[dsraven]

joedoh says it. figure out your system needs and go from there. check the rad top tank for size because that is the reservoir they built into the old stock type rads. anything beyond that would simply puke out on the ground. the rad cap would only have a seal on the end of the cap because it didn't have to seal past the filler neck or seal a system designed to draw coolant back into the rad. if you have a rad with the side tanks then the capacity to store more coolant in the rad is usually less so an overflow reservoir could be used. check your rad tank filler neck for depth, integrity of the seal surface for under the cap lid and overflow hose connection seal against the neck as these are important to seal with a closed system. then check to ensure you have the correct rad cap for the system you will run. the system that will recover excess coolant and draw it back in as needed will require a rad cap with dual sealing gaskets, one on the end of the cap that seals down in the bottom of the filler neck, and one that is right under the rad cap lid which will seal against the top of the filler neck. usually a modern system will hold 15 or 16 psi which means the system will start to get rid of excess coolant or air bubbles when it reaches that pressure. if using a stock heater core be sure to pressure test it to that pressure or it may become your weak link (and leave a mess on the floor of the cab). remember to use a recover/overflow tank that is big enough for your system. otherwise it will overfill, dump the excess on the ground, and leave your system low at some point.