In the past few months, my friend Paul McBride has written several very interesting and informative articles on “breaking in” an aircraft engine, including “How to break-in your engine”. I recommend that you read them when or if you have a new or rebuilt engine to install on your aircraft. There is a lot of excellent information in the articles and I would like to just add a few points to them.
At this year’s Oshkosh, I was thrilled to find some old friends who actually knew what they were talking about. One of them was Jim Tubbs of ECi engines. He showed me a set of pistons that were completely “coked up” due to overheating. I asked how many hours, and he said less than 100 horrs. What happened was that the engine had been rebuilt, and then ground run for about an hour as the mechanic did all of the adjustments and run it in.
The first point is never run an aircraft engine on the ground for more than about three minutes. If you need more time, shut the engine off and let it cool down until you can rest your hand on the cylinder, and then you can run it for another three minutes.
The problem here is twofold. A fresh engine runs much hotter before it is broken in due to poor ring to cylinder contact.
Also, an engine without the cowl will not force air to all of the areas that need air cooling. And, even with the cowl on, the prop moves a lot of air, but not near the hub. Aircraft depend on the ram effect of air being forced into the cowl by forward movement. This ram air builds up pressure above the engine, which forces air down around all of the engine parts to cool them. No forward movement means inadequate cooling.
The second point is what oil to use. As Paul points out, always follow the rebuilder’s recommendations. If they do not specify an oil, I recommend you break in using a straight mineral oil meeting the SAE 1966 and Mil L 6082 specification.
The third point is oil change interval during the break-in process. If you have a factory reman engine or one done by a shop that does a dyno run, the first oil change has been done. If your engine has not been run in, I suggest you do your ground checks and then do a couple of short flights. I usually like to do them around the airport so that I can be sure that everything is right before doing any long flights. After a few hours, if everything seems to be working well with no leaks, change the oil. Again fill with mineral oil, or the oil recommended by the builder, and change the filter.
You can now can start using your aircraft as you normally would, and I agree with Paul that you should not baby it.
I recommend that you fly about 20 hours or so on this oil change. Refill with the same spec oil and change the filter. Now fly about 25 or so hours on this oil.
When you change the oil, cut the oil filter apart. If you do not find metal particles of any significant amount or size, say larger than a pencil lead, your oil consumption is down to normal levels, and your oil and engine’s temperatures are normal, you can consider your engine broken in. You can then put in your normal oil, but I recommend that you continue to monitor your engine performance and cut the oil filters apart at each oil change.
I know this sounds like a lot of extra work, especially compared to what is required for a new car, but extra care and work now will definitely have a payout over the life of your aircraft engine.
Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at Visser@GeneralAviationNews.com.