"Stock" pistons have four valve reliefs AND a "front" notch, so the pistons on one bank can be installed with the wrist pin offset toward the outside of the block, and the other bank with the offset toward the valve lifter valley. As the piston approaches the top of the stroke, the connecting rod goes "over center" slightly before TDC. When the piston reaches the top, the rod is at a slight, but measureable angle, and the thrust of the compression pressure holds the piston skirt tight against the cylinder wall- - - -the "outside" wall on the passenger's side, and the "inside" wall on the driver's side cylinders. This allows the piston skirt to load itself against one cylinder wall due to compression slightly before the spark plug fires. When the plug fires, there's no "slap" as the thrust of firing reaches the piston. If a piston is installed with the offset wrist pin facing the wrong way, the piston is "floating" at the top of the stroke, and the piston skirt will make nasty noises as it hammers itself into the cylinder wall as the plug fires. The piston skirt, and/or cylinder wall will fail in a short time. If a V-8 piston is made with only one set of valve reliefs, it MUST have a different wrist pin offset for right and left cylinder banks.
Actually, a set of custom race pistons such as Keith Black forged pistons, will have four, not two different part numbers, because the valve reliefs will be custom-cut to match individual cylinders as well as being set up for right or left cylinder banks. The intake and exhaust valves are in different places in cylinders 1 and 3, for instance, and need different-sized relief cuts, also 5 and 7. Ditto for 2,4,6, and 8.
Jerry
Last edited by Hotrod Lincoln; 06/20/2010 3:07 AM.
