Will a Fluidyne radiator 1.1 cap on a Koyo aluminum radiator work properly?

I bought my koyo used from a member here on HT, but he sold it to me with a fluidyne cap, now that i finally got it running, i'm having boiling coolant in the overflow tank. i havent had the time to pick up a koyo cap or equivalent yet.

any advice is appreciated.

 

bah it makes no difference i bought a koyo cap, same problems..