kevinw

Oversized Cylinder IO-550

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Does anyone know if one cylinder on an IO550 is bored .010 inch over cause an imbalance or result in any premature engine wear? Also, is this common practice? Just never heard of this being done. Thanks

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I don’t know what the Continental overhaul manual says about the subject, Lycoming does not allow it.

Clarence

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I don’t know what the Continental overhaul manual says about the subject, Lycoming does not allow it.
Clarence

Thanks Clarence. Seems odd to me but I’m not a MX. The plane in question had this done at a MSC and Zephyr did the work. It must be allowed but I question if it’s the right thing to do.


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Complex flow challenges with the change of a single cylinder’s geometry....

The driving force to want to have one odd cylinder, would have to be economics...

The cost of a single installed replacement cylinder is in AMUs...

I had seen it done on an O360... where an AMU was a giant expense at the time.

What keeps you from wanting to do it... the small amount of air being drawn in on every stroke that is different than the other five cylinders.... leaning would be different... the one with more air would peak first...?   LOP would most likely be the same, each cylinder would be completely burning the same amount of fuel.... the one with excess air would have additional air cooling...

So, if you use your JPI all the time, and are familiar with the data... you will see the effect in the data... similar effect to having a slightly blocked fuel injector... now, could Gami supply a matching FI to improve things..? Sure... but now there is a balance of power/torque/vibration to consider... one cylinder producing more power than the other five has got to be a technical challenge for the crank?

A pilot that appreciates a finely balanced Gami spread, likes running a few degrees LOP, has paid extra to have a JPI and Gami injectors... will check the MX fund to make sure there is enough in there prior to ordering a replacement cylinder... :)

Definitely have the maintainer check if the oversized cylinder is allowed, and any other details that need to be included... like matching the other cylinders to balance things out...(?)

Is this question related to somebody’s PPI?

PP thoughts only, not a mechanic...

Best regards,

-a-

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One might want to do the calculations on what percentage that .010 oversize increases the displacement on one cylinder. I know that is not the only consideration, but it might give you an idea about what differences you might expect.

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Bore of an IO550 is 5.25" and stroke is 4.25". So volume of one cylinder is 92 in^3. With .010 oversize it is 92.3528 in^3. So the larger cylinder has about 0.4% more volume. Trivial.

Boring out a cylinder and installing oversize rings and piston is a common way to rework a cylinder. 

Skip

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Interesting, thanks for sharing the applied math skills, Skip and Don.

Including the head space, makes the actual % volume change even smaller...

 

What would cause the engine manufacturer’s displeasure?

Ring flutter, exacerbated by a larger gap?

PP curiosity as usual...

Best regards,

-a-

 

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I believe the concern is the balance of the 2 opposing pistons, not the volume difference.

Clarence

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Thanks for all the insight and also the math, Skip. This cylinder now has a slightly larger piston and rings than the other five so there shouldn’t be any problem operationally. There must have been some scoring on the wall and repairing this must have been the least expensive way to go. All six cylinders went off to Zephyr for repair and only one came back this way.


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Questions-

What is the allowable imbalance between opposing cylinders?

What is the tolerance, weight wise, when balancing cylinders moving parts?

Could have one cylinder at the min weight and the opposite at the max weight. Then the imballance between the two would be at its widest. Could be the two with one being .010 over might still be within the max differential. One wouldn't know unless one weighed each part on both cylinders. Final authority goes to the factory overhaul manual and service instructions. What does the book say? I don't have one handy. 

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A quick scan of the overhaul manual didn't show any reference whatsoever to installing one cylinder bored oversize. There are instructions for boring. Anyway, this is a field repair and not an overhaul. I sounds like the cylinders were sent in for reconditioning and the bore on one was worn beyond service limits. The options are: 1) replace the cylinder, 2) plate the cylinder to bring it back up to dimension, 3) bore it oversize. The time on the engine really impacts the economics of the decision. If you are trying to just get a couple of hundred hours until it's going to need an overhaul, then the cheapest route it the probably best. If the engine is low time, then a new cylinder may be a good option. 

Here's an old article from Mike Busch that goes into a lot of detail about cylinders. It also explains the origins of the head cracking issues of the early 90s that spawned the shock cooling hoopla. https://www.avweb.com/ownership/the-jug-jungle/

Skip

 

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So the plot thickens...as it turns out a small crack was discovered in one of the cylinders and is being sent back to Zephyr for them to look at. While my mx was talking to them about the crack he mentioned the one P10 oversized cylinder they did. The response was "we would never do that". As it turns out they oversized all six cylinders and Sarasota incorrectly logged it as only one. I suppose that's a good thing as they are all balanced and if the cracked one is replaced I can only assume it will be replaced with a P10 as well.

In case anyone is wondering why I started this thread, I sold my J and purchased a 98 R. All this was discovered during the PPI. Jason from JED-Aire in Benson, MN (a Mooney expert) is going to finish the annual, get a cylinder replaced and hopefully within two weeks I'll have my new bird home. I turn 48 tomorrow and owning an Ovation has been a dream up to this point. Possibly the last airplane I ever buy. Thanks everyone for your help. Attached are a few pics.

IMG_2548.jpg

IMG_2550.jpg

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Nice plane.  On the plus side, your engine likely gained an extra HP or two from the additional volume :D

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Nice plane.  On the plus side, your engine likely gained an extra HP or two from the additional volume

That’s what my mx said. Instead of 280hp, I probably have about 284. Lol!


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Congrats on the new bird...

Expect the FF got increased to get the extra four hp...

A good Gami spread will indicate that there is an even fuel distribution to the nicely balanced cylinder volumes...  all great for flying LOP...

A Long Body as a forever-plane... now we are talking!

Best regards,

-a-

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