Manually-operated by-pass valve which installs in-line of the two oil hoses to and from the cooler. In the open position, during warm-up, the oil will partially return to the tank without flowing through the cooler. As the oil temperature rises the amount of oil passing the cooler will gradually increase. With the by-pass valve manually closed all of the oil is forced to flow through the cooler. The latter is recommended only for high outside temperature.