Yes I will keep an eye on it, going to check it at the weekend after the OH has used it for work.
I was surprised how far down in the hole the thread started, the hole was full of black crud so no wonder the bolt sheared off as the engine was cold when I tried to remove it (the leakoff was so bad it refused to start)
With the drilling it was SKILL

(no just a lot of luck) to be able to drill that length of bolt right down the middle, a decent bit helped big time, it is still as sharp as it was new as well.