A thermal by-pass valve for a heat exchange circuit, comprising: a valve housing forming a valve chamber and having a first port for an oil supply line, a second port for an oil return line, a third port for a heat exchanger supply line and a fourth port for a heat exchanger return line, said first, second, third and fourth ports communicating with said chamber, and further having a by-pass valve port surrounded by a primary valve seat, said valve port being located in said chamber and providing fluid communication between said first and second ports when said valve port is open, said valve housing including a housing cap closing one end of said valve chamber; a thermally sensitive actuator including an actuator body having a shaft section and a movable piston extending from one end of said actuator body, said actuator being mounted in said chamber and movable therein in response to extension or retraction of said piston, said piston being extendible in order to engage said housing at or near one end of said chamber; a valve member mounted on said shaft section and movable by extension of said piston from a first position where said valve port is open to a second position wherein said valve port is closed by said valve member sealingly engaging said valve seat; and a return spring mounted in said housing and having one spring end engaging said shaft section so as to bias said shaft section away from said valve seat and to move said actuator away from said valve seat upon retraction of said piston; characterised in that a spacer member separate from said housing cap projecting from the actuator body and coaxial with said piston, said spacer member including a sleeve section forming a piston-receiving passage into which said piston extends during use of the by-pass valve, and a connection section connecting said spacer member to an end section of the actuator body, said piston being slidable in said passage, wherein said passage has a length and, during use of said valve, said spacer member acts to maintain said one end of the actuator body at least said length of the passage away from said one end of the chamber wherein said spacer member includes an annular plate section extending circumferentially around said sleeve section and forming a valve portion, and wherein said plate section is movable by said actuator between a valve closed position where flow of oil between said fourth port and said second port is blocked and a valve open position where flow of oil from said fourth port to said second port can take place. a valve housing forming a valve chamber and having a first port for an oil supply line, a second port for an oil return line, a third port for a heat exchanger supply line and a fourth port for a heat exchanger return line, said first, second, third and fourth ports communicating with said chamber, and further having a by-pass valve port surrounded by a primary valve seat, said valve port being located in said chamber and providing fluid communication between said first and second ports when said valve port is open, said valve housing including a housing cap closing one end of said valve chamber; a thermally sensitive actuator including an actuator body having a shaft section and a movable piston extending from one end of said actuator body, said actuator being mounted in said chamber and movable therein in response to extension or retraction of said piston, said piston being extendible in order to engage said housing at or near one end of said chamber; a valve member mounted on said shaft section and movable by extension of said piston from a first position where said valve port is open to a second position wherein said valve port is closed by said valve member sealingly engaging said valve seat; and a return spring mounted in said housing and having one spring end engaging said shaft section so as to bias said shaft section away from said valve seat and to move said actuator away from said valve seat upon retraction of said piston; characterised in that a spacer member separate from said housing cap projecting from the actuator body and coaxial with said piston, said spacer member including a sleeve section forming a piston-receiving passage into which said piston extends during use of the by-pass valve, and a connection section connecting said spacer member to an end section of the actuator body, said piston being slidable in said passage, wherein said passage has a length and, during use of said valve, said spacer member acts to maintain said one end of the actuator body at least said length of the passage away from said one end of the chamber wherein said spacer member includes an annular plate section extending circumferentially around said sleeve section and forming a valve portion, and wherein said plate section is movable by said actuator between a valve closed position where flow of oil between said fourth port and said second port is blocked and a valve open position where flow of oil from said fourth port to said second port can take place.