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Old 04-16-2019, 03:07 PM   #21
40 Deluxe
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Default Re: Installing piston rings

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Originally Posted by rotorwrench View Post
This would be harder to check on a car engine but not impossible. It would be easier on an 8BA with the oil filler and draft tubes blocked.

I find the rings line up at no particular interval but they do line up and give low readings now and then (less than 65 psi). I always test run the engine for a while and then recheck the low cylinder. 9 times out of 10, the compression will be back to normal. If it isn't, we will hover the aircraft for a while and then recheck. If it's still low then it's time to remove a cylinder and find out why.
If you use a leak down tester you don't need to block the filler and road draft tubes to check ring leakage.
Those air cooled aircraft engines must have huge, huge ring gaps for compression to drop below 656# when the gaps are in line! Even then, there is so much space in the lands between the rings that compression pressure immediately finds its way to the second ring gap no matter where it is on the piston. Could you be dealing with a stuck ring? Have you pulled a cylinder to verify the gaps are lined up, then staggered the gaps, reassembled and retested?
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Old 04-16-2019, 03:13 PM   #22
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Default Re: Installing piston rings

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Originally Posted by Gene1949 View Post
some thoughts;
I prefer to "clock" the rings per engine manufacturers instructions because of the initial startup to "load" them properly during the crucial 1st few minutes of the break in.

Even Grant publishes a "preferred" ring end gap in their packaging. Sure (as the writer claims) an engine will run and not burn too much oil with an excessive end gap but why go thru all the expense of building an engine only to get sloppy in the area of cylinder sealing??

I don't buy it, it's a matter of having an engine "run" vs owning one that will perform to the best of its ability and justify such an expensive project.
Most engine builders I know would laugh this guy right out of their clean room.
Well, you'll notice that the 1/16" ring gaps were a test only! No one suggested that was recommended!
Just curious: How can ring gap location affect the "load" on the rings during break-in? Compression/combustion pressure will be the same no matter how the rings are "clocked".
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Old 04-16-2019, 04:16 PM   #23
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Default Re: Installing piston rings

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Just curious: How can ring gap location affect the "load" on the rings during break-in? Compression/combustion pressure will be the same no matter how the rings are "clocked".
Don't believe that is the issue, but if you want to line the gaps up, I say go for it. Just not something I would do. The engineers who originally made these recommendations knew everything that we know (and most likely a lot more) about how rings function.
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Old 04-16-2019, 04:46 PM   #24
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Default Re: Installing piston rings

The recommendations are probably written by someone in the sales department.
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Old 04-16-2019, 05:34 PM   #25
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Default Re: Installing piston rings

Some folks just don't understand that a new set of rings does not fit the cylinder all that well yet. They have to bed themselves in to fit and that takes some miles running with a load on it. To align the ring gaps is not a very good practice in my book. There will already be blow by for a bit on new rings so there is no point in tempting fate. It takes very little extra effort to stagger the rings before assembly.

As someone was wondering about differential cylinder pressures I will explain a bit further. The pressure that is applied to the cylinder is only 80 psi. The differential is what leaks out with only 80 psi applied. 65 over 80 is the limit. Normal pressures will be anywhere from 68 to 78 over 80. It will run at 65 psi but it won't have as much power as it will with 75 if you catch my drift. When you find one at 60 over 80 psi then it's talking to you. How you want to listen is up to the individual. I'll always run it again until it proves that it will not come back up. I won't pull a cylinder unless I have to.

Some folks do pressure tests by turning the engine over and noting on a pressure gauge what it rises to but that would not give an indication of where leakage is coming from since you won't be able to hear squat with the engine turning over plus a person has to watch out for the propeller on an airplane. Aircraft engines still use a draft tube and the filler tube is capped with a dip stick that has screw threads to hold it in place. A person just has to put a thumb over the draft tube to feel the pressure build up with 80 psi in any given cylinder. There will always be some ring leakage but a person can get used to what is normal or abnormal.
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Old 04-16-2019, 08:47 PM   #26
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Default Re: Installing piston rings

