Grundfos 415531 CRT4-120/10 Chamber Stack. This Chamber Stack Kit is a genuine replacement part designed for CR Series Pumps and manufactured by Grundfos. This Grundfos 415531 CRT4-120/10 Chamber Stack features multiple cylinders and "Stacked" impellers individually designed to meet your pumps flow and pressure requirements. The Chamber Stack Kit mounts within the actual pump and provides the ultimate in-line multi-stage pump performance. The Chamber Stack also features an exterior body and exterior parts constructed entirely of Stainless Steel.