Is Engine Break-in Always Needed?

Is Engine Break-in Always Needed?

Fact or Fiction: Modern engines produced to precision tolerances don’t require break-in; just set the needle valve a little rich, and fly.

In today’s plug ‘n’ play society, you may be tempted to heed this advice. After all, you can buy an ARF and have it ready to fly the next day. Is it really necessary to spend time breaking in the engine when you could be out flying?

When all else fails, we can look at the engine manufacturer’s owner’s manual. There does not appear to be a consensus among engine makers regarding the importance of engine break-in. Some manufacturers downplay the importance, stating that no break-in is required, while others stress its importance and give very detailed break-in instructions.

It is true that many engines produced today are manufactured using high-tech, computer numerically controlled (CNC) equipment. CNC equipment can produce parts to very close tolerances, but there will still be some variation (albeit very small), and the only way to achieve the optimum clearances between the cylinder and the piston is through engine operation. Metals expand with increasing temperatures; therefore, the engine must be at operating temperature for the piston to seat properly. A proper break-in will ensure that these clearances are established.

There is another reason to carefully break in your engine. During its manufacture, stresses are built into the individual parts. This applies to all engines, from small 2-stroke glow units to full-scale piston and turbine engines. Turbine engines, which are probably manufactured to the highest standards of all, are given a thorough break-in. The break-in cycle is carefully designed to relieve stress on the parts. The cycle consists of incrementally increasing the temperature, with cooldown periods between each increase. If this process is not controlled, the internal stresses could distort the parts and cause engine damage.

If you are really pressed for time or you live in an area where the engine noise will disturb the neighbors, there are still some things you should consider doing. If possible, mount the engine on a test stand so that you can become more familiar with it. Running it on a test stand also makes the initial mixture adjustments easier. For most engines, when the top-end rpm hold steady for a reasonable period, the idle is reliable and the transition is fair, the engine should be ready to fly. Continue the break-in during your initial flights by making certain that the needle valve is set on the rich side. Also, vary the throttle to thermally cycle the engine. Your engine will appreciate it!


Updated: December 16, 2011 — 11:49 AM


  1. Debra,

    Interesting article. I have a related question. I have a few older engines that I acquired in swaps etc. that are fine but have cases that are in bad shape. Can you recommend a really good cleaner to restore there condition? Thanks.


  2. Debra,

    Interesting article. I have a related question. I have a few engines that I acquired used in swaps etc. that run fine but have very dirty and discolored cases. Can you recommend a good engine cleaner to help restore there condition? Thanks.

    1. Demon-Clean is a great one ( that Dave Gierke highly recommended. Varsol is a varnish that’s also recommended, but harder to find. Hope this helps!

  3. I have used “Dawn Power Dissolver” which I got from a Ace Hardware store. It works very well on burnt on castor. I just got done cleaning my friend’s OS .20 fourstroke. He had flown it several times (Crash) and just put it away still mounted on its’ motor mount. Just drives me crazy when people do this. The engine turned out real nice. Looks almost new.

  4. Demon -clean does work great, I have used it to restore some pretty ugly engines to like new condition. Can highly recommend it.

  5. Very important for breaking -in a Engine is also the use of the propper fuel with appropriate oil. Here in europe it is generally known that you do not use a fuel containing syntetic oils. Best for breaking in a engine is the use of a fuel containing 22 to 30 % of castor oil and 2 to 12 % nitromethane. The nitromethane is not absolutely essential but it helps to 4-stroke engines to run smother specialy at nedle valve set to the rich side.

  6. Hello,
    I like this article. It reminds nitro engine users that there is a break-in process. And there is a break-in process for gassers too.
    Any time I purchase a nitro or gasser, I have to have an instruction manual to know about break-in proceedures for that motor. And, if it is not known, there is a break-in for brushless motors too!!
    I have selected a type of nitro fuel for my airplanes and I break my motors in with that fuel and fly with it. I also run-up brusless motors to seat the rotor shaft to the bearings.

  7. i used car alloy wheel cleaner this works well just make sure you wash it off

  8. You can tell the quality of the engine fabrication by the break in instructions. Breaking in a reciprocating engine is the polishing off of the machined suface roughness. If you think that a few minutes of super rich operation will polish piston thousands of times, if that is not enough then the machining is low quality. This is why OS only need a quick break in and others need more.

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