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RC Engines Break-in and Tuning

RC Engines Break-in and Tuning

To get the most out of any RC airplane, you have to break in your engine, and pick the correct propeller for the plane’s intended use. For my fun fly Florio Flyer 60, I installed an O.S. 75AX for a little extra umphhh!  To fine tune my propeller selection I used an accurate digital tachometer to get the engine numbers…

Engine numbers:

  • Displacement: 0.75 cu in (12.3 cc)
  • Bore: 1.02 in (25.8 mm)
  • Stroke: 0.93 in (23.5 mm)
  • Practical rpm: 2,000-16,000
  • Output: 2.4 hp @ 15,000 rpm
  • Weight: 19.4 oz (550 g)
  • Glow Plug: O.S. #8
  • Fuel Used: Morgan Sidewinder Basher 20% Nitro / 16% Oil

For engine break ins I prefer to use a raised platform like a deck or a table to keep the engine and prop out of the dust and debris that can get kick up during extended engine runs. For the first I used the Evolution 14×6 prop to break in the engine and I ran three tanks through the engine running the engine slobbery rich for two minutes (4-stroking!) and then leaned it out to max rpm for 2 minutes. I then richen it up and repeated the cycle for the whole tank. I then let the engine cool off for 20 minutes.

Safety First!

I fired up the engine again and tweaked the needle again for max rpm and then backed off about a quarter turn. My Master Airscrew 14×6 prop earned about 9,200 rpm and idled a smooth and constant 2,900 rpm. The Evolution 14×6 was very close the Master Airscrew with a 9,100 top end and a 3,000 rpm idle.

I also tried a few different glow plugs which, for the best idle and overall good throttle transition turned out to be the O.S. #8 plug.  Next I switched to what I thought was my most promising propeller, the wood Xoar 14×4. Remember I am proping the engine for fun fly so I am looking for a good idle and transition and a powerful climb, not max rpm. Of course this will require a trip or two to the flying field to see what the ultimate winner is.

Finding the Sweet spot

I next tried the Xoar 14×4. And it spun up to 10,800 and had a very low idle of 2,100. The low end needed to be tweaked a little as the engine started to load up a little. I leaned it out about 1/16 turn. The engine died with at an idle of about 1,700.

So with this fun fly prop looking good I dropped the diameter to see what a sport flying prop could do. I went to the Xoar 13×6 and was pleased to see the engine rev up to 11,350 without any adjustments. Tweaking the needle a little got a solid 11,550 rpm with excellent throttle transition from a 2,400 idle. I think this is the prop I will be using for the first test flight.

Fuel consumption seems pretty good also, as it took only 3/4 of a 10 oz. tank to do all the prop tests. I used both the O.S. #8 and an Evolution glow plug but I could not see any obvious differences! My Glow Driver is the McDaniels Ni-Starter and I always have a back up one for the filed box.


At the Field

I’ve been using Morgan Cool Power glow fuel for years and since I wanted to see what would happen with a little more nitro, I switched to the Sidewinder Basher 20% Nitro, 16% oil blend. This is the fuel the RC truck guys use so I thought why not. Engine start up at the flying field was instantaneous and only took a couple of hand flips. After rechecking the range and control directions, I took off into the wind. Throttle response is smooth and reliable and the airplane was dialed in for straight and level at about 1/2 power.

ff15 (2)

Advancing the power to full, I pulled into a vertical climb and the FF 60 just kept on going! Unlimited is the best description. At a high altitude, I throttled back to idle and entered a flat spin. The model rotated and wound up tightly at first and then flattened out. After about 10 rotations, I exited the vertical down line and advanced the throttle. The model instantly responded and the engine sounded great and I had no over-heating issues at all.

ff17 (2)With several bursts of power, I did several snap rolls again with out any hesitation. On landing, the engine remained steady during the entire approach. After a touch and go, I hit the throttle and again the model rocketed skyward like it had Jato packs helping. Several low level loops but me back at the approach end of the field and I landed without even a bump. Overall, the extra power of the O.S. 75AX and using the Xoar 13×6 and the Xoar 14×4 make a big difference. Overall use the 13×6 but if you’ll be doing a lot of climbing, then the 14×4 will impress all your friends when you win the Climb and Spin event. If you want some serious fun fly performance, this is the plane and engine combo to try. You’ll love it.

FlorioFlyer60 (2)




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Add a Comment
  1. Ummm, sorry but I need to comment negatively on your post to hopefully prevent others from having problems. For one thing, you should NEVER run an ABC, ABN, ABL, etc. type, non-ringed engine blubbery rich, not even for break in. Doing so can easily damage them. Secondly, you should always break in this type engine with the exact same fuel you are going to fly it with, especially in regards to nitro percentage. Lastly, OS recommends this type engine should be run with a fuel containing castor oil such as Omega, instead of Cool Power. Your treatice would apply well to a ringed type engine, not, I am sorry to point out, engines of the OS 75AX type.

