Doing some test with various diffrent props and mufflers recently I noticed throught the exhaust port of my G20 a very small vertical scratch at the bottom of the piston skirt. It is about half the dia of a modelling pin and about 1/4" long and possibly a thou or so deep and is visible when the piston is at BDC at the bottom left hand side of the picture, see the attached picture. Though the engine has several gallons of fuel through it the machining marks on both the piston and the cylinder are still quite visible. Should I be concerned about this or not.

Karol

It sounds like a small piece of dirt or maybe carbon got caught momentarily in the piston skirt between it and the cylinder wall. I would not be concerned as it is, but I would watch to see if it gets bigger or longer. Do you have much carbon in the muffler used just before the occurrance? Do you filter your fuel? Do you use an air filter if you fly where dust or dirt debris could be sucked in the carb intake? Good luck, I hope it does not present itself as a real problem.

Sincerely,

Richard

ditto.
Probably the lack of an air filter. These devices are not very popular with the RC crowd.

Agreed.

Unfiltered RC engines always have scratched piston skirts. "Professionally" run engines that use air filters don't exhibit the same problem. For some reason people seem to think if they don't see debris or dust blowing in the wind they have clean air. Those same people should wonder why they have nostrils and sinus passages semi blocked with particualte matter on those clear sky days.

The muffler used extensively on this engine had no accumulated carbon and the fuel is filtered several times via felt clinks before it reaches the carb, and even though I fly from grass fields I have never used an air filter on any of my gas engines. Maybe it's time to think about using one on this engine as it is on a Stick model which is uncowled. Hopefully someone will point me to a possible filter source or suggest a method of adding a filter.

Karol

Bru Line used to make air filters that fit via a rubber boot over the carb intake. As I remember they were available in fine or coarse mesh. Other possibilities might be air filters as fitted to RC cars, particularly off road dirt trackers.

Sincerely,

Richard

<hr />I would not be too concerned...unless the piston ring shows a mark in it... in line with thescratch on piston. The ring is of more concern...it does the sealing.

I have a friend at the club that has made some real neat air cleaner intake systems. He puts them on his Saitos also. I will take a couple of photos &amp; post them. Capt,n

OK... ... ... ... ... ... ... ...

The view through the exhaust port is as is after removing the muffler, as no cleaning was done and as you can see it's quite carbon free, due I think to the Red Line oil used. The engine has about 10 gals of fuel through it and at approx. 25-35 mins. running per 8 oz tank depending on throttle management that a fair amount of running.

Karol

Piston cleanliness is not everything. It just shows how well the detergent in the oil is doing it's job. It is the surface mating in the engine that counts. Not by wearing the engine, but by rubbing without wearing. That is the mark of a good oil. That is why good synthetics hardly show wear.
Jaso FC class oils are excellent. The FD class added some extra hours to the grueling wear tests, and also added extra detergency (right word?), to clean out the carbon deposits that are formed in the normal combustion process. In other words, keep those particles in suspension until flushed away.
I wonder: "flushed away by what?" . By replenished oil at a 1:100 ratio? I.E., Burn one liter of fuel in half an hour, containing 10cc of oil. Most of this oil passes right through the engine without doing useful work. The oil particles that make it to the metal parts are few, and hardly capable of flushing anything. So they are engineered to cling to the surface like it's the last thing they need to do. The flushing takes place when starting with the choke on, and the engine is wetted with gas.
These carbon particles are extremely small, because they are not allowed to accumulate and bake together. (oil's job). Their size is so small, that the oil film is thicker. They thus cannot harm the engine.
The story is different with dust particles that float in the air, or are stirred up by the prop wash. These particles are thicker than the oil film, and so they start abrading the contact surfaces that should be burnished. Clean engine nevertheless, but wearing faster than it should be.

Now to return to air filters. These sometimes just cannot be accomodated in the plane, even though they at least double the engine's life expectancy. They also increase the engine's performance over time. The unfiltered air engine never realy runs in. It wears, and by doing so prevents proper running in.
As long as the engine survives we see no problem. Crash damage and "moving on to newer pastures" never let us wear out an engine. So air filtering gets no priority. It does not change engine useful life for our purposes. It does however provide cleaner planes and better running engines, until they are put on permanent store.

Pe,

As always an excellent post, and thanks for enlightening me. Wow, my post # 3600, boy oh boy they really do add up.

Karol

time to party?
The counter now is at *02!

Pe Reivers
I enjoyed reading your post on the oil,s job in a engine. I started doing what I read on a 2 cycle oil can about 25 years ago. I read it was good to at the end of day, to rev up the engine and flood it out. I first did this with a chainsaw. I was skeptical about fouling plugs at first. It did not fowl plugs, but I noticed the engine started far better the next time. sometimes with no choke needed. Then I tried it with my 338cc snowmobile that had a Yamaha Twin. I would have the sled on a stand where I could rev up the engine and clear all snow off the track. I then would place fullthrottle &amp; fullchoke and flood out the engine. The next day it would also start with no choke needed. It was then I noticed that on start-up I could see a lot off loose carbon blow out of the exhaust onto the white snow. Well I never fowled any plugs in that engine either. In fact it was the fastest stock sled exactly like it in the area.

