It has been months since I have visited this fine site and now I have a question. I hope this is the correct place to begin?

I bought a new OS .55AX engine and have followed the break-in procedures to the letter!

What my queston is now that I have run a gallon of 15% fuel with castor/synthetic fuel through the engine is when can I expect to get max power from this engine?

I am flying a Tower Hobbies .40 Kaos ARF with a 11/7 prop now that I have used the first gallon of fuel. I broke it in with the 12/8 that was one of the reccomended props for break-in.

I don't want to lean the engine out too soon so I am still running it on the rich side.

Any help or other tales of break-in of the fine engine would be most appreciated.

Philip in Austin,TX

Hi Iluv,

I would say that your engine should be pretty well broken in. May I also suggest that you not follow the break-in method listed on the manual. It is kind of a throw back to ringed engines. You will want to break in an ABC engine on a somewhat rich two stroke, not super rich four stroking. This has to do with allowing the sleeve at the top to quickly heat up and expand. I would suggest that you back off on the nitro content down to 10%. The castor/synthetic oil blend is ideal (the best of both worlds) as far as in protecting your engine against a lean run. IMHO, OS is suggesting too large of a prop for the 55AX - the 55AX put out alot of power (like a .60 size) with the foot print of a .46 but it is still not a .60 size engine. How heavy is the Kaos ARF? Are you going for speed or thrust (unfortunately, you can't have both, just a compromise). I have a 55AX on a Hangar 9 P51 Mustang PTS that weighs alittle over seven pounds. I like to run my engines a little rich to make them last longer and the egnine has about 14 tanks of fuel through it. I originally started with a APC 12-7 on the engine. I ran 6 tanks through the engine on the bench and six tanks in the air with the 12-7. I was hitting around 10,000 rmps (running rich). I made a decision that if I wanted speed, I would either drop down to a APC 11-7 (12,000 rmps) or go with more thrust (vertical performance) with a APC 12-6 at 11,000 to 11,200 rmps (still a little rich). I went with the 12-6 and have two flights and I must say that I love this engine. I used to have an Evolution .46NT in the Mustang and it was a poor performer. The plane flys faster with more authority with the 55AX at half throttle compared to the .46 full throttle. With the 11-7, your Kaos should haul butt (unless the thicker airfoil is holding it back). If I can help anymore, let me know.

Happy flying!

I disagree. Follow the break in procedure in the manual, which has you alternate between very rich and a little rich, like you did.

Yes, the engine will gain power over time, even though most may say it's 'broken in' after a few tanks - this has been my experience with 3 AX engines this summer. After a few gallons they all turned larger props at faster rpms.

I recently installed a 55 ax in my extra 300 and I broke it in by the book with one tank of fuel then tuned it and went flying. It may need retuning after a couple tanks of flight, but it won't hurt it to go flying after one tank of break in. I used a 13x7 and 12x7 and found that the plane has more speed with the 12x7. I think the 11x7 won't be enough prop for you, I would go up to a 12 incher at least. I run an 11x7 on my TT pro .46 and it screams. Experiment with 12x6 and 12x7 or even an 11x8. A guy at my field has a very fast venus with a 55ax and 11x8 prop.

so fax a 12x6 has been my favorite, next would be an 11x7, i am running a tuned pipe too

For a more quantitative answer, my 46AX was turning an 11x5 prop during break in at 11,500 rpm and got up to 12000. I tried a 12x5 and the rpm dropped back down to 11000. I switched back to the 11x5 for a few more gallons, and now the 12x5 spins up to 12,300rpm.

