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Tapered-Bore Engine Break-in - Upgraded

Old 05-27-2004, 10:18 AM
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DarZeelon
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Default Tapered-Bore Engine Break-in - Upgraded

This issue is revisited time and time again by many modelers. Every week, or two, another modeler opens a new thread, asking questions about this subject. I have referred many of them to the original thread.
Since the original thread was started, a little over one year ago, it was visited by nearly 6,000 RCU members.
Many have added some useful input and I have also reached some conclusions.

I thought it would be a good time to enter a new thread, based upon the old one, with some omissions and necessary additions, to further extend the useful life, power and reliability of the now very common tapered-bore engine.



Most engines with a ring-less high silicon aluminum piston, running in a plated cylinder sleeve, have a conically tapered bore.

This means the bore diameter is smaller at the top of the sleeve than it is at the bottom.

The purpose of this smaller internal diameter upper cylinder, is to cater for the differential heating (and expansion) of the cylinder, when the engine attains its normal operating temperatures.

The upper part of the sleeve, near the head and the combustion chamber, is much hotter than the lower part of the sleeve. Therefore it expands more and when at operating temperature; the part of the sleeve in which the piston reciprocates, becomes nearly cylindrical.

The piston in these engines is made of a low expansion, high silicon content, aluminium alloy (hyper-eutectic?), so it expands no more than the brass (or aluminium in AAC), base sleeve material, when heated.

All these engine's pistons have an interference fit at the top of the bore, so the engine "squeaks" or even seizes when turned over when cold. Even more so when it is new.

Because of this fit, these engines need special break-in procedures. Harry Higley, David Gierke, Clarence Lee, Dub Jett ([link=http://www.jettengineering.com]Jett Engineering web site[/link], read about break-in) and the late George Aldrich wrote about these principles and they work.

If such an engine is broken-in slobberingly rich and four-cycling, as you would normally do with with a ringed engine, or with a lapped meehanite (cast-iron) piston engine, it could be run-out, worn or broken, instead of being run-in.

If the sleeve is allowed to remain tapered, during the break-in, the interference fit at top stroke would get the piston and the plating scuffed, with ABN engines being more susceptible to plating damage.
Lower costs of waste product disposal are the only reason some engine manufacturers prefer nickel plating to hard chrome.

Also, in every non-firing revolution (every other revolution when four-cycling) the piston is pushed, into the seize at the top of the bore and then pulled back down, out of the seize, by the con-rod. These repeated compression-tension cycles can ultimately cause the con-rod to fail, due to metal fatigue (this was once common in some older K&B ABC QM engines).

Even if the con-rod remains in one piece, the loads imposed upon its bushings/ends by piston shoved in and out of the cold, tight sleeve top, could potentially hog out the con-rod ends (bushings). Much more so than would running a racing engine at a rich two-cycle, with normal fuel, from the start.

Even if no immediate failure occurs (and in most sport engines it will not), the piston's crown will be worn down, as much as it would by many hours of normal use.


Dub Jett writes that the actual target of the break-in is the bottom end.
Since ball-bearings don't need anything of the sort (as well as not requiring any more than 2% oil in the fuel), the targets are obviously the sliding bearings; the con-rod bottom end, the con-rod top end, the crank-pin, the wrist-pin and the wrist-pin bosses in the piston.

If the engine is a bushing bearing engine, like the Thunder Tiger GP, or the OS LA engines, the crankshaft main bearing (bushing) needs the same treatment that the con-rod ends are getting.


To protect these bottom-end parts during the break-in, without resorting to an extra rich mixture setting, that will potentially result in the problems described above, a special fuel formulation should be used for break-in.
Use fuel with degummed Castor oil (unless it is a YS two-stroke; then add synthetic) added to it, to a balance of 25% total oil.

This fuel will only be used for test-stand break-in, so you shouldn't care that the engine may not idle well and may provide less power. All engines will start, run and could even be peaked on such a fuel. You will only need about a quart of it, so the cost of preparing it will not be precariously high.