Now 25 years later I sold the saw real cheap to my son-in -law. He runs it just like I did flooding it out. This saw never needed a carb kit or any major work at all. Maybe 2-3 spark plugs &amp; a pickup hose in the fuel tank. You may have seen, the by now famous pile of wood my son-in-law cut with this same saw in 3 part-time days.

I believe the flooding out soaks any hard carbon forming...and on start-up the loose carbon leaves the engine.I never took head off to de-carbonize it. What is your response to this? Thanks Capt,n

Don't know what Pe is going to say, but more oil can't hurt a sitting engine for many reasons.

I am not sure that the flushing removes carbon. It stands however to reason that the flushing at the end of the hard work prevents carbon building.
Let's look at it a bit closer.
A while ago I did a heat evaporation test of all oils I had accumulated over the years. I used an electric iron for that. First I coated the iron with black ink, and turned the heat on max. With an IR gun I traced the hottest part at 220&deg;C. +- 2&deg; over 15 minutes. In the hot spot I Dremeled a shallow cup that could hold about two drops of oil. Diameter was about 8mm. (I must have pictures somewhere)
With iron cold and level I applied one drop of the oil in the cup, and then turn on the heat. At about 80&deg;C many general use 20-weight two stroke oil flash off, visible by a smoke plume. These are the hydrocarbons mixed in the oil to make it thin, and improve the oil/gas mixing. 30-weight two stroke oils also flashed, but at slightly higher temperatures. The thicker 50 weightall out racing oils did not flash off until well over 140&deg;!
During the heating up, one could observe how the oil crept over the edge of the cup. The amount of creep is a measure of the surface tension, and the ability of the oil to wet a surface.
Now to the interesting part.
Some oils flashed off completely before reaching 200&deg;c!! Gone! Nice and clean with hardly a trace but some ash. (metal salts)
The other oils thickened slowly, until completely reduced to a film of varnish (brown or black). The rate of evaporation and thickening again depending on oil quality. Castor and Elf racing showed the longest retention of tackiness, while Castor needed longest to turn all black.
When tacky at 220&deg;, and allowed to cool down,the oil was quite firm at low temperatures. Firm enough to hold a ring in place (ring sticking) in a cold engine. This could cause blowby, until the engine warms up. Blowby is detrimalto any engine. It scours the surface clean of oil.

Now imagine the choke stop method. Some care has to be taken on reed engines, not to do it from high rpm.
The still running and warm engine is flushed with a lot of fuel, containing some fresh oil. (some, not much). This thinned fresh oil is carried between all sliding surfaces by capillary action, and mixes with, or coatsthe thick goo that was formed in the hot engine, effectively preventing the rings to stick.
In the next cold start there is no thick goo between parts that prevents the flushing by the fresh engine choke, and the rings are free to move. The fuel/oil mix is free to enter all crevices and do it's job from the first engine revolution onward.
The same is more or less accomplished by the full choke start method. Choke on, ignition on, flip until burp. Choke off, and start.
The only drawback here is that the engine now is cold, and the old oil goo is much harder, so it will not as easily mix with the new charges. So the thickening process goes on, until carbon is formed.

This is my opinion and reasoning behind it.

Can't argue with that reasoning and experimentation Pe. Do you think this process would also take place at lower temps, say 100*C??
I have been killing all my 2 cycle engines with the choke at idle for years, after a cool down period.
On my dads ranch we never shut down any of the large diesels without a good cool down period.

Pe ,The chainsaw I had for 20+years, was a reed valve engine. It was a Sears Pro-series and had 4.2 cubic inches...ran a 24 inch bar with ease.I alywaysused hi-grade chainsaw oil in the mix. I always reved it up &amp; choked itso it would suck inas much fuel &amp; oil as possible! Well I got 2 more engines to work on....pressure washers...not as fun. Capt,n

Not all reeds are the same.  Steel reeds are very sturdy. Some carbon reeds are quite fragile.  Hence my warning.

Air cooled engines that are asked to do some work never run at such low temperatures, unless extremely rich. Under these conditions they will fail on you because of internal fouling due to the very dirty combustion process.

Maybe you misunderstood what I was saying. When I shut down most of my engines after a flight (or weeding) I usually let them idle and cool down. They usually are at around 240*F/ 120*C (or less) and heat soak very little, at that temp does this same process you explained earlier take place if I were to choke kill the engine at those temps. I would not think the same process (gumming up) would take place at those low temps.....

NOT A REPLY
Just try the method on a weed wip...leaf blower...chain saw or any 2 cycle engine you have. You will see it will start &amp; run better. Open your mind. That extra oil &amp; mix will keep the seals pliabe...instead of drying out. It will coat your bearings good with oil mix...prevents rust. It will keep rings loose in ring grove. Or go ahead and shut your engine down &amp; heat will keep baking everything in to stay! Your choise!........