Hi everybody,

I would think that if you are running the engine super rich, it has an exaggerated cooling effect. Therefore, since the sleeve is tapered at the top and you are running the engine in a four-stroke fashion, then you might not be getting enough heating of the upper sleeve to prevent the top of the piston head from slamming against the tapered end of the sleeve and prematurely wearing it out. The engine will break in but at what possible cost, reduced engine life? But, I will freely admit that if the engine gets hot enough by following the method in the manual to allow proper expansion of the sleeve, then I withdrawl my statement about the break-in method. Though, I think if you google it, more people would say not to break in an ABC engine by running it super rich. I remember when I got my 55ax (the first I bought - have a .46ax and 75ax also) it was extremely difficult to turn. I had to heat the cylinder head to get it to turn easier. Why, because of the piston head hitting the tapered sleeve. I am one that believes that the OS AX series of engines are good enough to be able to slap them on a plane, run a tank through it and start flying. I do think that after a gallon of fuel, it should be broken in unless he is not stressing the engine enough - really running it rich. Does anybody here agree that the 55AX should be ripping the Kaos all over the sky? Also, he never stated what he wants exactly, more speed or more power (for vertical). Has anybody looked at what prop size an OS 75AX engine will turn. The 55AX manual is stating that it will turn the same prop that the 75AX will. Does anybody find that odd or a little hard to believe? At what rpms does an engine usually produce it most power? At what rmps is the 55ax going to turn a 13-7 at (it being stock of course - no pipes)? Will it swing it at 12000-13000 rmps? I can tell you from tach readings, it will not. Of course, your mileage may vary. But again, I am not saying that it is impossible after some use / lots of flight as I only have 15 tanks through the engine. If it does, then I, Iluvflying and anybody else who has bought this excellant engine are in for a real treat.

Happy Flying!

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Dark oil is normal at break in, it has bits of metal in it that comes from breaking in an engine. When I put my new V8 in my mustang, I changed the oil in it after only 1000 miles because it also has fine metal material in it from the rings seating in the cylinder. Its a normal thing on a new engine, but if its doing it after a long while then you may have a problem.

This is true but not quite for the reasons normally assumed. I'd never really believed that running a new ABC rich could cause any damage but with nothing to back up my suspicions I kept quiet about it. Until the day I managed to get a brand new ABC for a very good price (yes, ebay so I decided to run an experiment. I ran it stinking rich for 45 minutes then stripped it down. The rod was running in nicely but the piston looked like new. It wouldn't run in let alone be damaged which was fine because I wasn't trying to run it in, I just wanted to find out if there'd be any damage at all. I wrote up the results [link=http://www.rcuniverse.com/forum/m_2875125/mpage_1/key_/tm.htm]here[/link] in RCU with photos.

It took me some time to figure out why there was no damage to the piston until it occurred to me that all that extra fuel overcools the interior of the piston which naturally cools all the piston as well so it can't expand as much as the top of the liner does. This is similar to heating up the cylinder head to reduce the pinch, the head gets hot but the piston stays cool.

OS has great service - if you follow the manual and something bad happens, they will replace it. The instructions for breaking in say to run rich for a few seconds, then lean out for a few seconds, which allows for heat build up, expansion, and full range motion.

My manual for the smaller AX engines say to have the low needle one full turn open from flush with the housing, but apparently that has been changed. This is the excerpt from the online manual:

Close the throttle rotor gradually from the fully opened position until it is just fully closed. (Do not turn further.) Then, screw in the Mixture Control Screw until it stops. Now unscrew the Mixture Control Screw approx. 3/4 turn. This is the basic position.

This is the same as my 120AX, and indeed 3/4 turn open is correct (for 15% nitro fuel).

When in doubt you can also call Hobby Services. They are very knowledgeable and kind, and most of them even fly planes.

Hi Downunder,

That is a very interesting way of looking at it. I know that the piston head does not expand nearly as much as compared to the sleeve. I never really thought that the piston head might not expand either due to the cooling. If this is true, then there should be no worry of the piston head wearing the sleeve down prematurely. To me, break-in is metal rubbing metal, not metal hammering metal. If by peoples comments here that they have suffered no ill effects by following the manual's method, then both methods work well for different reasons and we all benefit with long lasting powerful engines. I agree with Brett65 about the exhaust appearance - completely normal in the beginning - especially with ringed engines. Thanks for the engaging conversation.

Happy flying!