Yup, it's the clearance. Very few have actually had skirt or ring land failures with the stock Mahle pistons. The guys that have run into trouble were doing repeated high speed runs and scored the teflon off the pistons due to the tight tolerences and aggressive tunes. There are a couple of guys on ModularFords.com running around 1000RWHP on the stock block using turbos. I'm curious to see how well they hold up without the searing heat of the Eaton or Twin screws.