clearing out centre manifold stud thread

[JonE]

further inspection shows that all the vertical water 'wells' in the block go down to within 1cm of the top of the valve chest. The ominous centre one does not, so I'm intending pillar drilling down to clear the obstruction. At present, I'm imagining the water jacket in this "shelf" over the top of the valve chest as having the characteristics of perhaps an inch high, and perhaps 1/8" thick, and running the length. Or following Ian's comments and others' findings, is it more complex?