I got scammed on an internet deal recently. I bid on what was advertised as new CP Forged Pistons. When they arrived, they were used. The lousy POS that sold them to me won't return my e-mails. The pistons look to be in very good condition. No scuffing. No cracks, etc. I thought I might be able to salvage something from the situation by using them in a budget 400 SBC build up. I took them to work, mounted them on the lathe and checked run-out. Each one has a run out of .004 to .007 in the same area. I thought the things were junk but some local builder told me otherwise. He said that high end race forgings will not be concentric. They are built this way to compensate for distortion during high stress. Can anyone confirm this??

 

Pretty much all pistons (except REALLY cheap ones) are offset on one side. It's to try to keep the load more above the center of the rod journal. That way more force is applied directly to the crank.

If you look at the top of the piston, there's normally a notch or a dot of some sort. Those all face the front of the engine when you install them. It's a necessity that you install them properly.

 

Thanks AJ_92RS. I have built several performance motors but never had the occasion to measure concentricity. In the past, I just take the new pistons to the machine shop and as side from double checking clearances, that was that.