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  #1  
Old 12-29-2014, 09:29 AM
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Default Detonation

Does this look like a case of detonation or could it be something else? This was discovered during teardown for overhaul. I had to remove a thick layer of carbon build up and this was underneath. The other three pistons looked fine. [IMG][/IMG]
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  #2  
Old 12-29-2014, 10:49 AM
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Default No.

No not detonation. It's from ingesting something other then air or fuel (FOD) a rivet or screw maybe.
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  #3  
Old 12-29-2014, 11:07 AM
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Quote:
No not detonation. It's from ingesting something other then air or fuel (FOD) a rivet or screw maybe.
Probably that - what does the inside of the head look like?

Dan
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Old 12-29-2014, 11:11 AM
BillL BillL is online now
 
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Quote:
Originally Posted by Russ McCutcheon View Post
No not detonation. It's from ingesting something other then air or fuel (FOD) a rivet or screw maybe.
Yep, FOD, Foreign Object Damage. I am very surprised it stayed in the that long though. Count the bangs over a small area then calculate the piston and count the strokes. That was lot of stokes before spitting it out. With so many sharp edges it looks like it was steel, not aluminum.

Detonation looks more like erosion of the surface sort of like severe sand blasting.

The head will look the same. You didn't lose a ground tang on a spark plug did you? I might be concerned about stress risers in the head, but have no specific experience with this on flight engines.
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  #5  
Old 12-29-2014, 01:19 PM
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FOD does make more sense as the other ones were fine. I sent the cylinders off for overhaul but didn't notice any of them beat up. The #4 had a bad exhaust valve and guide, and also the exhaust port was cracked. I never flew behind the engine. I bought the plane as a project. It was cranked and run up to be sure the engine ran, maybe 5 min. total run time. According to the amount of carbon build up on top of the piston, this was not a recent event. Is there any danger of putting this piston back in service?
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  #6  
Old 12-29-2014, 01:42 PM
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While I originally thought of FOD also, I am surprised to see the damage almost uniform across the entire piston.

FOD damage is usually confined to the area of the squish band, at least from my personal experience.

I would hazard a guess that this may be due to someone chipping off the hardened carbon in a prior rebuild.

Do you know the history of this engine?? Any prior rebuild or top overhaul?
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  #7  
Old 12-29-2014, 03:12 PM
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Mike, this baffled me also so I called the guy who has my cylinders and he told me none are beat up like the piston. As for engine history, the only thing in the log book was an entry back in 1991 that documented a major overhaul. The Hobbs on the plane was at 410hrs total time. The engine was installed when the plane was built in '91. It set up for many years, thus ruining the cam, lifters, and accessory gears. I know this damage couldn't have happened when I test ran the engine. Too much old carbon build up on top. The funny thing is that during the pre buy, I cranked the engine to check oil pressure and mags, pulled the plugs, checked compressions, and did a visual on the oil. All looked good on the surface. I'm just glad I discovered these problems on the ground rather than at 100ft on takeoff.
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  #8  
Old 12-29-2014, 03:41 PM
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Does the 1991 entry for an overhaul state if new pistons were installed?

From what I have seen of detonation, it caused pretty sever damage, with a cratering appearance, not sharp edged damage/divots like in your photo. And often only in certain areas, not evenly spread all over the entire crown.

It looks like you used a wire wheel to clean off the piston, and that may have altered the look of the damage.

Just my best guess here, this is kinda puzzling.

Please report back if you find the cause.
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  #9  
Old 12-29-2014, 04:35 PM
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Default FOR Damage

Arlie,
Here is my FOD damage. Never found the culprit.

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  #10  
Old 12-29-2014, 04:58 PM
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Default Pistons

At just over $100 a piece pistons are some of the least expensive engine parts. I would not even consider re using a piston, especially with that damage.
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