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Engine Piston Rings

Engine Break-in

Any aircraft will need some form of power to keep them flying, to postpone the inevitable return to mother Earth as it were. Since the first flights by the Wright Brothers numerous types and models of engines have been tested, developed and flown.

New engines, be that factory or overhauled new, need to be break-in for proper piston ring to cylinder wall seating. This makes sure that all new parts 'settle in' and begin to function properly. Some engine manufacturers or overhaul shops do break-in engines before shipping.

This process will help seat the rings, reduces oil consumption and ensure long engine life for years to come...

Basic principles

The moving parts in the engine, mainly pistons in the cylinder, are mated together to an almost perfect fit. The seal is not tight as you might think. During movement up and down the engine needs its oil to lubricate and therefore the piston/ ring/ cylinder combination must have little room to move.

cylinder honing crosshatch

New cylinder walls have a crosshatch honed pattern to facilitate this and during the initial hours of operation tiny pieces of metal are 'scraped' for the perfect fit and seal. The piston rings will then be able to keep the gases above the pistons without too much oil blowing by.
See the image to the right from performanceracingengine.com. More information about cylinder honing at the next link about: bore honing from Production Machining.

Modern engines use nick carbide cylinders (Rotax uses Nikasil coatings) and these can reduce the break-in time required for proper ring seating. But always use the manufacturers recommendation to the letter for proper result.


Some say that you should take it easy on the engine for the first couple of hours, this is a common myth. The best thing to do after normal warm-up is to perform high power flying (close to maximum continuous RPM) without exceeding cylinder head temperatures as specified for your engine. Keep a close eye on the CHTs for at least two hours, as these temperatures should drop indicating that break-in is about to complete.

Run-in Oils

For conventional aircraft engines (Lycoming Continental, etc.) the use of a nondetergent mineral lubricant that contains no synthetics or anti-wear additives is recommended. Do follow manufacturers advise for this. Rotax is an exception, they advise to use the regular oil for break-in, mainly because these engines use a newer and different cylinder/ piston metallurgy than the classical engines.

Rotax 912iS Engine

Rotax Engines

These are test run at the factory but need extra care the first few hours. Make sure that the engine reaches operating temperatures and keep 75 % power the first few hours to complete the break-in process. Change oil and filter after the first 25 hours are done and you are good to go. The operating and maintenance manuals are leading, visit flyrotax.com for the latest versions.

Best Practices

To be able to perform the break-in procedure and not breach any temperature or pressure limits it is important that your engine monitor instruments are indicating correct temperatures and pressures. Have these instruments checked or calibrated before starting this process.

When starting your engine after it has been installed on your aircraft, make sure to follow the pre-oil procedure set forth by the engine manufacturer. Some have forgotten this and needed an overhaul, expensive to say the least.

Test Flights

After you start the engine, look for timely oil pressure and make sure its up to operating temperature before the runup is performed. Do limit these runups and go flying. After the first flight (usually around 30 minutes) remove the cowling and inspect everything for leaks or anything unusual. Then go off for the second flight for an hour or so. Keep a sharp eye on oil P/T's and try to prevent long descents which may cool off the engine too much.

Engine break-in is usually complete when oil consumption stabilizes and CHT temperatures remain constant. This may take several hours. For Rotax engines expect it to be complete after the first 25 hours have been flown off and oil is changed, if possible: use mogas to prevent any lead build-up from AVGAS.


When the break-in process is completed you may expect the engine to perform by the book numbers and have minimum oil consumption with normal pressures and temperatures. Engine life is maximized with many safe and enjoyable flight hours ahead.

Written by EAI.

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