Just want to bring to attention some very wise words from Andy about drilling an over size hole in your core in case anyone missed it fantastic advice below....
FROM ANDY
The lesson is to always make an epoxy plug by tearing out the balsa core with an Allen key in a drill then filling with epoxy so that the balsa is completely separated from the hole. Every time you put a fitting on the boat.

I agree over sizing the hole and filling with epoxy is a good technique. Im a bit perplexed on the use of the Allen key though. Why not just an ordinary drill or spade bit?

Yeah Toph. You are using the first small hole to gain access to the core and then you dig out the core for the surrounding 5 - 10 mm and fill with epoxy. You then drill the size hole you actually want and get left with a nice solid doughnut of hard stuff to tension your fixin' down onto...