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HomeAircraftHelicopterEngines Plugs Mufflers Fuel › Replacing piston/liner/ring
01-17-2009 05:02 PM  9 years agoPost 1
pgrbff

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Piemonte, Italy

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Assuming the piston is not too badly damaged, why replace it after bearing failure or lean run?
Surely its the new piston ring that provides compression on the new cylinder liner?

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01-17-2009 05:07 PM  9 years agoPost 2
jetfixrguy

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Florida

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If the piston still looks good with no cracks or broken pieces, I would keep it, clean it up and reuse it. Yes you are correct the ring is what makes the compression along with the liner.

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01-17-2009 05:23 PM  9 years agoPost 3
pgrbff

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Wasn't sure if there was some other reason. I see they often sell the 3 items as a set.

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01-17-2009 05:24 PM  9 years agoPost 4
jetfixrguy

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yes because most of the time the piston is damaged.

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01-17-2009 05:36 PM  9 years agoPost 5
pgrbff

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Mine just scratched from bearing breaking up. No major damage.

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01-17-2009 09:03 PM  9 years agoPost 6
dkshema

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Cedar Rapids, IA

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Quite often a really lean run, or bits of bearing going through the motor damage the piston skirt. Small scratches usually aren't a problem, but if you've really gone lean and killed the motor, or if a bearing decided that it wanted out, the skirt of the piston takes a beating. You'll see gouges, bits of embedded bearing steel, galling of the metal, and in many cases, the ring will be stuck in the groove as the ring groove is messed up.

In those cases, a new piston needs to be installed.

In cases where the piston is clean and undamaged, you may only need to replace the ring. If you do that, it's not a bad idea to use a small brake cylinder hone to remove the glaze from the cylinder walls and restore the cross-hatch pattern as it was from the factory. Then break-in as if it were a new motor.

These two pistons had their numbers punched and were replaced:

The first one is from an OS70 whose rear bearing wanted out of the motor during flight. There doesn't seem to be a lot of damage, but the ring groove was screwed up and smeared around the piston a bit.

The second one doesn't look that bad either, but it's from an ABN version of the OS 32 SX-H. It was run lean. It got replaced because the very top portion of the piston, just below the crown, got badly galled and smeared. Since this is an ABN motor and has no rings, this part of the piston is critical to getting a good compression seal. Since it's an ABN motor, the piston and sleeves are sold as matched sets.

With ringed motors, the piston and cylinder typically AREN'T matched sets and you can replace one without having to replace the other.

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01-17-2009 10:15 PM  9 years agoPost 7
jetfixrguy

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Florida

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ABN motor and has no rings, this part of the piston is critical to getting a good compression seal. Since it's an ABN motor, the piston and sleeves are sold as matched sets.

With ringed motors, the piston and cylinder typically AREN'T matched sets and you can replace one without having to replace the other.
Great info.

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01-17-2009 11:26 PM  9 years agoPost 8
airdodger

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Johnston USA

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If the engine was run lean the piston was probably damaged also since the heat transfer was not occuring properly, especially the top ring land.

Chris

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01-18-2009 12:00 PM  9 years agoPost 9
pgrbff

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I have no reason to suspect the engine has been run lean.

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01-18-2009 12:37 PM  9 years agoPost 10
baby_zyklon

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Singapore

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There is actually no need to replace the piston, if there are no deep scratches or chipping on the piston. Same goes for the cylinder liner. However, in most instance, a broken ring will minimally take the piston along with it.

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01-18-2009 02:20 PM  9 years agoPost 11
airdodger

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Johnston USA

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pgrbff You asked why, I answered the question. I did not state what your engine went through, did you forget your question? Surely the ring provides the seal, what holds the ring, the piston.

Chris

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