Does anyone know where  you can buy a new double acting thermostat for a 261 engine I have tried in the U.S apparently these keep the engine from develeping hot spots which they say can fry your engine. the one I have doesnt seem to work so I stuck a thermostat from chey 350 and drove around for a while . But I noticed the temp wont come up for a while then it seems to jump up to normal temp.if some one with a little knowledge on these I would much appreciate info thanks Gerry