Early cylinder head on a late(er) 4.2 XK engine


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.


Tap the two extra hole for the closest pipe thread size, either BSPT or NPT and install a socket head pipe plug. I think the size will be 1/8" or 1/4" pipe size. Iron or brass, either is fine. Make sure the head of the plug is no higher than the face of the block. If it sticks up, just run the tap in about half a turn until it threads tightly just flush. Use pipe dope or Teflon tape on the threads.

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I’ve done this i recall it was 1/4 pipe plugs. Installed a xj6 head on 3.4 block.

Thanks blokes, will go down to the bolt shop this afternoon.