Some of the 120 possible crankshaft configurations have useful properties, but all of them have a rocking imbalance of some kind which may or may not require a balance shaft, depending on the application. This is because the six pistons with six unique phases cannot be "paired" as in the four-stroke case. The Detroit engines used a configuration that, once the primary rocking couple was balanced out, was also perfectly balanced at all other rocking couples until 6th-order. Mercury came to use a configuration that canceled only the primary rocking couple and was run without a balancer.