If you fill the radiators up all the way to the neck, a little fluid WILL come
out when you get 'er hot. Gassers like the fluid just over the top of the
slotted tubes inside, not much more. Gives enough room for when the
coolant warms and expands (it expands quite a bit, all fluids do)
I do really like to run the plastic expansion tank that fits around the frame
steering tube, it gives plenty of room for the coolant to expand into. Some
have experienced plastic failures of the tank, though.....YRMV