I have a 1980 Z with a 350 just rebuilt to 383 about 5000ks ago. Last weekend after a quick 30min trip I was coasting through town in 2nd when a huge explosion occured and my rad blow about, I checked the rad cap it teast good and so did the head gaskets, any other ideas what would have caused the rad to blow apart?

 

What was the pressure rating of the cap? When testing, did you verify that the cap would release the radiator pressure at the rated pressure?

Original (old) radiator, or replaced at some point in time? Replaced with what material - copper, aluminum, plastic (caps)? Where did the radiator rupture - tube, cap, etc?