if you use the push and twist caps they are designed to lift under pressure coolant would escape down the tube to a place of your choosing unlike screw caps that vent to the atmosphere

Mgglep where did you get your exhaust headers they look really smart?

exhaust
 

the exhaust is from an MK they are expensive new and they dont fit very well as the angle is wrong luckily i found one on ebay i got the whole exhaust for £100 i then just cut it and adjusted each tube until the angle was good then welded up