Around the time of the introduction of the H.E. engine, a couple of changes were made to this pipe.  First, a filler neck was added to the right of center, giving the car two filler necks and the owners even more confusion about this cooling system.  Also, a small connection was added to the top of the tee -- and there's more there than meets the eye.  This small tube doesn't merely connect to the tee; it actually passes through the tee all the way downward and ends right at the pump inlet.  This connection is for the air purge system, and the designers clearly wanted all the suction they could muster.  This line draws coolant out of the header tank, which in turn causes air to be sucked into the header tank through other connections.

The OEM coolant crossover pipe is made of thin-walled steel tubing.  As such, it is prone to rusting, especially if the owner fails to maintain the cooling system properly.  Most owners don't notice the rust problem until the pipe develops a hole and loses all the coolant, but it is actually a problem before it gets to that point.  As long as the pipe is rusting, flakes of rust are breaking loose, circulating around in the cooling system, grating at the water pump seal, and eventually contributing to the plugging of the tiny tubes in the radiator.  See the page on cooling system filters for more on that issue.

When the owner is fed up with the rust problems, there are several options.  One is to get a new coolant crossover pipe from Jaguar -- and vow to maintain the coolant better.  Another is to obtain a replacement coolant crossover pipe from Cathouse Spares, who fabricate their pipes from stainless steel.  Cathouse Spares is in Sydney, Australia, which may sound like a long ways from Europe or the U.S. but their prices are more than fair.

Or, you can make something.  Hell, it's only plumbing.  In fact, the carburetted V12's had a system that looked more like plumbing -- a tee, hoses, clamps, etc.  With a little ingenuity, it should be possible to fab something out of, say, copper tubing and fittings.  Well, Tom Bennett took that suggestion and ran with it, and provided a few pictures of the stuff he's come up with.  Note that this stuff isn't actually brazed together yet; it's just slid together for picture-taking.

Where'd you get that tee, Tom?  "These are very standard European fittings and can be bought at any good plumbing supply shop.  The fitting shown is not the one I would have preferred; as you can see in the pic it is a capillary type, i.e. it already
has the solder in the fitting.  The non-capillary type would look better, same as the right angle bends that I used."

That small tube -- did you just drill a hole and insert it?  Do you think brazing will hold it securely enough?  "The original crossover pipe was brazed at this point, so I don't see a problem."

What's with that little brass ring around the tip, Tom?  "On a standard compression joint there is a brass ring which is compressed around the pipe by tightening the nut; we call that ring an "olive".  This pic shows a 28mm olive cut in half to make a pipe end."

Why didn't you merely cut a 1/8" ring off a copper fitting and solder it onto the end of the pipe?  "Well, I had the "olive" to hand; if I had cut a ring from a fitting I waste a fitting!  Also the olive once soldered/brazed in place helps to strengthen the pipe
at the end; an over zealous tightening of a hose clamp at this point could crush the pipe end."

Where'd you get that filler neck, Tom?  "I just walked into my local rad fixing store and asked for it.  If it had not been available I would have used the original."

I see that the line off the side is in the correct place below the radiator cap seat.  Did it come with a connection above the radiator cap seat?  "Both types of filler cap are available, one with a takeoff just above the cap seat and one with no takeoff at all."  Did you add the connection visible in the pic?  "Yes."

"All this stuff was available of the shelf, total cost so far less than $25."

OK, at this point Bennett got to talking with Kirby Palm about the whole thing, and Palm pointed out that the tube sticking out the top of the tee connects to a short piece of hose, bends 90° forward, then connects to a question-mark-shaped tube, which connects to yet another hose, which connects to the header tank.  Palm suggested that, if a 90° elbow were installed on top of the tee pointing forward, it would make the connection a little simpler and neater -- but, better yet, you could install two 90° elbows, the first turning forward and the second turning down to the left, and you could eliminate the question-mark-shaped tube entirely and connect the crossover pipe to the header tank with a simple straight hose and two clamps.

Well, Bennett thought about that, and didn't like it; he thought it'd look hokey.  But then he decided it would look less hokey if the fitting came out the front of the tee in the first place instead of coming out the top and then turning forward.  This, of course, means the first elbow would have to be inside the tee instead of above it.  So, back to the hardware store for more fittings and a new arrangement: