Not sure why you need an overflow bottle, that's certainly your business, but at one time I also thought the same. I replaced the original radiator of my '71 GTO with a Cold Case one. Filled it. Drove it. Almost every time while sitting overnight, a sizable puddle of antifreeze appeared on the floor. Filled it again. Same thing. Thought I then needed an overflow bottle. But then I thought, this car didn't come from the factory with one, at least I don't think it did, so why this problem now?? I posted here and found my problem on this forum. Unbeknownst to me, the radiator was puking out extra antifreeze it didn't need or want. It was actually seeking it's own level of fill. So, once cooled, I removed the cap and saw that it was down about 1.5" or so from the top. Started the motor with the cap off and watched it. Eventually the thermostat opened and the radiator filled up quite quickly. Had to be quick with the cap. I never put anymore coolant in it and it never puked again. Runs at 180-185 all day long. Just sharing my personal experience, Nicholas.