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  #1  
Old 04-12-2012, 11:28 PM
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jdeas jdeas is offline
 
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Default Marks on wrist pin

I pulled a jug off my engine a few weeks ago to check the cam shaft as the engine has been stored for many years. While the cam looked great I noticed these markings on the wrist pin and piston rods.
Are these normal markings for an engine with 150hrs?

https://picasaweb.google.com/1077309...eat=directlink

History: Engine has 150hrs SMOH. Was pulled from service, new cam installed then pickled for storage.
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  #2  
Old 04-13-2012, 10:05 AM
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rocketbob rocketbob is offline
 
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Default

No that is not normal but it doesn't look bad. I would suspect that the rod bushings weren't honed to the right size and/or no assembly lube was used. I would polish the wrist pin on a lathe with a piece of fine scotchbrite, and would pull the rod out and have it re-bushed. New rod bolts and nuts would have to go in.
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N9187P PA-24-260B Comanche, flying
N678X F1 Rocket, under const.
N244BJ RV-6 "victim of SNF tornado" 1200+ hrs, rebuilding
N8155F C150 flying
N7925P PA-24-250 Comanche, restoring
Not a thing I own is stock.

Last edited by rocketbob : 04-13-2012 at 10:08 AM.
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  #3  
Old 04-13-2012, 11:48 AM
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Default Any other explanations?

The rebuild was done by Penn Yan Aero so I have a high confidence it was originally done correctly. Any other takers? Pulling all the pins and rods is expensive if not warranted.
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  #4  
Old 04-13-2012, 11:54 AM
paul mosher
 
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Default OH Manual

Take a look in the overhaul manual and see what the limits are.
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  #5  
Old 04-13-2012, 12:55 PM
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hgerhardt hgerhardt is offline
 
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Default

Those parts look just fine. The "ring" on the wrist pin is the space between the piston and rod, the section where neither pin nor piston touch.

And the bushing probably wasn't honed properly, but the pin doesn't appear stressed by it so I'd leave it alone.
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  #6  
Old 04-13-2012, 01:17 PM
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dwrichey dwrichey is offline
 
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I hate to be the fly in the ointment, but something seems peculiar. An engine that needs a new camshaft 150 hrs after a major overhaul? Hmm. How does one replace a cam without removing all the barrels and splitting the case? If the case was split, the rebuilder then re-installed the wrist pin in that condition? Hmmm.
The first airplane I built had a suspicious engine, it REALLY took a lot of pleasure away from flying it. I suggest you have your favorite engine shop do a complete inspection. Yes, it will be expensive, but you will be thankful in the end.
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  #7  
Old 04-13-2012, 01:24 PM
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Default Split case

I was told by my A&P instructor that it was not uncommon to simply slip the wrist pins in a low hr engine so the case could be split without having to purchase then re-seat new rings. As for the re-ground/installed cam. My guess was lack of flying hrs caused corrosion. When I had the jug off I ran an inspection camera all around the interior of the case and couldn't find anything out of place. If fact all the interior parts seemed to still have a good coat of preservative on them. Amazing for the time this engine was on the shelf!
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  #8  
Old 04-14-2012, 09:10 AM
BillL BillL is offline
 
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Default Uneven Contact Pattern - odd

First, I am no expert on this engine type, but having been in 1000's of other engines, 2 things.

Like rocket bob, probably not "bad" but agree very suspicious. The wrist pin surface finish looks quite rough for 150 hrs, likely assembled that way, and the bushing has highly polished wear lines, not a more even wear pattern that is expected. So - - like others, recommend checking those bushings and ensure that the rod ends are within specification for concentricity before installing new bushings and reaming.

My 2 cents, not a final absolute judgement.
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  #9  
Old 04-14-2012, 09:22 AM
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When you see patterns on bearing surfaces like whats pictured on the small end of the rod, that means the clearance is too tight. Since the clearance is too tight not enough oil is passing around the wrist pin and so what's there is getting so hot it burns and leaves a deposit.
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N9187P PA-24-260B Comanche, flying
N678X F1 Rocket, under const.
N244BJ RV-6 "victim of SNF tornado" 1200+ hrs, rebuilding
N8155F C150 flying
N7925P PA-24-250 Comanche, restoring
Not a thing I own is stock.
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  #10  
Old 04-14-2012, 10:03 AM
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jdeas jdeas is offline
 
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Default Rod end more of a problem?

Quote:
Originally Posted by rocketbob View Post
When you see patterns on bearing surfaces like whats pictured on the small end of the rod, that means the clearance is too tight. Since the clearance is too tight not enough oil is passing around the wrist pin and so what's there is getting so hot it burns and leaves a deposit.
Are you saying that reaming the rod bushing and polishing the wrist pins may be an option? (If they fall withing tolerance). That could be done with the right tools and new cylinder gaskets. Much less invasive than removing the piston rods. What I am coming away with is this most likely isn't a problem but for an added margin of safety, polish the wrist pins and ream the bushings if they are undersized.
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RV-7 N314SY (KWHP)
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A&P


CANbus based trim/flaps and electrical

Last edited by jdeas : 04-14-2012 at 11:09 AM.
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