Using fuel with 25% oil will make any blubbering rich running unnecessary and will prevent the damage caused by any very rich and cold running period, without risking any bottom end damage.
Strangely, that short, blubbery rich minute, can cause the exact damage to the con-rod, that it is meant to prevent.

The high oil content prevents the need to choose between two evils, since you have both the two-cycle rich, operating temperature running, as needed for the piston and sleeve and the copious amounts of oil, needed to protect the bottom end.


Using the break-in formulated fuel, tapered-bore engines must be broken-in at a somewhat rich two-stroke mixture setting, beyond the point it starts to two-cycle, but not too lean.
It must be run with the throttle wide open and idling will achieve nothing during the break-in.

You may begin with short initial runs, but this is not really necessary.

Many sport type engines are virtually ready-to-use, just start and fly.
But I prefer treating my engines, as well as any engine I am entrusted with its break-in, as if it is an expensive, racing engine

Dub Jett recommends using a prop with the same pitch as the flight prop, but with one inch less diameter.
This, to lessen the load on the engine without compromising the cooling.


Run the engine initially for a total of 15 minutes, in a somewhat rich two-stroke mode.

After that the engine should be leaned gradually for short periods and then richened back (still two-cycle) to cool.
After about 5 minutes doing that, try to lean the engine carefully to maximum RPM. If the engine can hold maximum RPM without sagging, for 60 seconds straight, you have completed the break-in.
If it sags richen it immediately, to prevent damage, repeat the first part of this paragraph for 5 more minutes and then try again.

Repeat until the engine can hold peak RPM for 60 seconds.

The engine must not be allowed to run for any length of time during the break-in, at a four-cycle rich setting.
After the break-in is completed, there is no problem with four-cycling because the interference is minimized and is no longer a wear issue, even if the engine is run lukewarm.


After the break-in is completed, the TDC pinch should be retained. Not as tightly as when new, but it should be perceptible when the engine is turned over without a glow plug.

I have had ample experience with breaking-in ABC engines made by Rossi, MVVS, OS FSRs and various Chinese OS clones, as well as ABN engines made by OS, Thunder Tiger and Webra.


The only non-success was with my own OS.40FP, before I had adopted this break-in technique.
It peeled the nickel off about 25% of the bore.
It still ran for years afterward, though, until it broke in a bad crash.
Old 05-27-2004, 10:54 AM
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JPMacG
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Default RE: Tapered-Bore Engine Break-in - Upgraded

Thank You DarZeelon! I will follow your advice.
Old 05-27-2004, 11:30 AM
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Ed Smith
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Default RE: Tapered-Bore Engine Break-in - Upgraded

If I may add something. I use a smaller propeller than normal to get the operating temp and rpm up to where it would normaly be, while still at the aforementioned slightly rich two stroke.

ED S
Old 05-27-2004, 03:26 PM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

Dar, it is wonderful to have all of this information you've gathered for us now in one spot. I would not hesitate to refer anyone who asks me about break-in to this thread. You have obviously put much time, thought, and practical experience into this subject.

I ecpecially appreciate your willingness to share it with others. You will be blessed many times over!

Ernie
Old 05-27-2004, 04:39 PM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

DarZeelon,

I find you to be a wealth of information in these threads; I've followed your advice on several posts. There are those who think you believe you are the "KNOW-ALL, DO ALL" in here, but I, for one, appreciate the time you spend educating the RCU members and sharing the information you've accrued.

Thanks!

Gary
Maryland, USA
Old 06-04-2004, 11:05 AM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

Hey,

Just an FYI, a friend e-mailed me this thread.

I thought I would mention if you take out the baffle in the TT46PRO muffler (commonly done to get more power), it is physically impossible to 4-stroke this particular engine.

Most NIB engines today will not run 4-stroke for any significant amount of time without a constant application of the glo-starter. So a big hint, when you are breaking in an engine, is if you keep the glo plug on just so you can break it in, well then you are running a little too rich.

Hopefully this clears things up without going into the techno-babble.

