DarZeelon

Bob,


Most manufacturers of tapered-bore engines have now stopped recommending four-cycle running during break-in.

Yet the Jett Engineering web site still does recommend this; "The engine should be rich. Leave the battery on the glow plug for a while and let the engine run. After about one minute you my start to lean in the high speed needle and remove the glow plug battery. ".

This would suggest the engine should run for one minute at a very rich setting, which will cause the engine to run in a four-cycle mode and die if the battery power is removed from the plug.

Manufacturers no longer recommend this very rich running in tapered-bores, because it wear the piston's crown smaller, even if done for only one minute. This, in turn, will kill the TDC pinch, Dub goes out of his way to command, saying; "...by the time there is no "pinch" left (bump at top dead center) the engine is starting to lose some of its power.".

This initial break-in recommendation and the TDC pinch retention contradict each other.

Bottom end protection, without piston crown damage, is best achieved by using 25% oil, preferably Castor, instead of a two turns richer mixture setting. Same amount of oil protection for the bottom end, no alternative cost.

The web site says; "It never hurts to add about 4-6 oz. of oil to a gallon for the first few tanks." Adding oil to a total of 25% would require adding 6 ounces to the gallon of the recommended Power Master (7 ounces of oil to a gallon of other commercial fuels with 16-17% oil). Just make this imperative and lean the mixture to a rich, no glow driver, two-cycle immediately.

The result: No bottom end wear. No piston crown wear. TDC pinch remains.

bob27s

My Feedback: (19)

Hi,

Thanks for writing......

There is some good and bad information out there about break in proceedures for ABC type engines...... some info is a mixture of both. In general, a proceedure outline by an engine manufacturer should be followed as written.

Other engine manufacturers have their noted proceedures. The proceedures depend a lot on materials. Jett uses special alloy aluminum for all pistons, and all sleeves are TRUE chrome plated. For a Jett, that first minute of being rich does no damage to the piston or sleeve. It is no different than starting a Jett or any other ABC engine when cold and at idle. The proceedure outlined in the instructions and on the web site is very tried and true. Always follow it with a Jett engine, and the engine will run flawlessly.

Something of note. 'Taper bore' engines and AAC/ABC engines can be different. Some have a 'fixed' taper that does not change or changes unevenly. A well designed AAC/ABC engine has a taper and piston/sleeve fit that DOES change. Both the piston and sleeve change with temperature. When the engine is cold (at start) there is an interference that you can feel and hear turning over the engine... with a Jett, the audible 'squeek' is a good thing to hear! As soon as the engine starts (for sport engines, at idle) the engine begins to warm up, creating the appropriate fit between piston and sleeve. As you take the engine to full power and it reaches operating temperature, the fit becomes optimal, and you will hear the engine rpm increase to its full potential. If anything, you want it on the 'tight' side.

Keep in mind, that Dub runs EVERY engine before you ever get it. In fact, the low end needle is pre-set, and in some cases you may receive the engine with the high end needle set to an optimim setting too. Every engine must meet or exceed the published performance RPM before he ships it. The break-in proceedure you follow helps seat the rod ends. No 'wear' really occures on the piston or sleeve to achive a 'fit'.

I have used a proceedure similar to what Dub describes on my ABC engines for many years. That first run is important to allow the bearings, rod ends, wrist pin and crank pin to properly seat and loosen up. The first minute is rich, then I lean the engine up into a stable 2C and let it run at or near my intended operating rpm.

An example. Well after break-in, when we start our racing engines, it is important that they start wet and somewhat rich. You do not want a temperature spike the moment the engine fires... you want it to warm evenly, then after about 10-15 seconds you can slowly lean it to the optimum ground RPM for flight (for a Q-500 engine, this is about 600 rpm down from ground peak). Clearly from this, starting an ABC/AAC engine 'cool' at first does absolutely no harm, and does a lot of good.

For those interested..... the break-in instructions noted here can be found at
http://www.jettengineering.com/tech/break-in.doc

When Dub mentions "rich" in the break-in instructions, he has instructed that you have set needle valve initially at a very rich setting .... this to avoid any possibility of starting the engine lean. A Jett engine simply will not run blubbering rich (some other engines will) even with the glow plug driver on. It has to come up to a decent temperature and have a smooth mixture just to run stable. So by rich, he is stating that the engine is running, stable, with the glow driver on, clearly 'wet' sounding and clearly not peaked. This is, in most cases, a rich 2 cycle or just on a 4 cycle break.

