Kirby Palm's '83 XJ-S came with an old-fashioned push-and-turn style fuel filler with an aftermarket (and chincey) cap on it.  Also, the '83 doesn't have a lock on the door; the only way to make the filler secure (other than refitting the later style lockable door and repainting to match) is to install a locking gas cap.  Jaguar offers such a locking gas cap, CAC3565, but interestingly it is marked "Not USA"; apparently it doesn't meet EPA emissions regulations.

Palm purchased a generic locking cap.  It fit the filler pipe just fine, but it sat a little tall and the door wouldn't fully close.  This could be fixed by chopping on the brand new cap, but Palm decided not to go that route and returned the cap.

In hindsight, there is another idea that might work.  First, here's what the OEM filler pipe for the '83 XJ-S coupe looks like:

Yeah, it's rusty; fine British quality materials!  The little tang welded to the side is supposed to engage a tab on the car and prevent rotation as the cap is turned, but the tab was missing on Palm's car.  It's totally unnecessary anyway.

Note that the filler system for an XJ-S convertible is totally different, as is the filler for the Cabriolet and any other model Jaguar; this page is only applicable for the XJ-S coupe.

The smaller end of the filler pipe fits into the opening on the tank.  There is a really fat O-ring at this connection.  There is also a large hose that clamps onto the outside of the fitting on the tank and onto the large diameter portion of this pipe.  Why Jaguar decided on such a belt-and-suspenders attitude towards sealing is unknown; most other cars have only the clamped hose, no O-ring necessary.  On Palm's car, apparently the O-ring wasn't sealing anyway and the hose was the primary seal.

So, back to the idea that might allow an aftermarket locking gas cap to fit:  Loosen the clamp on the large hose connection to the tank itself and push the filler pipe a little farther into the car and retighten.  If it won't go in far enough, you might pull it out, cut a little off the small end, and try again.  The very tip of the small end may be bottoming in the fitting on the tank, so cutting it off will allow it to insert farther.

If you just don't like the OEM filler, Walter Acker IV came up with an idea:  replace the filler pipe entirely with a later-style turn-until-it-clicks filler scrounged from a junkyard.  Palm decided to tackle this mod while having the tank out for other work.  The following pictures illustrate one way to go about it.

Acker says, "It seems that most all the screw-on pipes are of the same dimension so that you can probably use almost any brand of screw-on filler pipe."  Palm perused his local junkyard and decided upon the filler pipe from a Subaru.  Just about every Subaru in the junkyard had exactly the same type filler pipe (except for the really old ones with the push-and-turn type), and one of the advantages to choosing the Subaru is that it's not too difficult to remove.  There is a locking door over the cap, but if the remote release inside the car doesn't work or you can't get into the car, you can simply pry the fuel filler door open and push the plastic latch back without even scratching the bodywork.  There are three Phillips screws around the filler, and then you can reach around into the right rear wheel well and remove three screws (10mm heads) to remove a sheet metal cover.  Under the cover, the filler pipe connects to a small vent hose and the larger filler hose.  The smaller hose has a spring clamp, and the larger hose has a worm-screw clamp.  Undo these, and the filler pipe comes right out.  Here's what you end up with:

Note that there isn't a spot of rust anywhere on this part; what you see in the picture is undercoating.  That isn't what keeps it from rusting, though; the entire filler pipe appears to be plated with something.

Now, here's what Palm did to it:

That's what you get when you cut it off just above the vent pipe connection, and cut/grind the mounting flange away.

You could go ahead and remove the little spring-loaded door.  It is required by the EPA to make it difficult to put leaded gasoline into a car that requires unleaded.  In other words, it's a relic; leaded fuel hasn't been available in the US for years.  Promise yourself that you'll be a good boy and only put unleaded fuel in the car, and the little trap door becomes superfluous.

It's done, folks.  That thing will install right where the OEM filler came out.  The outside diameter of this pipe is about 3mm smaller than the OEM filler, but that's close enough that the large diameter hose will clamp right down on it.  The large hose, firmly clamped at both ends, holds the filler securely enough to open and close the turn-until-it-clicks cap; there is no need for further anti-rotation features.  The O-ring can be simply removed and tossed, or you can just leave it in there.  The rubber grommet that sealed the OEM filler pipe to the bodywork will fit around this pipe just as well, actually positioning the opening a little deeper into the filler recess on the car -- just in case the aftermarket locking filler cap is a little taller than the Subaru cap.

Acker has other ideas on how to do this mod.  Apparently his method is to cut the top off the OEM filler pipe and have the chunk of the junkyard filler brazed into it.  That'll work, too.  Since the OD of the Subaru pipe is about 3mm smaller than the OEM pipe, it might just fit perfectly inside the cut-off OEM pipe, making brazing neat and easy.

Acker speaks to the benefits of the turn-until-it-clicks style filler:  "One thing that I like is the fact that when you go to a parts store they have a new style of gas cap which is a pressure release type so that you can release pressure buildup when removing the cap for fuel filling."  Of course, if you have any significant pressure buildup, you probably should look into your tank vent system.

Acker notes that some areas with emissions inspections may scoff at this mod.  That'd be kinda funny, since the turn-until-it-clicks filler is far superior to the OEM filler from an emissions standpoint.  But inspectors often don't care if the car pollutes or not; they only care if the emission controls have been altered, and replacing the filler constitutes an alteration.