Of course, once an engine gets loose and is broken in well, you can 4-stroke it quite easily by richening up the high speed needle.

I am a real lazy *******. My preferential method of breakin an engine in, is to run it for half tank rich until it runs semi-reliable. After that, I am done. I fly the plane/heli with the engine rich. After many weeks/flights later, until I can't stand the lack of power, I start to lean it out a little at a time.

The idea here, is to give an idea of what is 4-stroking rich, and 2-stroking rich. On an OS46FX, 1.5 turns out from optimum is stinking rich but still 2 stroke, 3.5 turns out is 4 stroking. On an TT46PRO, from optimum, 2 turns out from optimum is stinking rich, 5 turns out, and it's stinking rich and still 2 stroking, 7 turns out and it starts to 4 stroke, without the baffle, and it simply won't 4 stroke.



Jimmy
Old 06-04-2004, 11:46 AM
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DarZeelon
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Default RE: Tapered-Bore Engine Break-in - Upgraded

Jimmy,

In a tapered-bore engine, you would not want it to run at a four-cycle, rich setting, for the duration on the break-in.
Please read the thread carefully.

So the fact that you cannot make to rich enough to four-cycle, is not very important.
You must, however, be able to richen it enough to spin about 500 RPM less than peak.

If it is not very rich and four-cycling, you do not need to keep the glow driver on, for it to run.

The Thunder Tiger .46 PRO is a $75 ABN engine, which is much less likely than an OS.46FX, to peel its nickel. But if this should happen, a new piston+sleeve will cost $68 - 90% of the engine's total price, not including shipping...


Welcome to RC Universe.
Old 06-08-2004, 02:29 AM
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justahobbie
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Default RE: Tapered-Bore Engine Break-in - Upgraded

Great Info. It will be printed to take to the club field bullitan board

Ryan
Old 06-08-2004, 11:39 AM
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liorgozlan
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Default RE: Tapered-Bore Engine Break-in - Upgraded

Hey Dar,

1.At the original break in thread as well as in an article you wrote in Hebrew you suggested the first 15 min running should be in a rich two-stroke mode for a 10-20 short cycles starting from 10 seconds and gradually getting longer.

2.I broke in my rossi 45 according to your instructions and it performs great.

3.Now, I wish to break in my mvvs 91 and reading the new upgraded thread I noticed that you wrote: “Run the engine initially for a total of 15 minutes, in a somewhat rich two-stroke mode.”

4.Does it mean I should run it continuously for 15 minutes or should I stick to the short gradually getting longer cycles?


Thx

Lior Gozlan
Old 06-08-2004, 11:49 AM
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DarZeelon
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Default RE: Tapered-Bore Engine Break-in - Upgraded

Hi Lior,

After writing the original article in '2fly.co.il' as well as the original thread in this forum, I was convinced that the multiple 10-60 second cycles will do no damage, but will not be better for the engine than just doing several small tanks, 5 minutes each, with cooling in between them.

The 'heat cycling' is imperative for lapped, meehanite (cast iron) piston engines, like some still offered by Fox and by Enya.

But in the high silicon aluminium piston in ABC, AAC, ABN and Plasma Ceramic engines, the heat cycling will achieve very little.
It will do no harm, however, as you noticed in your Rossi.

Even with tapered-bore engines, I do a couple of short runs, before extending the cycles to 5 minutes.
Old 06-08-2004, 12:02 PM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

Dar,

If anything you got it backwards, there is no effective heat treatment for cast iron and still till temps reach 600 degrees or so. For aluminum and brass it is lower. As discussed on other threads no one can prove that cast iron heat and steel heat cycles at our engine's operating temperatures. Despite the voracity of some peoples statements.
Old 06-08-2004, 12:24 PM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

Hugh,

It was agreed here, in other threads, that the process termed 'annealing', needs temperatures of 600 degrees Celsius.
Therefore it cannot take place in any type of engine (except in jets...).

But in those meehanite piston engines, low heat cycling does achieve the longest possible life expectancy.
I do not know what to term the process, but a superior seal and superb wear resistance are the result.

