Short guide to breaking in a new piston aircraft engine

Mike Busch, author of “the big book on piston engines,” has a helpful article in the Jan/Feb 2019 issue of COPA Pilot, the Cirrus owners’ group’s magazine. Busch explains why engines need to be run hard for a few initial hours and then offers a concrete procedure:

Break in the engine by running it as close to maximum continuous power as possible without allowing any CHT to exceed 420F for Continental cylinders or 440F for Lycoming cylinders. Run it this hard for an hour or two until you see the CHTs come down noticeably , indicating that the lion’s share of the break-in is complete.

This requires running at nearly full throttle at a low altitude (the engine won’t generate more than about 75 percent power after climbing to an ordinary cruising altitude of 6,000 or 7,000′ due to the lower density of air molecules up there).

This is timely for me because we’ll be breaking in a new engine for the Cirrus SR20. After 14 years and roughly 2,000 flight hours it seems prudent to swap out the engine, despite the fact that it hasn’t given any signs of ill health (this is contrary to Busch’s recommendation to wait until the engine tells you it is sick and then maybe only replace one cylinder).

Maintenance shops say that they hate dealing with Continental and love Lycoming, which might be one reason why Cirrus has switched to the Lycoming IO-360 engine for the more recent SR20s. Our little cluster of T-hangars also contains at least one story of a Continental engine that failed after a few hundred hours due to manufacturing defects and terrible support from Continental (instead of sending out a new replacement engine for the nearly new aircraft, the best that Continental would do is take the old engine back, try to fix it, etc. That would have resulted in months of downtime at a minimum. So far the wisdom of the shops has been proven correct. We had some issues with our order and couldn’t even get a return phone call from Ernesto Rodriguez, the customer support manager at Continental. I kind of like the idea of the 200 hp 6-cylinder Continental engine in the SR20 as offering greater smoothness than a 4-cylinder engine (what Cirrus is buying now from Lycoming), but one notable feature is that the per-mile cost of engine reserve actually becomes higher for the older SR20 compared to the SR22 (they both use 6-cylinder Continental engines and the cost of overhaul or swap-with-reman or swap-with-new prices are almost identical between the 200 hp SR20 engine and the 310 hp SR22 engine, as you might expect given that the mechanical configuration is pretty similar other than dimensions). To minimize interactions with Continental and save about $12,000, perhaps the smarter thing to have done would have been to wait for the engine to get sick and then ship it to one of Busch’s favorite field overhaul shops (see previous post:

Why do engines need breaking in? Busch has some interesting figures showing the grooves in a new cylinder that are supposed to make the barrel “oil-wettable”. These have a spiky top that you want to grind down with hard usage to slightly flatter. Busch says that ordinary Philips 20W-50 “might be a better choice for break-in oil” than the traditional straight-weight Aeroshell W100.

13 thoughts on “Short guide to breaking in a new piston aircraft engine

  1. Why is it important to grind down these tips all at once in the beginning rather than letting them grind down on their own time through normal use? Isn’t this the reverse procedure from breaking in a car, where you drive it gently until the piston rings have conformed to the cylinder walls?

  2. Andrea: Busch says that a perfectly smooth newly machined surface is not “oil-wettable” because the oil just beads up. So it gets roughened with “220-grit stones” and that leaves sharp peaks that make the engine run hot due to friction. As noted above, the goal is to grind off the sharp peaks quickly.

    Consistent with what I’ve heard before, Busch says that “running a freshly honed cylinder at low power for any significant length of time can cause a condition known as ‘glazing’ in which a tough residue of carbonized oil builds up on the cylinder walls and stops the break-in process… Once the cylinder has become glazed, it’s no longer oil-wettable, and the only solution may be to remove and re-hone the cylinder and start the break-in process all over again.”

    Why don’t people have to do this with car engines? I don’t know! covers this a bit. Maybe the car engine factories actually do run the engine hard for a few minutes before delivery? Or, more likely, using more sophisticated equipment, they are able to machine the cylinder walls with exactly the right structure in the first place?

