Cross link - mounting early tach generator on XJ head

Hello all - perhaps the XK engine forum would have been the correct place to post, but I’m hoping to get some answers on how deep to drill the required mounting holes in the head for mounting the tach generator and also any tips on drilling the cam to mount the tach drive bits. Thanks in advance! Mounting tach gen on late XK (XJ) head

The original heads had the tach drive mounting holes drilled and tapped all the way through into the oil drain-back area…this is the cavity where the cam shaft oil slingers reside…but there are only TWO mounting bolts so I would not worry about making the bottom hole functional since all of the early heads did not have this third(bottom) hole. Of course, getting the swarf out of that area could be problematic, so if you have a bottoming tap, you might avoid drilling all the way through into the oil drain-back area and just use very short bolts…as has been already mentioned.

I did tap a 3/8" fine-thread hole into one of my spare 4.2 cams and I can tell you it was not all that tough to do.