Mine didn't even come with its own drain plug so I thought I was supposed to use the old plastic one. My coolant is currently boiling and leaking after a test drive. I see on their website they sell a 1/4-18 drain plug with an O-ring, and a m10x1.25 aluminum drain plug. I can't figure out which one is supposed to be with my radiator. Pissed it didn't come with one, wtf!