A Stroker’s Biggest Enemy
Not all pistons are created equally. Wiseco has been working closely with Lunati to ensure that their pistons are designed properly for stroker duty. Toss in a piston that’s not up to the task and the results can get expensive fast. The key is in the design of the skirt taper for LS-based 4- and 4.125-inch strokes. The basic premise is that the piston must still be at full diameter when it reaches bottom-dead-center (BDC) even if a lot of the skirt is hanging out of the bottom of the bore. This way the piston won’t wobble and tear itself apart as it’s forced up the bore, and past the sharp edges at the bottom of the cylinder.
According to Brian Nutter at Wiseco Piston, “LS blocks have varying cylinder lengths ranging from 5.420-inch in the 6.0L block, 5.455 to 5.475-inch in the LS1, 2, 3, and 92 blocks, LSX at 5.575-inch and LS7 at 5.900-inch. Most pistons are tapered approximately .050-inch from bottom to top to accommodate for thermal expansion, but it’s important that a piston skirt be at full diameter at BDC. The necessary taper must be introduced at a point above this. When the piston is at full diameter at BDC, it won’t rock, and the bottom of the cylinder will not dig into the skirt, causing rapid wear. Reducing piston rock also keeps the rings perpendicular to the bore and oil control is much better. To further reduce wear, Wiseco recommends that the bottoms of the cylinders be deburred with a cartridge roll around the circumference. Forged piston manufacturers have different specifications for a piston’s skirt taper depending on the application. It’s important that a customer speaks with them directly to verify proper design.”