Hi, they are supposed to be appx .0015-.002 under when measured at the lower part of the skirt. However, if measured just below the oilring it is closer to .003-.004" so they are not that different from other pistons. Used KB pistons tend to have more clearance at the lowest skirt point, often .002"-.003" more than should. The skirt edges are kinda weak. If you try to straighten back they can easily break (I have tried!). I am not sure if they make more noise when worn, if the skirts are long as for a typical 440 piston.