When a dish-top piston can be replaced by a flattop, it's simpler, and less expensive to do that substitution, especially if pistons are being bought anyway in the course of a rebuild. The term "performance" pistons might be a little misleading. Racing pistons are custom-made, and don't meet some of the needs of a street engine, specifically quiet operation and long life. Street-engine pistons are designed to give tens of thousands of miles of trouble-free operation, and do it quietly. In order to run with minimum clearance, cast pistons have an expansion-limiting piece of metal called a "strut" cast into the wrist pin bosses. This piece cannot be incorporated into a forged piston, so running clearance must be increased when forged pistons are used. Most pistons are "cam-ground" to minimize the clearance necessary between the piston skirt and the cylinder wall. As the piston reaches operating temperature, it gets rounder, and clearance is equalized all the way around. Even street-type forged pistons have somewhat of a cam shape, but the lack of a stabilizing strut makes the cam shape less effective. A lot of race-type forged pistons are round, or nearly so, and must have lots of clearance. They rattle, and don't last long. If a street-type forged piston is run, and clearanced properly, it will work OK. If a racing piston is used in a street application, it will be noisy, short-lived, and contribute to rapid cylinder wear and oil consumption.
Jerry
