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Engine basics => Engine set up tips => Topic started by: frank mccune on April 25, 2013, 08:04:34 AM
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Hello:
The other day I had about a two minute flight on my Fox .15 baffle engine and when I tried to restart it it had almost no compression. The engine was running in a very rich 2 or 4 cycle when the plane ran out of fuel. The prop was a Cox 8x4 gray plastic one from the 70's.
This engine had a new "oversize" piston fit to the new sleeve by carefully lapping the tow together. There was a perfect fit with great compression with no leakage. The piston was tight above the exhaust ports with a good taper. When turned over, it gave a very strong POP. Of course it would one flip start both hot or cold!
The engine was carefully broken in and the fuel had a large amount of Castor oil added.
I am aware that these engines have been deemed as unworthy by many but I always liked them for use on .15 airplanes! They were easy to start, fairly powerful and best of all, very lightweight! I am at a loss as to why this engine died an early death. I have a thought but that is all it is without any data to support it.
What are your thoughts as to why this engine had such a short life?
Tia,
Frank
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Did it get better when it cooled down? I have some iron piston engines that lose compression when hot.
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Put some light oil in it and make sure you didnt blow a head gasket or the plug is leaking
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As has been said, check for leaks. Glow plugs can blow the seal, head gaskets blow, bolts come loose, back plate can leak, etc..
BIG Bear
RNMM/AMM
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How long ago did you lap it to perfection? The last time I heard about a "wore out in one flight" incident the guy hadn't washed all the lapping compound off.
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Put some light oil in it and make sure you didnt blow a head gasket or the plug is leaking
Or it doesn't have a hole in the piston. If it was fine through break-in but failed in one flight, there's something interesting going on.
There's a reason they are Hurled(tm).
Brett
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No, there are no leaks.
I thought that I had removed all of the lapping compound out of the p&c before I broke it in again. It ran great on the test stand! Perhaps I did not get it all. The lapping compound that I used was supposed to be 1500 grit aluminum oxide. This material is not supposed to embed into the metal. I am beginning to think that I did NOT remove all of the lapping media!
The compression did not return after the engine cooled.
Oh well! Can you say,"Hello Enya?"
Thanks for the replies,
Frank
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I recommend not lapping in your Enya. They can take some time to break in as well but resist the urge. 8)
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No, there are no leaks.
I thought that I had removed all of the lapping compound out of the p&c before I broke it in again. It ran great on the test stand! Perhaps I did not get it all. The lapping compound that I used was supposed to be 1500 grit aluminum oxide. This material is not supposed to embed into the metal. I am beginning to think that I did NOT remove all of the lapping media!
The compression did not return after the engine cooled.
Oh well! Can you say,"Hello Enya?"
Thanks for the replies,
Frank
Hi Frank
If you lap in an engine you should always take it back apart and thoroughly clean it, use soap and water and a stiff brush, tooth brush bottle brush pipe brush... it will work better for ya that way
Randy
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I would bet on a blown glowplug seal if it's not a lapping related problem. I would expect a lapping compound contamionation to show itself progressively during the break-in period rather than all at once in the first flight.
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That's my thoughts also. Leaking glo plug or head gasket. I have one with many hours of running that still has great compression.
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Back in the mid to late 60's, I bought a new Enya 35 stunt, based on recommendations from Jack Sheeks. It was a precision motor that came with two heads and three venturi inserts. One of the advertised features was the Enya was a hand lapped engine. It was, without a doubt, the smoothest rotating engine I ever felt. I don't know if the ones sold today are lapped at the factory. Perhaps improvements in manufacturing and materials have done away with the need for lapping.
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my experience with the fox 15 is if you lap a p/l set to were it "Feels Right", you get almost instant wear out. These suckers seem to need to be really tight and then run in.
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Do you recall if the castor oil residue was clear or dark colored? Clear = Good, Dark or Black = Bad.
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my experience with the fox 15 is if you lap a p/l set to were it "Feels Right", you get almost instant wear out. These suckers seem to need to be really tight and then run in.
Lapping it beyond the bare minimum to make it safe to run is probably too much, that's for sure. Probably fit well, got a heat cycle, the liner grew and the piston didn't, dead.
Brett
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I would never recommend lapping a piston into a liner. On all my engine builds the liner is lapped first, then the piston lapped to fit. The means a liner lap and a piston lap. If you think about it, the fits we require in these engines is VERY close, in the 100 thousanths or even millionths of an inch clearance regions. Consider the size of a 1000 grit particle, its about .0004 inches (1500 is a bit smaller) that times 2 is .0008 inches, nearly a thousandth of an inch. What we want is about a decimal point further to the left! In order to get the piston to move in the liner with lapping compound in it and feel "just right" you've already made the piston undersize. The seal you had was the oil in the peaks of the surface finish and with very little running the peaks were worn away and could no longer hold any oil, your fit was gone. Typically lapped engines are fitted tightly so that as they are run the piston and liner wall "seat" together, this cannot be accomplished if they are lapped one to the other, they will always break in to a looser then desired fit. Further, well done fits are tapered, liner tighter at the top then the bottom and piston larger at the crown then the skirt. This does 2 things, it accounts to some degree for the thermal expansion of the liner which is hotter at the top then at the bottom and with the piston it minimizes the skirt friction that does nothing but sap power. A piston and cylinder lapped together will lack both of these very desirable characteristics. They will conform one to the other and the total surface area of the piston circumference will contact the liner, if the liner is tapered in the wrong direction due to wear it gets looser at the top rather then tighter and you will not be able to maintain compression.
Flame suit on.
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you can use a pipe cutter to make a small groove in the piston.
I have seen some motors running very fast with this groove.
Greetings Robert-Jan
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you can use a pipe cutter to make a small groove in the piston.
I have seen some motors running very fast with this groove.
Greetings Robert-Jan
Indeed they will, but its short lived
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Indeed they will, but its short lived
Or, it will pop the top of the piston off.
Brett
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My favorite nobler with a fox .15 "wore out" pre-maturely.....melted a hole in the piston. I reckon the glowplug was wrong for the engine. SH^
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Or, it will pop the top of the piston off.
Brett
Yeah that too!!!!
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Oh, Oh! If you used 1500 grit lapping compound, you might have gone too far. That stuff is OK for automobile valve grinding, but our engines work with very close piston/cylinder fits.
Floyd
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I use DuPont White polishing compound. I lap to a drop fit from the exhaust port down, a light drag fit from the exhaust port to a little before the top, then leave the top untouched.
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I use DuPont White polishing compound. I lap to a drop fit from the exhaust port down, a light drag fit from the exhaust port to a little before the top, then leave the top untouched.
Note that this might - repeat might - be tolerable on an old-technology iron-piston engine that is exceptionally too tight, but you should not ever do anything like this on a current-era ABC/ABN/AAC/AAO engine, ever! At the very best it would be pointless, and at the worst it will mean getting a new piston/liner.
Brett
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I just remembered an event some years ago. Aaron, Kent Tysor, and I were practicing. I was using one of the most reliable engines we have used in this event, an OS .40VF. Had a great engine run and the other two put up flights. Fueled mine back up, flipped it to prime and "NO" compression, zero, nada, zip.......
We couldn't figure out what happened other than a gasket or something muxt have blown out. After a few minutes of the three of us trouble shooting, I put in a new plug for some reason. Outcome: it was the first time I had ever experienced the seal blowing out of a glow plug! Run for many more hours, still running good today.
BIG Bear
RNMM/AMM