When the radiator is mounted below an engine’s water outlet, air bubbles can get in the cooling system. That’s when you need an expansion tank, and the tank gets mounted higher in the chassis and has the radiator pressure cap on it. Jeg’s has some new ones that look old-time and cool. We like the stamped aluminum ones best, but billet is also sold.
Here are the part numbers:
511055, billet, natural
511056, billet, polished
511050, stamped,natural
511051, stamped, polished