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I always wondered if on the stock radiator of the roadrunner you want it filled to the top or leave it a little low ( like an inch below the cap).


From the Service Manual page 7-5 under Installation: "(3) Fill cooling system to 1 1/4" below the filler neck seat with water and rust resistor or water and anti-freeze, as required. After warm-up, re-check coolent level.



Which are fine instructions for a cooling system without an overflow/coolant recovery bottle (which was how Stu's vintage RR was originally delivered). The intent was to provide some headroom in the radiator for the expansion as things heated up, without puking some out onto the ground.

As stated before, with the addition of an overflow bottle, radiators should be filled to the top, with the "headroom" essentially being between the cold and hot lines on the overflow bottle.

To make it even simpler, if you start with a properly constructed overflow system and the radiator anywhere in the neighborhood of filled, a few cycles of going from a cold system to a hot system and then back to a cold system will take care of the level inside the radiator, provided you watch the level in the overflow bottle and adjust things there.

Down to just a blue car now.