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Nov 04, 2017, 10:54 AM
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New piston ring. How many tanks to break-in?

Just rebuilt an OS FS 70 ultimate. Few new parts and new piston ring. So how many tanks (300cc) to break-in before I dial it in to its optimum?
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Nov 04, 2017, 11:52 AM
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Hard to say:
If the engine is a complete rebuild with multiple new mechanical parts (like rod, or piston, or such), treat it as a new engine. In other words, approx 1~2 hours of gradually increasing load with frequent cool down runs inbetween.

If it is ONLY the ring that has been replaced, 5 minutes of very short (10 sec) all out full load bursts (with peaked needle setting) and 10~20 sec rich idle periods should be enough to use the engine unlimited at a slightly richer setting than normal. One hour later it can be peaked as normal.

That is how I would do it, but only you know how extensive the re-build has been.

No experience, but I am told that the rings of after market suppliers like SwarfMonkey or Bjorn Baal seat considerably quicker than most original replacement rings.
How much quicker, I have no idea.
Nov 04, 2017, 12:59 PM
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SeismicCWave's Avatar
Originally Posted by DYORD
Just rebuilt an OS FS 70 ultimate. Few new parts and new piston ring. So how many tanks (300cc) to break-in before I dial it in to its optimum?
Most OS instruction manuals I have read said that their engines do not require break-in on the bench. Just fly the engine is what they recommend. However I like to test run the engine on the bench so that I can actually see the engine perform properly before putting it in an airplane.

Once in the airplane I fly it slightly rich for a tank or two. Ok I sometimes screw up and it was slobbering rich which was bad. Then I lean it out a bit and fly gently for the next few flights.

It is next to impossible to say when an engine reach an optimal state. My question back to you is that if an engine reach an optimal state will it start to deteriorate into a less than optimal state from that point on or would it hold the optimal state for a long time? I don't know and I believe you will not really get a proper answer on that. We are modelers and not scientists. As such we play and not work at documenting model engines.

Anyhow the reason why I say it is hard to decide when the engine is optimal is because sometimes it takes a while for the engine to really break-in. It all depends on the engine, the metal use on the cylinder, ring, liner, fuel use, oil content and percentage in fuel etc. etc.

That's why every on advices on tuning an engine needle by leaning it then richen a little. Running a slightly rich engine never really hurt an engine. Running an ABC engine slobbering rich is not good but that is another story.

So my suggestion is to run one tank on the ground to observe the engine behavior. Don't run it full bore for the whole duration of the tank. Start off with a rich setting on high idle. Tune the low speed needle for a decent transition and vary the engine speed during the first tank without staying on WOT for a long duration.

If the engine behaves properly then install the engine in an airplane and fly it gently with a slightly rich setting. Focus on varying the throttle setting.

As you fly more and more you can actually feel the engine breaking in yourself and then you can lean out for better performance slowly. Also stay a little on the rich side.
Nov 04, 2017, 02:50 PM
Hope to get out of life alive
kenh3497's Avatar
This technique has worked for me since 1973. It's nothing new. The article is a little embellished but the process is sound.

Nov 04, 2017, 02:50 PM
REAL engine men know.
1QwkSport2.5r's Avatar
Rings don't seat nearly as quickly at less than full throttle. I prefer to run the engine right at the 2/4 (or 4/8 for a 4-stroke) break so it is occasionally missing. This setting is rich enough to keep the ring from overheating and lean enough to actually build some heat which is what you want to do. I don't buy into any fancy frilly stuff - like idling for the first tank or shutting it down to cool between tanks. Anytime I've done that stuff, the rings seemed to take forever to break-in and they never seemed to have as good of a seal. The last few ringed engines I broke in, I ran them at that 2/4 break (2-stroke) for 4 tanks on the ground before peaking them out. If at that point they hold a full lean setting (500rpm rich from peak) for 20-30 seconds, they're good to go.

