cylinder question

[jim cooper]

Re: cylinder question

I'm not a mechanic but I vote for Richard Pearson's expertise. These little engines are CRUDE! 1920's-30's type technology! Loose tolerances, low compression, low RPMS! I'd Bolt it on and fly it! We are not talking Porsche Carrera here. JC

[flyguy]

Re: cylinder question

RE: flat four: never having looked closely at a flat four crank I'll have to take a powder on that one. I don't know. Sure have piqued my curiosity now. Will have to find at least a photo of a flat four crank.
Darryl

Edit: anyone have a good clear photo of a flat four crank, or a link to a good photo?

Second Edit: Just found one courtesy of Sacto Sky Ranch. Lycoming 4. Adjacent throw pistons, one goes up, matched one goes down, should be interesting dynamics.

Ah well, where is that crow I have to eat. Fair trade since I learned something. Chuckle.
DC

[jgerard]

Re: cylinder question

Sorry guys, I'm a licensed mechanic and do this stuff for a living and there is no way I would mix an oversized cylinder with stock ones. There is no approval from continental to do so.

[Hank Jarrett]

Re: cylinder question

There is a HUGE difference between what you are "approved" to do and what you can "get away with". Some things I can "get away with" I consider doing, if I KNOW it is safe. A good example is the hardware store slot head screws holding my upholstery together in my plane. My boss (also an Aero type) had a FIT when he saw "non-aerospace" fasteners in my plane. I would NEVER use a hardware store bolt in a load path, but come on! The screws holding the fabric grommets in were hardware store grade when the plane was built! I just replaced the rusty originals with exactly the same thing, minus the rust.
On the other hand, when I did the major on my 45 engine (an A-65 series 8) I talked to the engine shop first and got their approval to balance it. I then tore the engine down and took all of the parts to an engine guy who builds custom engines for NASCAR. He precision balanced the crank and cam (which required him to take a light cut on each to bring them in balance). He then dynamic balanced each by spinning them again on a balancing lathe. Next he measured and weighed all the pistons, wrist pins, caps and rods (both ends). We found the pistons all weighed different with one WAY under weight. It was just on the edge of the allowable. The IA allowed me to take several boxes of new pistons he had to the builder and we found a fourth pistons close to the rest. He then took a light cut on the inside of the skirt of three pistons bringing them all to the same weight.
Next step was to mix and match the parts to match the weights as close to the same as possible by putting the lightest pin with the heaviest piston, and so on, on the neatest scale I have seen since school. We matched parts up in sets so everything was as close as possible. The rods were a whole story themselves with the weight and CG being matched. When I fly this engine, I expect it to be as smooth as an electric motor (yes, we did match up the cylinder displacements too).
I AM NOT suggesting this is a good thing to do and DON'T recommend it for anyone else. First off, if ANY part didn't meet spec after we messed with it, it would have gone into the trash (none did). ALL of the parts were checked by the IA AFTER the mods to be sure we didn't mess anything up. I did this because I got a GREAT deal (seems like the whole mess cost me under $300!) and I wanted to learn how to balance an engine. It was worth it because I got to work with a real engine artist and it was a once in a life time opportunity.
What the engine builder told me was that all of the things we were doing would let the engine spin from 10 to 20k no problem (not on it's own), but it was a complete waste to balance an engine as slow turning at mine. We did it because HE WANTED TO TRY IT and I WANTED TO LEARN.
This is the opposite extreme of anyone wanting to intentionally fly an engine with different size and weight pistons. Yes you might be able to "get away with it", but WHY? Something in me that loves and respects fine machinery just CRINGES when I see fine machinery being needlessly beaten up. (I have never been able to bring myself to watch a NASCAR race) Running an out of balance engine DOES beat it up, even if it can take it for decades before it breaks. Look at H.D. Motorcycles!
That is the main reason (to me) to correct this problem at the next top (if not sooner). The other reason is if there is an accident of incident, and the FAA finds this, SOMEONE is going to loose their head, even if it had NOTHING to do with the failure. That's the way bureaucracies work. If I was the A&P or IA and knew the jugs were mismatched, I'm afraid I would refuse to sign off the engine for flight, even if I knew I could "get away with it".
Hank

Just my opinion, and all of you know how anal I am.