For a brand new engine this is basically to allow the engine to warm up slowly so you don't toast it right off the bat. As noted, this is just for the first minute or so. In about the time it takes to lean the engine a bit so it stays running, put the starter back in the field box, and move so you are located behind the engine for subsequent adjustments, you have expended the appropriate amount of time.

You then lean the engine so it is clearly performing well, running smoothly, and you can remove the glow plug driver. At this point it is not 'lean' or anywhere near peak rpm. It is rich, but trust me, there is no damage or wear going on with the piston and sleeve fit.

Jett's break-in proceedure recommends that you install a smaller diameter (or cut down) prop for the first runs. You want to achieve RPM, not heat. Starting with that very first tank of fuel, go to full throttle, and lean the engine until it reaches the recommeded target RPM, and it will still be slightly rich at that setting. That is exactly what you want. Do not take the engine to peak RPM with the smaller prop.

After those intial few runs (about 5 to 10 minutes each), you should install the prop you choose to fly with. Again, back the needle out about 1 turn to start the engine slightly rich, warm it up for a bit at full throttle..... then lean it out until you find peak rpm (by sound or with a tach), then back off the needle immediately 1/4 to 1/2 turn. Note the needle position, and you can use that sound/rpm as a reference. (my proceedure... is to then shut the engine down, and let it cool for no less than 5 minutes, then restart it without touching the needle). I typically suggest that Jett owners run two tanks of fuel with the flying prop on the test stand...this allows you to become familiar with the performance, rpm, and throttle response............ then it is best to install it in the plane, an fly it.

Following that, you should rarely have to set the high end needle more than 1/4 turn in either direction to adjust the top end mixture. If you find you have to make a major needle or adjustment or the engine does not run correctly once installed in the plane, you should immediately suspect a fuel system leak, line kink, fuel foam or similar. We have noted on occation a 100-200 rpm drop transitioning from the test stand to the aircraft. This has to do with stiffness and engergy transfer... and it is completely normal.

The last thing you want on initial break-in runs is to get the engine HOT! A hot engine results in engine damage, lost piston seal, blow plugs, burnt head buttons. About the only thing you can do to a Jett engine to damage it is run it lean! In fact, every Jett engine owner will tell you if you consistantly set the engine rich by 400-800 rpm before every flight, the engine will run for a LONG time without ever loosing its fit.

As for the extra oil in the first few tanks.... some people do it, some don't. I run powermaster or omega 15% all of the time, and use the same fuel for break-in without any additional oil. The extra oil sometimes helps those who push the engine a bit too hard on its first few runs, where it might get too hot.

The bottom line, with Jett engines (and in my personal experience over 25 years, this is true with most ABC type engines): Follow that break-in proceedure, always keep the engine slightly rich, and you can not go wrong. ABC and AAC engines, especially our BSE engines, require very little run-in time for 'fit' reasons.....the bench time is mainly to allow the owner to learn how to operate it without burning it up

I hope this has been useful

Bob

DarZeelon

Bob,

Thank you for your very elaborate reply.

DarZeelon

Bob,

You wrote that an engine manufacturer's instructions should be followed.

Yet what many manufacturers write may not not be as clear as Jett's.
Also, specifically for OS, the manual says to cycle between four-cycle and two cycle, only to be contradicted by the OS Q&A, Bill Baxter and AnnMarie Cross, who say not to four-cycle it at all during break-in, if the engine is a tapered-bore.

I would say the break-in method should most often be determined by the type of engine, more than by its often unclear, or erroneous manufacturer.

bob27s

My Feedback: (19)

sometimes i get carried away I didnt realize how long it was until I posted it.

I hope the information proves useful.

DarZeelon

Bob,


These two paragraphs in your post cleared it up completely.

Other manufacturers give inaccurate, unclear, or incorrect instructions, BTW; so it may not the best advice to follow them.


It may be good if you added the end of the first paragraph above, to the break-in instructions, so it will be clearer to the user.