Why mineral oil for break-in?

Paul McBride, an expert on engines, retired after almost 40 years with Lycoming.

Q: I’ve seen this in Lycoming literature — “Keep in mind that all turbocharged Lycoming engines must be broken-in on ashless dispersant oil only” — but why is it true? My engine overhaul shop, Central Cylinder in Omaha, Neb., insists, in writing, that for its warranty to be valid, I must break my turbo’d IO-360 in on mineral oil.

PAUL MILLNER via e-mail

A: This requirement has caused some confusion for Paul and I bet he’s not alone. Lycoming Service Instruction 1014M recommends that “All turbocharged engines must be broken-in and operated with ashless dispersant oil only.” I’d say the direction is quite clear, but let’s look a little deeper into this important factory recommendation.

You mentioned your engine was an IO-360, so right away I’m thinking it’s a normally aspirated engine judging by the model designation beginning with “IO” because, if it were a Lycoming factory turbocharged model, the designation would have begun with a “TIO-360.” Since you mentioned the overhaul facility, I decided to check directly with them just to be certain I wasn’t overlooking something here. I’ve known George and his sons for a long time, so it was good to have a reason to touch base with them again. I spoke to son Dan and mentioned why I was calling and he immediately knew the engine I was referring to. He told me that the engine was equipped with an aftermarket turbo system, so now I understand why you said “my turbo’d IO-360.” He went on to tell me the company policy is that any engine overhauled by them must be broken-in on mineral base oil. He explained they’ve never experienced a problem with this and, as a matter of fact, it has worked very well for their overhauled engines over the many years they’ve been in business.

Now, who’s right and who’s wrong? The answer in this case is simple. Central Cylinder is the overhaul facility and signed the engine off and provides warranty on the engine, therefore it can dictate which oils it prefers to have the engine broken-in with. This would hold true for any field overhaul facility because they have first-hand experience with what has proved best for their engines following overhaul.

If no requirement such as this is mentioned by the overhaul facility, then I’d certainly follow the Lycoming SI 1014 recommendation. I’d also follow this SI if your aircraft is powered by a Lycoming factory built turbocharged engine.

But I’m sure you want more of an answer than “because they said so,” so let me cover the importance of proper engine break-in and what has proven to work over many years. This basic rule applies to most normally aspirated Lycoming engines and it is: Use a straight weight mineral base oil of the proper viscosity for engine break-in. What this allows is for the engine, when operated properly during the break-in period (don’t baby it), to permit the piston rings to seat with the cylinder wall. Keeping the power up during break-in allows the pressure in the cylinder to force the rings against the cylinder wall, which causes the seating process to take place. Failure to use full power for takeoff and climb could be detrimental to the seating process, however engine temperatures should be monitored closely in order to avoid overheating the engine. Preferably using 75% power during cruise and not less than 65% will ensure adequate cylinder pressures are produced to permit proper seating of the piston rings for good engine break-in.

I know you’re probably thinking that other oils may allow the same results and it may be so, but I’m from the old school and believe it’s hard to argue with success. Breaking engines in with mineral oil has been almost foolproof, providing the engine is operated properly during the break-in period. If the engine is not broken-in properly and the rings don’t seat, the typical result is an increase in oil consumption, which eventually will require the cylinders to be removed, rehoned, and new rings installed to start the process of break-in all over again after spending lots of money that could have been avoided.

One more thing comes to mind on this subject and that is the use of ashless dispersant oils for breaking in the O-320-H and the O/LO-360-E series Lycoming engines where either straight mineral oil or AD oil may be used. These engines have a higher compression ratio, which equates to higher cylinder pressures, which help force the rings out against the cylinder walls, therefore AD oils are not deterred from seating when operated properly. When it comes to Lycoming turbocharged engines, the increased heat and cylinder pressures resulting from turbocharging provide the best seating environment while using AD oils.

Send your questions to: AskPaul@GeneralAviationNews.com.


  1. Don Nelson says

    I just had my O540-B4B5 engine overhauled by Victor Avaition. The breakin procedures provided were Lycoming SI No. 1427B. The recommended procedure calls for the power to be set at 75% for the first hour and to alternate power settings between 65% and 75% for for the second hour. My question is how do I determine the percentage power setting on my PA28-235? My aircraft has a fixed pitch prop and no MAP guage. The owners manual does not define the percentage power settings although it does have charts utilizing them.

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