Duke Fox reputedly ran a .35 through many, many gallons of fuel, without any noticeable wear on the engine.

I don't think there is any heat treatment process that takes place in high silicon (hyper-eutectic) aluminium pistons.

Besides, lets start a new thread, based on Mr. Higley's and the late Mr. Aldrich's notes on break-in of lapped engines.
Since I am less experienced in these engine types, maybe another distinguished RCU member will volunteer to start it.
Old 06-08-2004, 12:37 PM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

But in those meehanite piston engines, low heat cycling does achieve the longest possible life expectancy.
It was the brand and castor that achieved the longest expectancy. No body talked about heat cycling ever. A typical break in for a Fox was to buy a quart of fuel, punch a couple of holes in the cap and run the whole quart of fuel through at a rich four cycle. I recall some Fox's were so tight that you had to put lapping compound in the engine, then turn the crank with a drill.

No one talked about heat cycling or heat treatment till Supertigre came out with their ABC engines. The first ones needed some stress relieving by putting the sleeves in an oven at full blast, then turn off the oven and let it cool down over night. At least that is how I recall it.

I linked articles discussing heat growth and tempering of cast iron. Growth took place just below 1400 degrees F. Tempering at about 600 degree F. Both well below our operating temps.
Old 06-08-2004, 01:45 PM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

Hugh,

Usability is a crucial issue here.

I believe the number of members in RCU would be 1/10 of what it is now, if every engine that you purchased required you to disassemble it and to repeatedly 'bake' its parts in the oven and then allow them to cool.

Same goes for lapping compounds (tooth paste was mentioned) and drills...

The easiest thing to live with in a tapered-bore engine, is the 15 minute rich, two-cycle break-in.

Some (most) of the new modelers are not as technically inclined and are likely to damage an engine, if they have to take it apart.

Despite what Super Tigre wrote about their first ABC engines, I don't know of any heat treatment process that these engine undergo during break-in. The piston for sure, and the sleeve, being made of brass which is basically a copper and zinc alloy, is unlikely to undergo anything, at oven temperatures (less than 300 degrees C). So what gives?

Despite the wanting OS manual (I think even the AX still has the same one), the [link=http://www.osengines.com/faq/product-faq.html#q2]Q&A page in their web site[/link] specifies a two-cycle rich break-in for their ABN engines.
Old 06-08-2004, 02:43 PM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

Dar,

The point I am making is that you and others are trying to make break in a complicated black art. The point I was trying to point out with the lapping compound is to show that break in is essentially similar, it is a final wearing in of parts! Just run it, keep it from overheating, don't expect much from it till some fuel is burn't, and stay happy!
Old 06-08-2004, 05:06 PM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

The reason for short cycle run times on model engines has nothing to do with heat treating. As the engine runs and comes temperature, the engine heats up unevenly. This is due to many factors; cool fuel flowing thru passages to the combustion chamber, exhaust exiting, and cooling air are some of the reasons. Thus there is great stress put on the engine parts. The heating and cooling of the engine in rather short cycles is done to relieve the stresses.
Old 06-08-2004, 10:32 PM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

ORIGINAL: DarZeelon
Duke Fox reputedly ran a .35 through many, many gallons of fuel, without any noticeable wear on the engine.
What he did was to hook up a 44 gallon (or whatever the American equivalent is) drum of fuel, start the .35 every morning when he came to work and walk off and leave it running for the rest of the day. Obviously the drum fed a chicken hopper tank so the head of fuel in the drum didn't affect the tune. After 2000 hours he pulled the engine to pieces, found no discernible wear so he figured there was no point in further running. This story was related in another forum by a guy who worked for Fox and saw the test.

A point to be noted about this test though is that the engine only had around 200 cold starts (assuming he worked a 10 hour day). He also did a similar test on another Fox .35 but this time using normal RC fuel. It lasted a couple of minutes and was destroyed.
Old 06-08-2004, 11:42 PM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

Hugh,

The point I am trying to make (well, one of them), is that many people do still regard the break-in as witchcraft and run their tapered-bore engines at a very slobbery four-cycle, which may damage it and will certainly shorten its life.