Weedeaters and leaf blowers aren't subjected to any frilly break-in and they're subjected to far harsher environments than our toy engines are... I do like running mine on the ground a bit in a controlled way to be assured the engine runs properly in a controlled environment free of any airframe related variables. This way if the engine acts up in the plane but didnt on he bench, I know the problem lies within the plane.
Nov 04, 2017, 03:35 PM
Registered User
Originally Posted by kenh3497
This technique has worked for me since 1973. It's nothing new. The article is a little embellished but the process is sound.

Yup... that is what I meant in poost#2, but that procedure is a bit hard to translate to model use.

Ringed engines I start, let it idle for a few minutes, then give it a couple of minutes of hard accelerations and decelerations then continue the break in as per manufacturer.

On the other hand: motorbikes are quite different animals compared to glow engines, and a model fourstroke is even depending on the ring not entirely sealing but allowing some oil to pass, for its bottom end lubrication.
Nov 04, 2017, 05:05 PM
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I TOBOR's Avatar
300 cc equals 10.1442 oz, that won't quite run it for 40 minutes but it would break in the ring easily, reduced speeds at first then longer spurts at higher speeds. The cylinder is already broken in, it will seat the rings easily.
Nov 04, 2017, 08:48 PM
Registered User
Got the engine as scrap. Few parts are missing.

Here are the new parts:

Piston ring
Head gasket
Push rod
Push rod cover

So basically, only the piston ring needs a little attention. I've finished 2 tanks on a very rich setting already. I'll try to peak it on its 3rd tank today.
Nov 05, 2017, 04:42 AM
Registered User

In that case, basically all you needed to do is Start it, idle it untill warmed through (1 or 2 minutes)
Peak at WOT and give it 10 seconds bursts of WOT,10 sec idle for a few minutes, back it off to slightly rich (like, a few RPM down extra) and go fly it.
After 2 or 3 tanks, you could adjust it as you normally would.

As you have allready run it rich for 2 tanks now, I would not waste more fuel on it and just go fly it like you normally would, and enjoy the engine.
Nov 05, 2017, 06:00 AM
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The PIPE's Avatar

I'm not EXACTLY certain, IF a model four-stroker can even DO that...

Originally Posted by 1QwkSport2.5r
Rings don't seat nearly as quickly at less than full throttle. I prefer to run the engine right at the 2/4 (or 4/8 for a 4-stroke) break [?!?] so it is occasionally missing.
Dear 1QwkSport2.5r:

The PIPE Here...since forever "leaving behind" any use of two-stroke "noisy mill" model engines in February of 1985 for my own aeromodeling needs, I've only operated four-strokers ever since then, and have been around other RC fliers' four-stroke engines a bit longer than that...

...and I've NEVER heard of a four-stroke model engine actually DOING that so-called "4/8 for a 4-stroke" style of running at any time - especially for noticeable periods of time, however short - in my RC flying past.

"Something" like that WAS what some full-scale, stationary four-stroke engines used for powering farm equipment, water pumps, woodworking tools and stationary AC generators (or should that be "alternators"?) did, several decades ago, for controlling such a four-stroke's engine speed, through a steam-engine style governor that affected the four-stroker's firing cycle...since that governor made the engine deliberately misfire to sometimes slow it down to maintain a more-or-less "steady" speed, that sort of stationary four-stroker became known as a "hit-and-miss" engine.

Our model four-strokers ARE intended (unless one's a CL flier) to be throttleable, and are not "usually" used for the same sorts of needs as the vintage "hit & miss" engines are. There's always the possibility of some 4-stroke "misfiring" when one's idling one of 'em, but only momentarily, as the poppet valves in most of our four-strokers do a really good job - if the four-stroker mill involved is in good repair - of NOT allowing such "4/8 running", as you've described it, especially with glow ignition.

