clearing out centre manifold stud thread

[JonE]

I'm just drilling to tap an old 1/4 stud out of the centre manifold position. Its right below a smaller water channel junction into the head (why so small out of interest?) but I've drilled and at present can see there is head stud beyond.. but no apparent pathway for water anywhere. What is best practice here - do I need to remove the head stud too in order to see passage beyond?

Whilst sorting tap to do this, came across one marked "A Nr.2" (that's it) Its about 4.4mm. Do I take it that this could be 2BA? Or something more weird? Its very fine of thread, but no marking to say.