The compressor gently compresses the piston rings into position reducing the risk of damage to components during installation. Key operated with ratchet mechanism and safety valve for combining safe rapid compression with positive control. Made from tempered spring steel complete with steel key. PRC-540 For passenger car vehicle engines. Capacity: 57 - 125mm (2 1/4 - 5). Height: 80mm (3 1/4). PRC-730 For car/light commercial vehicle engines. Capacity: 90 - 175mm (3 1/2 - 7). Height: 100mm (4). PRC-765 For heavy commercial vehicle engines. Capacity: 90 - 175mm (3 1/2 - 7). Height: 165mm (6 1/2).