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Core plugs are sealing the casting holes on the cooling system, they usually corrode and leak rather than pop out due to lack of antifreeze.
It could have been corroded and popped out but in light of what has happened I would be checking the thermostat opens and the cooling system for over pressurising
Fixedwheelnut
@mashton
Welcome to the second monthly installment of "mashton has a hole in his engine".
I took the MG out for a spin in the lanes yesterday evening. Nothing extravagant, as I have been driving pretty conservatively after the piston blew.
Accelerating out of a roundabout on my way home, just entering West Wickham, and I heard a loud rattle of metal from under the car. Sounded exactly like I had gone over a small piece of metallic rubbish that I hadn't seen. But worse.
100m later and there is the suggested of smoke but no smell that I can detect. Pull over, bonnet up. Shitting myself.
As most of you know, my knowledge of cars is, shall we say, limited. I was telling around with a torch to find the oil dipstick when I saw a big, slightly ragged, circular hole in the side of the engine. Massive adrenaline shot and huge buyers remorse. Swear a lot and call the RAC.
But, the good (as in not catastrophic) news is that it was a blown core plug. Which I am told is no where near as bad as I thought it was. Put it on the back of a lorry and it is sitting outside the MG doctors, waiting for Monday morning.
What is interesting me now is why it blew and if there is any possibility of an underlying cause with the piston failure. The plug is the rear one, right next to cylinder 4, the one that blew. Is there any root cause that would lead to reduced cooling on that cylinder, or maybe the valves in particular, and subsequently to a blown core plug? I am well beyond my knowledge here, as usual, but it seems that it must have blown out by high pressure steam, which implies that coolant was reaching it. Temperature had been sitting pretty all through the drive.