The scope light is too bright to show anything when directly over the crack- I think I can switch the light and/ or camera to the side of the scope so I shall try that.
If you can, swirl a fine piece of cotton rag, on the end of a coat hanger wire, over the piston crowns, saturated in acetone: if it erases parts of the “crack,” that would give some peace of mind.
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Before we all freak out, if these were cast pistons, the top surface would not be machined, I’ve seen patterns like this that will fool you to thinking that they are cracks when they’re simply part of the metal flow when the Pistons were cast.
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That is what I’ve been thinking all along. Glad someone brought it up.
One can buy dyes the help detect cracks in aluminum.
With aluminum castings, pistons, connecting rods and rocker arms, magnetic particle detection is useless because aluminum is not magnetic. Another method must be used to find cracks. Penetrating dye works well for detecting surface cracks on aluminum parts because the oil that carries the dye is drawn into the crack. Ultraviolet dyes make it easier to see small cracks, but this requires a special UV light to illuminate the dye.
Porosity leaks are common in both new and used aluminum castings. Pinpoint holes can be hard to detect with dye, so pressure testing is often the best technique for checking the integrity of an aluminum casting (iron too). Pressure testing requires special plates and plugs to close off all of the coolant passages on a head or block. It also requires a large tank so the casting can be submerged under water. When air pressure is applied to the coolant jackets (30 to 40 PSI max), you should see no bubbles coming anywhere from the casting. Hidden cracks in intake and exhaust ports will bubble air out from the port. Bubbles from any surface means there is a crack or a pinhole porosity leak that needs to be addressed.
Not so: most certainly the flat part of the crown was/is machined…you can see the machining marks.
It looks like the arrow is cast in, though, so the machining ends before the edge.
Does the crack go through the arrow . Doesn’t look like it.
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And on closer inspection it looks like it does
Every arrow I ever saw looked like a stamped-in one.
Hmmm. Maybe it is. Optical illusion?
Here are my thoughts.
For the first time ever I differ with Nick… Because piston 5 is machined to the edge. I think the arrow is stamped.
I think that there is a light coloured coating that got baked onto the piston top and due to heat/cooling has shrunk and cracked and that carbon has filled the crack.
Dennis
69 OTS
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Penetrant dye tests are relatively cheap, easily applied and work on aluminum. Interpreting the result is trickier. Many brands are available. Magnaflux, the company, not their namesake process, also sells kits.
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Thanks to all for the comments and speculation as to whether it is a crack or not.
To me it looks like a crack possibly emanating from around the top ring groove but I can not understand how it could have happened.
I hope that this is all just a bit of needless concern and that it is a deposit on the top of the piston as some have suggested.
Unfortunately I can not get back to the engine until tomorrow but I shall certainly attempt a thorough clean and if necessary a dye check.
I have used and inspected dye check cracks many times in my previous employment so I am familiar with the technique. Just not sure how easy it will be to do it through the spark plug hole.
Thanks again for all the input - I shall let you know what I find.
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If it is a crack, it almost undoubtedly happened when someone put the pistons in the bore, and a ring hung up going into the cylinder. It’s the only way I can think of it happening.
Possibly rings too tight, either in the bore or the groove?
Hmmm… if too wide for land depth… perhaps.
Upon second thought, if the ring was too large a diameter to go in… the piston wouldn’t go in.
Is your peace of mind worth pulling the head? If those are indeed cracks the head will have to come off anyway.
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I can not imagine anyone pounding a piston in hard enough to crack it! Does that happen?
Tom
But if gapped too tightly, when expanded at running temperature, would that be enough?