    1. thank you Mr. R. Horning for important data regarding engine break-in for O.S.
      engine(s) especially, the use of the ” old castor oil ” which has been, an important lubricant, since 2-cycle & 4-cycle engines were engineered, to power
      FF & R/C aircraft, since the dawn, of R/C flight and future engineering advancement. Geez ! I hope, I’m not too ! over-the-hump ( man’s eternal plague
      to advance, man’s imagination ) Well ! anyway, my reply, is advanced, as well
      as, need to investigate O.S. engine company’s info as, to engine break-in
      proceedures & what to expect from the results, of these proceedures. I have
      the inquiry, to ask Mr. R.Horning, if he has a downloadable text, as to what
      is needed, to break-in an O.S. engine or other R/C engines, so we ( R/C aviators
      can continue, to ” fly the skies ) _ I’ll be waiting, your reply !!

    2. Couldn’t agree more than with Mr Horning. Ringless engines absolutely require the treatment mentioned in his post – you can kill one through alloy delamination by 4 stroke blubbering rich! Break in is not the time to play with a variety of props or fuels. Choose a prop at the lower end of the recommended range so as not to load the engine whilst it is “wearing in”.

      Snap roll with a 75 size engine with a wing held on by rubber bands – Really!!!

  2. I have been in the rc hobby for 42 years.this engine break in is a myth!!!! install motor start tune it and just fly!!!! I still have motors that are 18 years old and still running!! I never had a problem with a brand motor..no break in is necessary.i don’t know why people think this!!! I have proof!!!!!!

    1. Your engines will run better if you break them in properly. You can get away with not doing so only because the materials and machining are quite good these days but even the best machining leaved some roughness and breaking them in properly will seat the various parts better than not doing so. Working them to hard right out of the box tends to tear the high spots off rather than wearing them down smooth. It does make a difference. But if you don’t care then it doesn’t matter.

  3. You didn’t say if the engine is ringed or not, ABS or not. That makes a big difference in proper break in. Never 4 stroke an ABS engine or really any non ringed engine during break in.
    You also didn’t go into any theory as to why you used any particular method of Break in. The article sounded more like an ad for the props and fuel than a serious article on breaking in engines. It also sounded like you were guessing which props to use.
    Of course what do I know. I just flew my first model airplane only 60 years ago.

  4. An oddity I’ve wondered about over the years. The engine is rated max horsepower at 15,000 rpm and practical to 16,000 rpm yet I’ve never seen anyone prop the engine to turn that kind of rpm. I have done a bit of experimentation over the years and have come to the conclusion that the manufacturers over hype both the horsepower and practical rpm ratings just like the car manufacturers did back in the 50s up to 1974 when they switched to Net horsepower. I have to wonder what props they are using to get those numbers.
    The most important feature of an engine/prop combo is how much actual thrust they generate.

  5. Hello friends,
    Michael second might have a point when he says that engine break-in is un necessary. Modern engines such as OS are manufactured to such tolerances break-in is un-necessary. Some-none ringed ABC applications starts to wear out from the beginning and by the time loose compression. Especially engines suck ac Nova Rossi for RC-cars.

    On the other hand it is good to get to know your engine at home with plenty of time, rather than when you are out on your airfield. Obviously testing and checking first at home to carefully hammer out any snags on your plane, is the key to success on the airfield.

    There are some glow engine fuel you should beware of such it that may cause over-heating and resulting in lost compression on certain engines. I have a OS 46LA which lost compression when I used a fuel with to much nitro (20%) and castor oil. I suspect detonations occurred, as I noticed a change in engine noise during full throttle operations. Detonations is a violent combustion phenomena causing sharp pressure rise and heat energy release.

    Four stroke engines such as SAITO may even be more susceptible to detonations, running on castor oil. It can sound like the ringing of an old analoge telephone.

    In my opinion fuel blend containing synthetic oil is more preferable with lets says 5% nitro compared to castor oil and high contents of nitro.

    Carl Stålberg
    Stockholm, Sweden

  6. Gerry Yarrish,
    I enjoyed reading your article.
    Regards Carl Stålberg

  7. Because a blubbery rich running two stroke engine does not come up to design operating temperature, with a ringless, ABC, ABN, and similar engines the piston will be “sticking” in the bore at TDC. Besides potentially damaging the piston and the cylinder bore, this also GREATLY increases connecting rod wear, and can actually cause the rod to break!

    You really can break in an ABC type engine in the air with no problems, just be sure that you never run it lean. Personally, I run mine on the bench for one tank full, doing 2 minute runs with a full cool down between runs. During each run, I set the mixture at a rich TWO STROKE setting, and lean the engine to peak for about 5 to 10 seconds at a time with 30 seconds between those leanouts. Then after that tank, I fill the tank half full, and fine tune the mixtures for flight. The engine is then ready to fly as normal, while avoiding extended high power/slow airspeed flight modes for the first few flights.

    And once again, with an ABC type engine, pick ONE nitro percentage, and stick with it from first flip through DOI. (Dead on Impact)


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