I don't drill into the rocker shaft. I did warn that the clearances were tight. It can be done but it does need a lot of care, shortening of the standard union and of the top nut, which also needs tapering. To drill the hole in the rocker block you need a square bottom drill - I use a milling cutter.
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.