    • Sound like you’d get a lot of exhaust contamination into the oil after doing this… I guess oil changes are more frequent on airplanes?

  3. Engine break in seems very finnicky, I wonder why they don’t break them in at the factory, especially for airplane engines.

    I wonder how F1 handles it? F1 engines need to be pre-warmed with hot oil before they can be started. F1 teams also wouldn’t want to waste track time breaking an engine in.

    Can anyone reference good scientific articles? Especially in the SAE journal?

  4. I’m reading the Busch book via your recommendation on this blog, while I’m working on my PPL. The history is fascinating. As a technology minded person, is kind of incredible to me that 60 years on Cirrus looked at all the options in the market and decided that the IO-360 is the best choice for the SR20 G6. How is it possible that there has been so little innovation in general aviation aircraft engines brought to mainstream market over more than half a century? (or actually the last one and a half centuries if you go back to the Otto design)

    Of course the obvious answers are regulation/certification and liability, but not sure I see how even these could cause such a glacial pace of innovation given the amount of money at stake in buying and maintaining and fuelling such an engine across the GA fleet.

    • People say that the Porsche engine from the 1980s (see ) was a genuine breakthrough in terms of smoothness, noise, and ease of operation. The Wikipedia page notes that it was heavier and draggier (a word?), however.

      Without new materials, is there an obvious path to making a better engine? Plainly we could have much better controls for existing engines (but maybe they would just reduce pilot workload, not increase efficiency). But absent some kind of amazing ceramic that enables a high-compression diesel engine to be light, what would lead to a better reciprocating engine?

  5. I recently came across this site for the first time. I was searching fuel mpg of an airliner. I read lots of technical articles, possibly to an obsessive level…
    These blogs are interesting enough for you to possibly write books on light airplane engines and aircraft mechanism in general.
    Re. breaking-in engines. Since 1980, I must have rebuilt at least 300 Citroen Diesel engines. Even with those with star-ship mileages on the odometer, the original honing marks patterns can clearly be seen. I think that other than avoiding the dreaded bore-glazing (ask VW) you mentioned, traditional break-in is now an obsolete ritual.
    Thanks for another great website to keep me from restoring my own Citroen diesel : -)
    Who’d have known fifteen year ago that old people – like myself – would be reading stuff on a computer screen!!
    I’ve learnt more in five years than the last fifty. Thank you.

  6. @Paddy: it’s a trade off between performance and durability. 15,000rpm F1 engines are only engineered to last 3000km (15 hours?).

    Similarly, 9000rpm Porsche GT3 engines are raced immediately with no break-in, but require rebuilds every 25-50 hours (=$45k, +$30k for transmission rebuild too). This gives GT3 race cars operating costs/hour similar to a large business jet.

  7. But absent some kind of amazing ceramic that enables a high-compression diesel engine to be light, what would lead to a better reciprocating engine?

    Automotive engines have gotten much better in the last 50 years. Large 5 liter V-8s (after emission controls were introduced) made as little as 140 hp or less than 30 hp/liter. Nowadays, 100 hp/liter is not uncommon. You can either keep the same size engine and have a lot more power than in the past or else go with a smaller /lighter engine for the same amount of power. How did the do this? Electronic fuel injection (and more recently, direct injection), electronic ignition, overhead cam instead of overhead valve, turbo charging, etc.

  8. In auto engines, the modern goal is to achieve a “plateau finish” before the engine is even run. This finish has the microscopic valleys that are needed to retain oil, but the mountain peaks are pre-worn off so that less break-in is necessary. Instead of mountains and valleys like saw teeth you have plateaus (where the tips have been shaved off the mountains) and valleys.

    I don’t know if the plateau finish has not made its way to the aircraft sector.

    Regarding what Donald said, even in a high mileage engine, the crossed hatched valleys created by honing will still be visible but the peaks will have worn away – this would not be visible to the naked eye.

  9. The advice I’ve always gotten regarding a new or rebuilt automobile engine was to run it hard during the break-in period.

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