What would cause This???

[jogle1023]

223637223636What would cause this? There is no marks inside the cyl or on the head at all. Rings and the cyl are perfect.
223635

[flabum1017]

porosity in the casting.... defective piston maybe....

[wrechin2]

The last one I saw like that was out of a 92 150 faststrike on cylinder 1. It was cracked behind the sleeve into the water chest (under the regulator) and was allowing water in the cylinder. I took it to mar-fab and had them repair it.It was kinocking badly but the only sign of water was a clean piston similiar to what I can tell from your picture. It had a wiseco in it and the very center was bowed in like you laid a round bar on it and hit it. just my experiences.

[jogle1023]

I cleaned it off with a wirewheel before the pics to read the numbers off of it. No cracks in the block. Its a 2005 Johnson 175 with about 100hrs on it. It has me scratching my head. It still ran good just did a comp check and found this one low(#4 Piston).

[mcyama]

Look's like a Yammi 3.540 piston? Maybe a model 64d00. There have been several updates and piston modifications since it was produced. There are several reasons for this type of failure. Just to name a few, poor octane fuel, low RPM at wot, poor quality oil and of course the easy one,a piston configuration that the dome was too thin to properly transfer heat to the rings so to be cooled by the cylinder walls. Food for thought, reduce internal temps. by either riching fuel mix, reduceing ign timing or my favorite go with the 64d03 pistons, that can be found in most late model 150 and 200 carb and efi motors. Well after looking a little more it seems to not to be an oem piston but the fix will still be the same Thanks Martin

[Superbender]

heat did that,low octane/lean fuel mixture/marginal cooling

[jogle1023]

I cleaned it off with a wirewheel before the pics to read the numbers off of it. No cracks in the block. Its a 2005 Johnson 175 with about 100hrs on it. It has me scratching my head. It still ran good just did a comp check and found this one low(#4 Piston).

[jogle1023]

Ok. I just am trying to figure it out. I am replacing 2 of the 6 pistons and just wanted to make sure. All the other holes still had 110psi. #4 piston had 80psi and #5 had 100.