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Originally Posted by rotorwrench View Post
Some folks just don't understand that a new set of rings does not fit the cylinder all that well yet. They have to bed themselves in to fit and that takes some miles running with a load on it. To align the ring gaps is not a very good practice in my book. There will already be blow by for a bit on new rings so there is no point in tempting fate. It takes very little extra effort to stagger the rings before assembly..
I do stagger the gaps when assembling because I read to do so about 60 years ago, and I still do it out of habit even after I learned it is not necessary.
Regarding your leakdown tests, ring gap position will not affect the results. Air leaking past the top ring gap will fill the (relatively large) area between the top and second rings and immediately flow around the piston to wherever the second ring gap is and then flow through that gap at a given rate no matter where the gaps are; in line or diametrically opposite. There is no restriction to pressure flow between top and second ring gaps, no matter where they are. Especially during a static test where time is not a factor (constant incoming pressure instead of a brief compression stroke).
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Old 04-16-2019, 10:37 PM   #27
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Default Re: Installing piston rings

The report wasn't written by Grant Rings or another ring manufacturer.
So the info isn't that reliable at all,actually put together by an Engineer for what that is worth. I would really ask a ring company what they advise on piston ring installation.


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Old 04-17-2019, 09:11 AM   #28
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Default Re: Installing piston rings

Written by an engineer is good enough for me.
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Old 04-17-2019, 09:24 AM   #29
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Default Re: Installing piston rings

http://www.diagnosticengineers.org/j...dge%20Gaps.php


Then at the same time you don't want to recognize an engineer from a ring manufacturer.


Isn't that good enough as they are engineers also.
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Old 04-17-2019, 09:42 AM   #30
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Default Re: Installing piston rings

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Originally Posted by 40 Deluxe View Post
I do stagger the gaps when assembling because I read to do so about 60 years ago, and I still do it out of habit even after I learned it is not necessary.
Regarding your leakdown tests, ring gap position will not affect the results. Air leaking past the top ring gap will fill the (relatively large) area between the top and second rings and immediately flow around the piston to wherever the second ring gap is and then flow through that gap at a given rate no matter where the gaps are; in line or diametrically opposite. There is no restriction to pressure flow between top and second ring gaps, no matter where they are. Especially during a static test where time is not a factor (constant incoming pressure instead of a brief compression stroke).

I can't really compare an aircraft engine to a car engine. For one they are air cooled so they are more like a giant motorcycle engine. The different IO-360 engines I work on have a bore of 5 1/8" & stroke of 4 3/8" so they have some big slugs in there. A 360 cubic inch 4-cylinder is a pretty big 4-banger. The IO-540 6-cylinders just have two more cylinders all the same dimensions. Apply any air pressure to a slug that big and it will have some force on it. We set them up to exactly TDC to keep them from turning during the check. It can be difficult to do that sometimes. These are two compression ring and one oil control ring pack set ups so really only the compression rings hold any pressure. They are also choke bore cylinders with about .010" of taper as ground at manufacture.

From the previous description, you can tell there is a lot of difference but as far as holding compression, they still do it the same way. When a ring aligns, some of the hot combustion pressure is forced through those small ring gaps. If this continues for any length of time, it will eventually burn a crease into the side of the piston. Fortunately, they seldom align for any length of time for the most part or engines wouldn't last for squat. As the rings wear, the gap gets wider and the tension decreases on the cylinder wall so the engine starts losing compression. If a ring breaks, it will burn a big crease in the piston and the cylinder wall if a person keeps running it that way. This is why we do compression checks every 100-hours on aircraft. It's harder to pull over to the side of the road when things go south. Helicopters are better than airplanes in that respect but the pucker factor of an auto rotation due to engine failure will wake a person up pretty quick.
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Old 04-17-2019, 06:45 PM   #31
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Default Re: Installing piston rings

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Originally Posted by Ronnie View Post
http://www.diagnosticengineers.org/j...dge%20Gaps.php


Then at the same time you don't want to recognize an engineer from a ring manufacturer.


Isn't that good enough as they are engineers also.
Don't understand your point. I'm not questioning the above report.
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Old 04-17-2019, 07:43 PM   #32
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Default Re: Installing piston rings

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Don't understand your point. I'm not questioning the above report.



Sounds good to me
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