I went to a mechanic and he showed me this custom piston made out of molten car pistons (there was no other option in this case). My question is: Is the scratch from the side of the piston going to affect its quality in any way? I figure that the most important part of the combustion happens above the top of the piston (and the top of the rings), so the bottom is not that important, as long as it does not scratch the cylinder.
I do not see anything that I would call a "scratch". A scratch would be into the piston ( below the surface ) so should not hurt an iron , steel , or even an aluminum cylinder wall. However , I see a relatively large amount of gas porosity on the surface from hydrogen gas dissolved into the liquid aluminum . This implies more gas porosity so I would be concerned about the strength of the piston . Assuming radiographic examination is not possible ; "run it and see what happens" may be the only choice.