i don't know exactly how i cracked it, i had used nitrous, and this is the second time i shattered part of a piston. the thing is the guys at my shop are saying it is cracked, but it doesn't looked cracked. i don't have a picture, but it is about 4 inches long, and it doesn't just go straight up and down, it varies slightly from a straight line. what do you mean by weld it? its not a very big crack, maybe like the width of the wire from a paper clip and the same on depth and i didn't freeze a connecting rod or anything, just shattered part of a piston. i shut the car down as soon as i knew something was wrong, within seconds of it happening. what i think it might be if not a crack, it could have been from the large chunks of piston that were flying around for a few seconds