Strange Scratch Marks on my Pistons, what could be the cause?

[Darkbound 7]

Hey guys, I finally took my engine apart to hunt for the culprit of the knocking noise that I had, second time now.

I have very strange scratch markings on my pistons, ALL of them have them to a different degree, but same pattern.

 

These pistons were installed in the car 10k miles ago they were taken from another vr6 engine. My old pistons do NOT have these scratches. What could be the reason for such scratches? There is nothing that I can see on the cylinder walls/bottom that could haev caused such pattern, I am taking everything to am achine shop in tuesday, but still this baffles me. Anyone any ideas?

[mattvr6 133]

Is that the thrust side?

[Darkbound 7]

Its the "smaller" side

[mattvr6 133]

Smaller side?

[Darkbound 7]

This side (this is not my piston, random image from google)

[mattvr6 133]

Assuming the bores were homed our etc before fitting these pistons?

[Darkbound 7]

They were not, anyway I took everything today to a machine shop to get it measured. Everything is perfect, bores are perfect, rods are also perfect not twist/stretched, perfectly balanced within 1g of each other, they said that the pistons (aside from these scratch marks) are also good, we still cant find any explanation for my knocking problem. Crankshaft is also perfect....

Im getting 1mm oversize pistons along with forged rods and r36 rod bearings, and to hell with it.