[jim cooper]

Re: cylinder question

Pardon my ignorance but I always thought the reason a race engine was balanced, ported, releived, etc. was to make it smoother and more powerful. I read an article some years back that told how old man Honda insisted that each engine be balanced CAREFULLY and that was a major reason they last so long. American manufacturers have never, as I understand, gone to the trouble that Honda did consequently they didn't last as long or run as well. True? My question is: How far out of balance would this engine be? Big time? or just a very little bit like Richard says. Its an interesting topic to me and hope someone with much more knowledge will explain it. Would a good machine shop say it was ok to fly, or not? JC

[Hank Jarrett]

Re: cylinder question

An easy way to picture how much power an out of balance engine looses is to watch one vibrate on the mount (all engines bounce around a little). Now with the engine stopped, push it over by hand on the mount the same distance. That shove you had to apply is the energy that your engine spends moving the engine that same distance every time the prop goes around. That is energy that should have gone to turning your prop and it was wasted shaking the engine. Pushing it once may not seem like a lot, but shake it as fast as it vibrates while running and that is a LOT of energy. Not only are you loosing power, but you are beating up the mount bushing and fatiguing all of the metal parts. Smoother is always better. Too rough, and it will break something, and that never seems to happen at a good time.
Hank
It takes some test equipment and math to calculate exactly how much you are loosing to out of balance, but it isn't insignificant.

[Edwin Otha]

Re: cylinder question

I have not come across any Continental information that prohibits the installation of a single oversize cylinder. It may exist but I haven't got a copy.
The factory put out a revision in the early 70's for piston and rod weights ( M72-17R1). This allows a 1/2oz difference between opposing rods and 1/2oz between opposing pistons. The engine doesn't care how big it is only that the weights are the same.
EO

[Pearson]

Re: cylinder question

An engine doesn't vibrate soley because it is out of balance. A lobe on the cam being slightly more worn than others, a slight difference in airflow through the intake tubes, or more resistance to exhaust flow through the exhaust valve or exhaust pipes, are all just examples of many more things that can cause an engine to vibrate. These things affect the vibration because they are causing variances in power output of individual cylinders.

This is why it is pointless to balance an engine if you are not going to equal the flow of fuel/air into, AND exhaust gases out of an engine.

I did some ROUGH calculations and came up with a difference in weight between a standard and a 0.015" oversize piston of ABOUT 13 grams. That is well below the 1/2 ounce figure mentioned in the previous post. I want to emphasize, these were just rough calculations.

There is absolutely NO DOUBT, to me at least, that an engine with one cylinder 0.015" oversize is going to be slightly out of balance. Is it within limits. I don't know what the limits are, other than what has been posted just above this one.

Will that engine, with the one oversize cylinder, shake more than an engine with all cylinders equal? It depends! It depends on if all the other stuff I mentioned above is all equal. More than likely it isn't. So now it is really just a throw of the dice. If the one oversized cylinder is the one that has the best gas flow into and out of, it is going to shake like heck. Because that cylinder will be producing more power. What are the odds of that being the case? I personally think slim to almost none.

It may not be approved, legal, or sanctioned by anyone, but I would be surprised if one of these little low compression, slow turning engines with one oversized cylinder shook any more than it did with a standard one.

I have to add that this has been one of the more interesting threads.

Happy Thanksgiving to All!!!

[NY86]

Re: cylinder question

Even if it is approved, legal and sanctioned, it is also crude, sloppy and unprofessional. This is an aircraft engine, not a lawn tractor. Do it right or don't do it at all is my feeling. Oh, I also am a trained, certified professional A&P.

[flyguy]

Re: cylinder question

Considering how appallingly crude some the parts of the engines we are discussing are (eg rockers that will not align with the valves), it is amazing that they hold up as well as they do. IMHO these kinds of aircraft engines don't fail to blow up or quit because of any elegance or precision of their engineering, but rather because they are simple and incredibly strong.

I am forever amazed at starting up an internal combustion engine of any type, considering the violence going on inside it, that it doesn't throw pieces of broken engine all over the area.

But if I consider the strength of the steel parts involved (you could likely lift your entire house with one of the rods, for example), I understand why things don't come apart under the incredible stress.

Having said that I will admit to believing in doing things right and I suspect that if we do it the best we can it will not be perfect enough to be the enemy of good enough.

DC

PS: Of course, in flying, there are at least 2 or 3 and likely a half-dozen or so things that will try to kill you long before a balance problem with your engine even rears it's ugly head.

[LostnSpace]

Re: cylinder question

Well, like Robbie asked, I have not given my response, but it sure would be interesting to know what "his A&P" gave for an answer, to be sure, whatever he said was in agreement with someone here Happy Thanksgiving everyone, I am indeed Thankful for this site and everyone posting here, LnS.

[robbie]

Re: cylinder question

My AP/IA said " Not an issue with that engine." So I asked 4 other AP's in the area, plus one Cly shop and they said the same.
They all agreed that and A-75 or A-80, they would want to weight the two pistons before doing it.
Well the cylinder did not pass a standard inspection and is being bored .015 over so it will be the same as the rest.
This was a very interesting thread, thanks for all the input.
And yes if the cylinder could of stay standard. I would of keep it that way.
Back in the 60's and early 70's I built three 327's up and they were all ballanced. But the continual piston and rods were all over in weight as I remember.(18 years ago) It ran 2500 hours that way so I personly would do it.
I under stand the point of all who would not.