A bypass system comprising: a source of a fluid; a bypass valve mounted on a line to selectively bypass the fluid prior to reaching a downstream component, said bypass valve including a valve poppet a valve seat , a piston , a valve housing and a wax portion, said piston is adapted to move along an axis allowing movement of said valve poppet toward and away from said valve seat , said piston is adaped to move within said valve housing , said wax portion, in operation, expands as a temperature increases, and causes said valve poppet to move against said valve seat , said wax portion, in operation, cools if the temperature lowers such that said valve poppet moves away from said valve seat ; and there being a sealing surface on said valve poppet in sealing contact with said valve seat and said sealing surface being part-circular in a plane in which said axis lies; wherein said valve poppet is part-spherical about said axis, and wherein said valve seat has a sharp break point selectively in contact with said valve poppet to provide said seal, or wherein said bypass system further comprises a cylindrical seal and wherein the cylindrical seal is received in a recess of said valve poppet such that said seal provides said part-circular portion of said valve poppet , and wherein said recess is formed in a frusto-conical surface of said valve poppet ; the system being characterised in that : said valve poppet is threadably received on threads on said valve housing to facilitate replacement of said valve poppet . a source of a fluid; a bypass valve mounted on a line to selectively bypass the fluid prior to reaching a downstream component, said bypass valve including a valve poppet a valve seat , a piston , a valve housing and a wax portion, said piston is adapted to move along an axis allowing movement of said valve poppet toward and away from said valve seat , said piston is adaped to move within said valve housing , said wax portion, in operation, expands as a temperature increases, and causes said valve poppet to move against said valve seat , said wax portion, in operation, cools if the temperature lowers such that said valve poppet moves away from said valve seat ; and there being a sealing surface on said valve poppet in sealing contact with said valve seat and said sealing surface being part-circular in a plane in which said axis lies; wherein said valve poppet is part-spherical about said axis, and wherein said valve seat has a sharp break point selectively in contact with said valve poppet to provide said seal, or wherein said bypass system further comprises a cylindrical seal and wherein the cylindrical seal is received in a recess of said valve poppet such that said seal provides said part-circular portion of said valve poppet , and wherein said recess is formed in a frusto-conical surface of said valve poppet ; the system being characterised in that : said valve poppet is threadably received on threads on said valve housing to facilitate replacement of said valve poppet . wherein said valve poppet is part-spherical about said axis, and wherein said valve seat has a sharp break point selectively in contact with said valve poppet to provide said seal, or wherein said bypass system further comprises a cylindrical seal and wherein the cylindrical seal is received in a recess of said valve poppet such that said seal provides said part-circular portion of said valve poppet , and wherein said recess is formed in a frusto-conical surface of said valve poppet ; the system being characterised in that : said valve poppet is threadably received on threads on said valve housing to facilitate replacement of said valve poppet . <EOS>