I have here a 4.2l XK, 7l block from a series 1 XJ6. I also have a fully rebuilt cylinder head from a 1966 420 Jaguar, which is slightly shorter than the 7l block, and omits the two coolant ports directly behind the cylinder head at the rear of the engine.
I have heard of these being interchangeable, with the holes on either the block or the head being filled to block coolant.
My question is, how is this done in practice?
I do not want to weld it, I am thinking tapping a very shallow thread with a grub screw and a polyurethane fill over would do it. I still have the 7l cylinder head, but it’s not in great shape and I have a gleaming 420 head here. Ideally I will swap in the 420 head, rebuilt the 7l and put it away for a rainy day.