My recent purchase

[Rubbond]

My ignorance no doubt but I've never heard of a core plug? The name suggests
it may be to release the sand after the block has been cast. Or maybe a
design feature to prevent the block from cracking if the coolant were to
freeze? Or to assist with flushing the engine?
But in any case, the radiator cap cannot really fail. It's just a
spring-loaded seal that would leak, rather than get stuck. And surely the
radiator core would be the next thing to burst, or the coolant hoses if
cracked.
I have on many occasions had problems with engines overheating (come to
think of it most recently with a vintage outboard engine). It was always the
thermostat, or in the case of the outboard, blocked water channels, but the
steam always came out of the radiator cap. If these cores "pop out" at less
than 15 psi, then might they not just drop out going over a speed bump?
Andrew.