oversized cylinder

[fearofpavement]

Been having a discussion regarding using an oversize cylinder on an engine that has stock cylinders. I believe it to be acceptable and common practice if a situation arises which requires a cylinder replacement. Others disagree citing potential vibration issues due to the different displacement/horsepower. My position is that the differences in weight, power, and displacement by going .010 over (or .015 for Continentals) would be so miniscule that it is insignificant in a low rpm airplane engine. Is anyone aware of any specific data regarding cylinder sizes and mixing them on the same engine?
Thanks.

[Hank Jarrett]

Re: oversized cylinder

When I rebuilt my A-65-8 I took the moving parts to a gentleman who builds up NASCAR engines for a full balance (yes, BEFORE it was checked by the certified engine shop). He put my crankshaft on a balancing lathe and spun it up over 20K! Scared me to death. He then balanced all the pistons, rods wrist pins and caps (mixed them up so they were balanced as sets). Last step was to "CC" the combustion chamber. You make sure the volume of the combustion chambers is exactly the same. That way each chamber gives exactly the same "push" to the crank. All together these things add up to an engine as smooth as an electric motor.
I don't think I would want to intentionally introduce an out of balance condition, but I have no idea how much effect each balancing activity has. I'm sure not ever going to see 20,000 RPM!
Hank

[fearofpavement]

Re: oversized cylinder

Well, regarding engine balance, on a high rpm engine I can see where this would make a difference. But airplane engines really run at low speed compared to many other types of engines. So if we were spinning it at maybe 9000 rpm like a motorcycle can turn it would be a different story than the 2000-2800 that direct drive aircraft engines typically operate at. If someone wanted to bother doing the math I think the difference in weight and displacement would be small enough to ignore.

A crankshaft turning 20,000 rpm would be neat to see (from another room through thick lexan windows)...

[Forrest Barber]

Re: oversized cylinder

We used to balance and it does work, mixing .015 with standard has been done. I had a Harley once with a .080 overbore and piston on front cylinder, made it smoother.

[Hank Jarrett]

Re: oversized cylinder

Mismatching the displacement on a Harley to help correct a "designed in" out of balance condition is actually funny. I can see how it would help but don't know for the life of me how to optomize the effect.
I LOVE Harleys. The sound is great and they are a ball to work on, but I would NEVER ride one. Too many knuckleheads out there (and I'm NOT talking about the engine). If everybody else rode them I would love it, but I get enough people who can't see me in my Miata. Idiots take the fun out of motorcycles and airplanes. Less of them to mess with up in the air.
Hank

[Forrest Barber]

Re: oversized cylinder

The guys in the Club said I spun my rear tire in increments upon acceleration.
Ah the good old days; First bike was a 1928 Harley , Last Harley was a 1957 XLCH right from CA ; ah to have ridden just once with Sonny Barger. Now have a 1993 Goldwing SE , 108,000 Never made Sturgis.
Got to fly yesterday, not so mean today, got a lot done. bye

[Ragwing nut]

Re: oversized cylinder

If you install one oversize clyinder, you should install an oversized on the opposite throw for balance. If I do one I do them all now.

[Forrest Barber]

Re: oversized cylinder

Of course it is best to do them all! But you can get clever with the balancing without another piston "necessity is the mother of invention"

[Ron Coleman]

Re: oversized cylinder

Depending on which cylinder has the greater the displacement it may make the engine run smoother or have a tiny bit of imbalance. Mo matter how perfectly balanced the engine is, carb/manifold inducted engines allmost invariably do not deliver the same fuel mix/quantity to all cylilnders so the engines almost never run without some soft vibration.
Typical of v-twin engines in Harley or Indian motorcycles, because the carb sits between the cyllinders , fuel has to be delivered forward to one cylinder and to the rear on the other cylinder, durring acceleration, the rear cylinder will get a richer mix than the front cylinder. That is one reason that although the front cylinder gets more cooling air, it usually runs hotter (leaner) than the rear cylinder. IF one of the cylinders fail, it will usually be the front one. Running a slightly larger displacement on the front cylinder may correct the problem.
Variation in intake runners may also be why some small continentals run soft on one cylinder. I have found that some intake runners have casting lumps near the opening at the cylinder. I had to grind away the casting scabs inside the runner of the spider.
You want to argue about why rear cylinder gets a richer mix? Fuel is jerked through through the manifold in a rapid start stop motiion which causes fuel droplets to be deposited in the mainfold where durring acceleration they slide back the manifold to the rear cylinder. The reverse could happen during braking, but the cylinder is not working.


RonC

[robbie]

Re: oversized cylinder

if your engine is stock not balanced just put it on. You will not notice it
these old engine rock and roll and only runs at 2200 rpm's.
Just a fast idle for a car.
I have a old a&p who has worked on these engines since 1952, said you will never know if one cly is 015 over.

[tawadc95]

Re: oversized cylinder

had to chime in on the harley stuff...nothing like a Honda mine is a smooth hard running 158 hp at the wheel with the brakes to accompany it
kind of the alter ego of a tcraft!!