This ritual comes from the meehanite piston past and thinking that if it is good for the venerable cast iron piston, it must also be good for the tapered-bore. It is not and it is wrong!

For ringed engines, this hotter break-in is potentially damaging.
Old 06-09-2004, 07:37 AM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

The heating and cooling of the engine in rather short cycles is done to relieve the stresses.
You are confusing the reason for tempering metal. Tempering relieves the internal crystal stresses in the metal, not between parts. When heat treating steel for example the crystal formation switches from cubic centered to face centered, during the process many of the carbon atom gets squeezed part way through and many crystals are stressed, this creates a hard but very brittle steel. Tempering allows the atoms to fully align and relieves the atomic level stresses. There are no stresses between parts which heat cycling relieves.
Old 06-09-2004, 07:59 AM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

run their tapered-bore engines at a very slobbery four-cycle
Just saw an older gentleman break in his LA that way and it worked fine. The C/L people run their ABC engines that way all the time. Still Jett, K&B, and others build their ABC engines very tight and they could throw a rod if broken in on a rich four cycle. I suggest we read the manufactures instructions and break them in accordingly, if it says to four cycle their ABC engine, so be it. If the rod breaks it's under warrenty.
Old 06-09-2004, 08:03 AM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

He also did a similar test on another Fox .35 but this time using normal RC fuel. It lasted a couple of minutes and was destroyed.
Odd, I have ran them a lot longer then a few minutes without any problem, on normal R/C fuel. Don't hot restart well, and it didn't want to break back to a four cycle, but back to normal when I figured out I broght the wrong fuel. It ran better with fuel that had castor added to bring the total volume to 25% oil.
Old 06-09-2004, 08:23 AM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

Hugh,

We will have to agree to each hold on to his own views on this.

Even if a tapered-bore engine stays in one piece after a slobbery rich break-in (and most sport engines will not immediately self destruct), the amount of wear incurred by the piston crown as a result, is equal to many hours of normal use.
Much more wear than a rich, two stroke break-in will result in.

Even Dub Jett advises of a very short four-cycle period (1 minute, I believe), just to take the edge off the extreme, finger breaking TDC tightness, his engines are manufactured with. Most sport engines don't need this at all, but I have seen some MVVSs that did.

He is also the strongest supporter of making sure a TDC pinch always exists in any engine considered to be in good shape.
Old 06-09-2004, 09:43 AM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

I agree with your statements on tempering. However, tempering has nothing to do with thermal stresses caused by uneven heating. Take a piece of annealed steel plate and heat it with a torch in the center and watch it deform. Heat it evenly in an oven and the deformation will be much less. It is this thermal stress that the heating cycles attemps to mitigate.
Old 06-09-2004, 09:50 AM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

ORIGINAL: DarZeelon
Even Dub Jett advises of a very short four-cycle period (1 minute, I believe), just to take the edge off the extreme, finger breaking TDC tightness, his engines are manufactured with.
Dar, while I commend you for trying to get this running in process sorted out I think you've made an assumption as to why Jett recommends the initial very rich run (yes, about 1 minute) to bolster your case. In fact he doesn't give any indication on his site as to why he recommends it. It could just as well be to ensure the rod ends are thoroughly lubricated before leaning out to high speed.

I think it's safe to assume that if OS or other lesser engines can hold tolerances to the point that those engines have similar pinch when new then Jett's don't need to rely on a "bad" rich initial run to hammer the piston down to an acceptable fit.
Old 06-09-2004, 10:02 AM
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Default RE: Tapered-Bore Engine Break-in - Upgraded

Brian,

It is an assumption.

The assumption results from comparing the fit of Jett engines to most others.

I suppose it is to break the extremely tight TDC bind, because 1 whole minute is not needed to get the bottom end wet enough with oil; this takes only a second, or two at most.

Then why don't we ask Dub, or Bob27, his representative in RCU, about the purpose of the full 1 minute rich run?

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