface temperature at the piston ring face and cylinder wall interface will be greater with high B.M.E.P. than with low B.M.E.P. This is because we are pushing the ring harder against the rough cylinder wall surface causing high amounts of friction and thus heat. The primary deterrent of break in is this heat. Allowing to much heat to build up at the ring to cylinder wall interface will cause the lubricating oil that is present to break down and glaze the cylinder wall surface. This glaze will prevent any further seating of the piston rings. If glazing is allowed to happen break in will never occur. We must achieve a happy medium where we are pushing on the ring hard enough to wear it in but not hard enough to generate enough heat to cause glazing. If glazing should occur, the only remedy is to remove the effected cylinder, re-hone it and replace the piston rings and start the whole process over again.
Understanding what happens in the engine during break in allows us to comprehend the ideas behind how we should operate the engine after piston rings have been changed. The normal prescribed flight procedure after ring replacement is to keep ground running to a minimum, take off at full power and reduce to climb power at the first available safe altitude, all while keeping the climb angle flat and the climb airspeed higher to promote the best cooling possible. At cruise altitude we should use 65% to 75% power and run the engine richer then normal. At all times we are to remember that heat is the greatest enemy of engine break in, we should try to maintain all engine temperatures in the green, well away from the top of the green arc or red line. This means step climbing the aircraft if necessary, operating with the cowl flaps open or in trail position during cruise flight and being generous with the fuel allocation for the engine. We should not run the engine above 75% power in cruise flight because the B.M.E.P is too great and the likelihood of glazing increases. As you can see, keeping the engine as cool as is practical and at a conducive power setting is the best combination for successful engine break in.
After an engine is overhauled or has a major repair it is run in a test cell to ensure operating characteristics and to begin the break in process. However this process may take as long as 100 hours of operation to complete. You, the pilot, are in control of engine break in for 98 % of the time that it takes to occur. This is a serious responsibility when you consider the expense and aggravation of having to remove, re-hone and re-ring cylinders that have glazed and not broken in.
Hopefully, understanding what engine break in is, as well as what is happening in the engine while the rings are seating and how our flight procedures effect the break in process, will help us to achieve the quickest and most efficient break in after piston rings are replaced.