the invention is a device for controlling the temperature and thus the viscosity of the lubricating fluid in a rear axle or differential . a first embodiment of the device comprehends a curved bi - metal strip or plate disposed proximate the ring gear and closely conforming to it . the strip extends about the periphery of the ring gear in the direction of rotation of the gear when the vehicle is moving forward . as the temperature of the lubricating fluid increases or decreases , the bi - metal strip or plate moves to direct a larger or smaller flow of the lubricating fluid toward the housing through which heat is transferred to the atmosphere . in another embodiment , a bi - metal baffle disposed adjacent the ring gear includes a plurality of flaps which open upon a rise in lubricating fluid temperature and direct more fluid to the housing to assist heat dissipation .