To sum it up, what you were describing as such a "4/8 running" RC four-stroke mill's "operating mode" just isn't something I've ever really "heard" coming out of any model four-stroke mill's exhaust at an RC flying field or event at any time...there ARE those vintage "hit & miss"-ers demonstrated at some museums, and even still in use on some very remote farmsteads in the US & Canada, but our model 4-strokers just don't run like that, when one's got an engine in a decent state of repair, for noticeable periods of time.

Yours Sincerely,

The PIPE....!!
Nov 05, 2017, 06:34 AM
REAL engine men know.
1QwkSport2.5r's Avatar
PIPE - that "4/8 running" I describe is used only for breaking in and is a carburetor setting so rich that the engine barely runs over 4000-5000rpm at WOT. Here's a somewhat crappy example:
Enya R120-4C Break-in pt. 1 (1 min 13 sec)

You can run a 4-cycle engine so rich it doesn't fire every other revolution, but rather maybe every 4th revolution on and off.

I have since changed my process such that the engine is running just rich enough to occasionally miss at WOT. For the three 4-cycle ENYA engines I have, I have used Dave's method that is 4000rpm for 10min, 5000rpm for 10 min and so on. I changed how I accomplish this though - I run the throttle wide open and set the needle richer and use glow heat as needed. I only do this on 4-cycles that were new.
Last edited by 1QwkSport2.5r; Nov 05, 2017 at 07:04 AM.
Nov 05, 2017, 06:47 AM
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I TOBOR's Avatar
Does not apply to Saito's, page #5 of the Saito single's manual states, 4000 rpm with throttle closed to 1/4 to 1/3.
Nov 05, 2017, 07:03 AM
Registered User
For any fourstroke, it is better to control break in RPM by means of throttle, than by means of ultra rich mixture:

In a glow engine the mixture passes the crankcase anyway, so rich or lean, the moving parts see the same amount of fuel "pollution" of the oil. The oil refresh rate is also fairly high, because ALL oil fed to the engine passes the crankcase.

But a fourstroke when running properly, the fuel and oil separates in the combustion chamber (fuel burns, oil does not) and only a very small amount of that oil passes the ring and lubricates the bottom end.
If you run the engine extremely rich, the fuel that does not burn in the missed strokes, partly condenses in the oil film on the liner and the oil trickling down to the case gets polluted with fuel.
Fuel reduces lubricity of the oil.

It might not be big differences or huge amounts, but given that oil refresh rate in the crankcase of a model can be extremely low (it can be several minutes between oil passing the ring and getting expelled via the breather and I personally have observed refresh rates of 30 minutes in extreme cases) you want that oil to have the best lubricity it can have.

It is not likely to immediately harm an engine, but for me it is reason enough to just use the throttle, not the needle, when breaking in a fourstroke.
Nov 05, 2017, 07:17 AM
REAL engine men know.
1QwkSport2.5r's Avatar
I've got three fantastic running 4-stroke engines that for whatever reason run faster rpm than what other guys have typically obtained they've with the same engines and similar fuels. Static ring seal almost rivals a ringless ABC engine .. I have only had two 2-stroke engines that had a static ring seal that good.

I'm not saying either of us is wrong, just that I am not reading that far into it. I think some "theories" are more of just that - theory. I do not think these simple toy engines are THAT sensitive. My examples sure aren't. Maybe YS is more sensitive to break-in being such a high quality engine?

More of those Internet forum theories that I think has more fear mongering incorporated into it that just isn't necessary.
Last edited by 1QwkSport2.5r; Nov 05, 2017 at 07:47 AM.
Nov 05, 2017, 02:31 PM
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I TOBOR's Avatar
Maybe YS is more sensitive to break-in being such a high quality engine?

A YS will never reach that ABC like compression holding ability that a Saito reaches in about 30 to 40 minutes, honest.

Bert, you should try a little experiment that I do once in a while, You may get a surprise, attach a 4 oz tank to the vent of a fourstroke, no matter the brand. All in fun of course.

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