Breaking In a New Engine

Do you break it in with muffler on or off? Prop size? Fuel (I am breaking in the Fox on 29/5 fuel)? Time for each run? Time between runs? Total time? How do you know when it is ready to put on a plane? Do any hand-lapping first, or just take it out of the box? Break it in straight or pressure? RPM range -- higher or lower than what you expect in flight? Let it scream for a while, or run 4 cycle, or mix it up? What else? Most importantly, what do you do to make sure you don't fry the the new toy on the bench.

Buying fuel seems to be the hardest part of putting a plane in the air for us poor souls out here in the hinterlands. Ritch's Brew has a nice web page, but I have sent two e-mails and and two phone messages (my expense -- not 800) and got zip. You have to wonder why people bother to put up web pages and ignore all responses. Fox sells fuel, and I ordered some from them today. Hopefully Fox is alive and well, and a legitimate business unlike Ritch.

I am not sure how to describe to y'all how frustrated I am at the seemingly simple process of buying fuel.

Break it in on the fuel you are going to run it on. 29% all castor is fine.

Use the same prop you are going to fly it on, or one slightly smaller.

With the iron piston version, run it on a rich 4-cycle at first, then at a slightly leaner 4-cycle, right on the verge of the "break". I usually do about 5 minute runs at a time, with about the same amount of cooling in between. After a couple of runs, continue the 4-cycle break in, but lean it for a short burst a couple of times in each run after that. I usually simply pinch the fuel line to give it that short burst of leaner running. That is faster and easier than adjusting the needle. You may have to adjust the needle somewhat during each run anyway, however, for as the tank runs out it will tend to lean out and you will need to adjust it richer to keep it in a 4-cycle mode.

Muffler pressure is not needed as long as you are there monitoring it, and it really makes no difference whether you choose to use the muffler or not during the break in process. I usually do simply because of the noise factor, and the fact that I am going to use a muffler to fly on anyway.

How long to do this is always a guess. With the iron piston engines I will normally do at least an hour on the bench. More does not hurt. I would rather do too much proper break in on the bench where I can monitor it, than try to fly too soon and get a hot run that could ruin the engine.

How do you keep from frying it? By breaking it in on the bench and monitoring each run to ensure that it stays in that lean 4-cycle mode constantly except for those few short bursts of 2-cycle that you give it. You will hear some people say to simply break it in in the air. That, to me, is a sure road to destruction. If you get a lean run--just one in the air before it is fully broken in--you can destroy the engine beyond repair. And lean runs can happen because the piston is too tight (not broken in yet) or because a little piece of "junk" comes out of the tank and gets through the filter and plugs up the needle valve--just enough to lean the mixture.

Careful bench running is one of the best pieces of insurance you have towards a long life for your engine.

And, oh, yes, do not try to hand lap the engine unless you know exactly what you are doing. These engines are hand lapped at the factory already, and should be ready to run. Sure, a "trained professional" (whoever that is) may be able to get a little better fit by carefully hand lapping the parts together, but all he is doing is shortening the break in time with an even greater chance of ruining the engine. Like the stunts of Robby Kneivel, "Don't try this at home."

Sometimes I can't remember what we used to do. Your methods seem right on based on my memory.

Never apoligse for speling. Too of the smartest peeple I know cant spell cat without a dictanary. It's your ideas that cownt.

Really, never ever let anyone intimidate you on spelling. Never fail to post because of a fear of making spelling errors. I have seen a study that confirms there is no correlation between the ability to spell and smarts. My personal experience confirms that.

"Poor Spellers of the World, Untie!"

Whether breaking in or a mature engine, we don't want a ridge to form at the top of the liner. That is, we know the rod will stretch a millisomething at higher RPMs. Running on smaller props at break-in is good insurance. In the air, the engine will unload and develop more RPMs than on the bench with the same prop. Running a mature engine on a smaller than normal prop every now and then prevents forming a ridge.

I do 3 or 4 minute runs initially, letting the engine cool between runs. If you're using a "quick change" type of mount, such as an E Z Just, you can break in another engine while you're letting Duke's pride and joy cool. Then you're not just waiting around. After a few runs, I gradually introduce a click or two of leanness each run, observing and listening to the engine each time for tightness and overheating. This is the conventional wisdom, and I cannot fault it.

Even after a long breakin period on the bench, you will find your Fox continuing to loosen even after many hours of flying. The best ones will let the prop flop back and forth about 6 or 8 times, when you flip it dry in your hand. (This is even with excellent compression). One of the best Foxes I own is an old 4 bolt Sandcast that does exactly this. I've wondered what its provenance really is, how many owners it has had, and how many models it has been on. It's not the prettiest thing in the world, with both broken lugs JB Welded (epoxied) back with washers as added support. And the head is getting a nice crud buildup. It's on a Yak. But I digress...Don't get me started on these.

Just getting broken in, I'd say.

I would recommend that you should break any engine in with a muffler if you are going to fly it with a muffler. The reason is that the heat will distribute differently with out the muffler than with it, therefore changing the 'shape' that the piston/liner finally settles into. If you first start out without a muffle, then fly it with a muffler, you may have to re-break in the engine when you start flying it. On the iron piston Fox this may not be as much of a problem as on an ABC type engine.
Let the flames begin....
Jerry