O.K., I was reading the sale listing for a “Forecast” brand 5.3 cap from one of the major car parts suppliers online (not a Jag specialist, btw). They specifically mention in their “fitment” notes the fact that the Marelli caps are no longer available. Their Forecast brand looks to be very similar to the Facet brand aftermarket cap, and is about the same price ($100.00 or so). Something else mentioned in the listing is that, supposedly, the supplier had been advised “by a Jag technician” that to help avoid the fire issue with aftermarket caps that the plugs should be gapped to .025 instead of .035. First I ever heard of this. Has anyone else? 
The plugs are supposed to be gapped at .025". Period.
Hey, it’s 2019. Is there any way we can 3D-print caps and rotors for the Marelli? I presume fitting the metal terminals into the plastic cap and rotor would be tricky.
If someone were to go this route, I’d suggest changing the OEM design by making both the cap and the rotor about .100" taller, putting the entire .100" toward making the plastic rotor that much thicker directly above the distributor shaft.
It’d also be good to make the cap a bit thicker all around to prevent spark breakouts to the mounting screws, etc.
Page 234 of the Driver’s Handbook for my wife’s 1990 XJ-S convertible (5.3L V12 with Marelli ignition) specifies a spark plug gap of 0.025 inches. That is what I have been using for the past 14 years. Why would anyone do anything else? What possible good could come from setting the spark plug gap at anything other than what Jaguar specified for this car, especially opening it up to 0.035 inches?
I suspect that whoever told you that didn’t know what they were talking about. 0.025 inches is the correct gap to use and 0.035 isn’t.
Paul
I"m wondering if the supplier (btw, this was AutopartsGeek) got this model mixed up with the X-300s somehow (?) IIRC, seems there was some issue with the AJ16 engine’s coil-on-plugs’ coils overheating and going blinky. To help prevent that, it was advised (in a TSB IIRC) that the gap be either increased from .025 to .035 or decreased vice versa (whichever would make coils operate cooler than OEM). 
Here is APG’s fitment note (the original is all in red font, btw):
Notes: w/ Marelli Distributor Original cap is " . Unfortunately Marelli of Italy is no longer producing it. The Jaguar Dealer, as well as the aftermarket can only offer a less substantial version of the original. As per info from a independent Jag repair tech, you can avoid distributor cap failure by doing the following: Gap the spark plugs at .025 instead of .035 - And Always Replace the Ingition Wires, Cap, and Rotor
Or they or their customers could have gotten confused because some the Jaguar 4.2L XK engines and some of the Jaguar 5.3L V12 engines with Lucas ignition gap their spark plugs at 0.035 inches. I have five different models of Jaguars and when working on them I go to the technical data for the one that I am working on at the moment to make sure sure I am doing it correct.
Paul
O.K., Kirby … you’re grounded - no XJS for a week … 
I was just looking at the Book and there is a paragraph in there relating how some owners have found that some of the major auto parts stores were (are?) incorrectly showing, and even “insisting”, that the gap for 5.3 engine plugs to be .035". The paragraph also mentions that gaping the plugs to this .035" incorrect setting contributes to the likelihood of engine fires in the Marelli-ignition engines … 
I have a fairly simple 3D printer in the factory for making small parts. We often need parts in batches of 5 to 50 so that tooling cost is a major concern.
The 3D platen is big enough to make an item dizzy size.
Our printer would take all day to make one, not that it matters much, and the type of plastic used is not really engineering grade stuff.
There are better printers which could use a better plastic.
For other things we need in small quantities we have them made outside, CNC machined from solid material. That gives a good choice of suitable plastics. I have a few Luca dizzies, but not a Marelli one to look at. I would guess you pay around US$100 each for a batch of 10 providing there is nothing in a Marelli requiring special cutting tools.
Another technique we use is low cost resin casting, which is very popular for the hobby and model making market. We have things made this way too, my guess is an economic batch would be 25 and cost US$40 each. Resin casting can give fairly good detail and tolerances, good enough for a dizzy.
Once needing an item looking like it was really injection moulded, although we only needed 100, we found a local toolmaker who specialised in making injection die sets from aluminium. Most toolmakers can’t get it out of their head anybody would only want less than 100,000 pieces, and they stick to making tooling from high quality die steel which is expensive and hard to machine.
The aluminium die is 10 times faster to machine and gives pretty good results. The only drawback is it wears out after 1,000 to 10,000 items and cannot match the really tight tolerance capable with a die steel tool. My guess is tooling for a Marelli would be US$6000 and parts cost US$6 each. That is on the basis of a straightforward cavity die set. There is a fair choice of plastics to be used.
Of course on top of the dizzy moulding you need the posts and other hardware.
All in all I don’t see anybody rushing to make a better aftermarket Marelli dizzy.
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Ooooh, that’s an idea too!
Still thinking about the terminals. I’m envisioning a screw head down inside the spark plug wire socket. Still not sure what it’d screw into, though.
No Jag V12 experience here. Decades ago, a lot of them, I owned a 34 Packard V12 for a very short time. I was a penniless college student. My after class job was at a full service station. Customer and boss could not get the firing order right and start the V12. I squandered my next semester tuition and bought it!!! Gee, I was pretty smart. I got the firing order from a tech at the Packard dealership. Boss perplexed !! It ran. alas, even with the cheap gas and discount, beyond my means… Sold. got my tuition money back and a little more. These old critters not that desirable at the time???
Decades later, another century… Hot Rod of the 40’s project. I wanted a different distributor cap for the flat head V8. Known as the crab style. I found a repro. Cap decent, rotor awful!! Used an old rotor. Just cleaned it up. Ran great…
Relevance. Old stuff can be cleaned up and be better than substandard new stuff.
I have a little tool to clean female towers on distributor
caps. Merely bent coat hanger wire to secure a strip of emery cloth
So, have a good look at the genuine,. but old cap and rotor. Cleaned and any cracks “fixed”.
3d print a crab" style cap for the V12 ???
Carl