REFLECTOR: Piston Crack -- Dye Penetrant Testing

Mon Jun 15 16:56:10 CDT 2015

Steve, Mike & Kerry,

Thanks for your ideas and inputs.  The engine only has 75 hours on it and no
pre-ignition, but one short detonation 'event'.  I do run LOP a lot, but LOP
(and even some occasional light detonation are supposed to be good for an
engine according to Busch and Deakin and Tornado Alley) in terms of keeping
the carbon deposits down.  They all also say you can't hurt an engine by
running it too lean, provided you're too lean on the lean side of peak --
the engine just eventually flames out!

In the inspections I've done, I have not seen a 'sand blasted' look, just
relatively clean surfaces.  I did the this inspection because of the one
detonation event, but that was only with this one cylinder.  None of the
other cylinders detonated and the valves and pistons look just like this
one.

Also, several of the other cylinders have similar dark colored 'track' lines
on the pistons that also look like cracks.  We're going to try to do a dye
penetrant test, but it won't be easy through the spark plug holes.  

I'm still thinking these dark lines must be some kind of oil 'tracks'
because after my test flight, the oil 'crack' didn't re-appear, but there
was a new similar dark line in another area of the piston top, where there
were no lines the flight before.

My MAIN QUESTION is:  has anyone else ever noticed dark 'track' lines (as in
my photos below) that look like cracks on their piston tops before???

Thanks,

Bob

From: steve korney [mailto:s_korney at hotmail.com] 
Sent: Monday, June 15, 2015 11:02 AM
To: Bob Jackson
Subject: RE: REFLECTOR: Piston Crack -- Dye Penetrant Testing

Hi Bob...

IMHO...... Your running that cylinder too lean... The carbon on the top of
the piston has been eaten away by the excessive heat... After you cleaned
the piston and went out to fly again the carbon has all but burned off...
Looks like the pistons and the valves have  been sand blasted... Detonation
and pre-ignition cause that...

Steve 

  _____  

To: reflector at tvbf.org
Date: Mon, 15 Jun 2015 10:12:46 -0400
CC: bobj at computer.org
Subject: REFLECTOR: Piston Crack -- Dye Penetrant Testing
From: reflector at tvbf.org

While checking the valves (with my new eBay Dental camera 'borescope'), I
happened to rotate it 180 degrees and noticed what appeared to be a crack in
the top of the #3 piston.  The valves looked great, but a piston crack is
scary.  The engine only has 75 hours on it and has not been abused
operationally.  The cylinders and pistons were brand new from Continental
(for our TNIO-550N).

Cyl-3_Piston_Before-2.jpg

  Cyl-3_Piston_Before-4.jpg  Cyl-3_Piston_Before-5.jpg

The lighter gray marks between the crack and piston part # in the photo is
where the top of the (plastic) dental camera hit the piston top when I was
looking at the valves.  Here's the #3 exhaust and intake valves.

Cyl-3_ExhaustValve-4.jpg  Cyl-3_ IntakeValve-1.jpg

After a few days of head scratching and worry, we used a small wire brush
and some AVGAS to clean off the top of the piston along the 'crack' line --
then took it out for an hour long flight (overhead the field at 6000').  I
wanted to see if the crack line would re-appear in the exact same spot or
not.  But after looking again after the test flight, there was no evidence
of any line in the 'after' video -- the top of the piston looked smooth and
clean where the stain/crack had been before.

My questions are:

.         has anyone seen similar 'crack's or 'oil tracks' on the tops of
pistons?

.         could it be a splash or 'seep' line of oil or fuel that appears
possibly just as the engine is shutting down?

.         has anyone ever done dye penetrant testing of pistons before
through the spark plug holes?

.         would the dye penetrant residuals likely damage anything or
contaminate the oil?

We're trying to gather some data and perspective before talking to
Continental and the rebuilder and would appreciate your comments or
suggestions.

Thanks,

Bob Jackson

Velocity XL/RG/XC

TNIO-550N

1000 hrs

N2XF

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