"A Tale of Two Spark Plug (Types)"

Finally got around to r/ring Superblue’s plugs last night, and it was no easy task, in part b/c this time, unlike the first, I decided to use the “extended spark plug wrench” that comes with the V-12s instead of a standard 5/8 socket + long extension. The first time was “no problemo”, as I r/red the valve cover gasket (and half moon seals) at the same time, making “long reach” unnecessary for r/ring the plugs. This time though (and my not having a long extension of the 5/8 handy :blush: ) I had no choice but to use the V-12 wrench. Oddly enough, the Jag factory literature seems to indicate this is the same tool to be used on the 4.0s, although it does not come with one from the factory unlike the V-12s did, at least in some parts of the world.

So, I approached the task, unfamiliar with the V-12 plug wrench (from Superblack’s tool bag, btw). I also kept hearing Palmdude’s words in my head, which were not exactly kind to that tool, from his past postings. :angry: Anyway, I eventually did get the plugs in and out, but feared at one or two points I had maybe damaged or stripped the threads in the cylinder head’s holes :open_mouth: (aren’t plug threads made softer than the cylinder head’s, for that very reason ?) I think what was going on was that the V-12 wrench has like 16 points contact and not the more substantial 8-point of the 5/8 wrenches, allowing for some “slippage” when tightening up (or loosening) the plugs. And then there was the problem of getting the damn socket to let loose of the plugs when it came time to withdraw the wrench after tightening (I see now why the socket part is attached to the rod). Funny thing is that when the plug was finally loose, the socket wouldn’t adhere to them at all - I ended up using a pair of scissors (all I had at the time) to gently snatch the plug and lift it out of the well. :crazy_face: Keep in mind the first time I changed the plugs I used quite a bit of “anti-seize” on the replacements. This time I used maybe a little less (had only one of those individual use packets this time for a spark plug r/r job). Anyway, this time I put in the OEM type Champion copper (plus) RC9YCs, after seeing several posters favor same on an earlier thread about the recommended plugs for our XJSes. The ones I took out from my first changing, less than 5K miles ago, were Champion 3344s, which are platinum. Before that, with about 20K miles on them, I removed the ones the PO had in her, which were NGK BKR6ESs, which are also platinum. Here are my observations as to the condition of these last two types upon removal:

1- General condition - both the NGK and Champions appeared in good/normal condition - not overheated (i.e. damaged, “baked”, melted) nor underheated (i.e. sooty, oily). Insulator near tip in both is white with very slight “reddish” tint which = fuel/additive deposits. The heat range for both types of plugs, btw, is a “9”, btw, which are fairly cool. As to the two, IMHO the NGKs appeared in less “used” condition, despite having 4x the mileage on them of the Champions;

2- “Coronal ring”. This is something there is a TSB out there somewhere on (Motorman?). The TSB advises techs that the appearance of this brown ring around the base of the insulator is normal and nothing to be concerned about, apparently caused by the direct proximity of the high discharge electricity to that part of the insulator, or some such. The Champions appeared to have a bit more of this ring than did the NGKs, the latter’s being quite slight.

The plugs I put in this time, the Champion RC9YCs, not only being copper rather than platinum, have a noticeably hotter heat rating than the other two types, being a “6”. It will be interesting to see how the condition of these plugs compare to the other two when it comes time to r/r them, esp. given Superblue’s issue of worn exhaust valve stem seals (i.e. blue smoke often on start up) - will the hotter heat range of the coppers lessen this condition and/or keep the plugs “cleaner”?. I do find it curious that on some vendor’s websites, on which I had my Jag’s info. entered, the seller advised that neither of the earlier types of plugs “fit your '94 XJS 4.0”. Not sure if they were referring to size, as the thread and hex measurements are pretty much identical to those of the Champion coppers, or the fact that they are platinum vs. copper. ? IIRC, some of our members reported that copper should be used in our XJSes and NOT platinum, based on their own experiences. btw, so far, I can’t really tell any difference as to running with the coppers, but my MPG does seem to have increased a bit. :+1:

(aren’t plug threads made softer than the cylinder head’s, for that very reason ?)
Erm steel threads V aluminium, generally steel will do Ali every time.

And I take it the threads in the head are the aluminum ones? :open_mouth: Why in the world don’t they make plug threads softer than aluminum, then? Or is that not possible due to the heat and high pressure constraints? Oh well, I guess in the event the head threads do get crossed or stripped, they have those special Heliocoil-type repair gizmos to correct the situation easily and inexpensively (vs. replacing the head) … :relieved:

I wonder why they would make the threads in the head from aluminium. Can’t be because the head is aluminium. They should have machined the heads from cold rolled steel for extra durability.
The spark plug can’t be soft, it would disintegrate and then goodbye. The threads usually live much longer than the car, and you put the antiseize there for a reason, right?

Good point - I don’t know why the general shift to aluminum cylinder heads in recent years (which I would think would make them also even more susceptible to warpage in an overheating situation vs. steel :grimacing: ). I guess it’s just another “weight saving” measure … ? :confused:

Would you believe that your engine is mostly aluminium, and for the better?


I find the suggested method of putting a rubber hose tightly around the plug tip (where boot goes) and screwing in by hand avoids any possible stripping. I also ALWAYS spray carb cleaner down plug holes to clean threads best i can (first making sure no foreign objects, like a screw, are sitting near by) I now use a thread chaser on my V12 plug holes, because you SHOULD be able to screw the plugs in by hand all the way. Dirty threads can make you think the plug is seated, when it’s not. Then i just give a wee torque, like 1/16th turn.

And i always count, i measured about 10 3/4 turns (full revolutions) to seat plug.

And although Champion is the original suggested plug, The Book and many here go with NGK. Including me. Champion has lost a bit of its reputation, and NGK has an excellent one. Japanese tech.

One thing I learned from doing my “research” on the job was that apparently the V-12 uses tapered plugs, requiring only 1/8 turn to seat after making contact. The 4.0 plugs are “flat” plugs (i.e. with sealing washers). Supposedly they require 1/2 turn, although it seemed like some of my plugs wouldn’t turn tighter than 1/4 or so turn. :grimacing: I didn’t dare try to force them past that point, due to “strippage” concerns.

How so (other than less weight)? :confused: I just remember aluminum engines/blocks/heads back in the '70s generally being problematic, with oil leaks, etc. The cheaper cars tended to have them, it seemed. :angry: But then maybe technology has improved since then ?

Yeah, the V12 is the first car I’ve had with tapered plugs. I think my sticker on the hood said 1/16th turn. 1/8th is quite a lot. There is also a torque of about 8ft lbs, 1/16th of a turn is plenty.

That’s why it must be so clean there? The taper is making a seal with the head. Any dirt or junk in there, and you’ve got a combustion leak? I once had a plug not fully seated, and I may have been getting a slight misfire from that cylinder.

The problem with flat plugs, supposedly you’re not to re-use the crush washer. I’ve re-used mine without any issues, but I could see if I removed/replaced plug several times (checking gap, compression, etc), the washer may be deformed and not form a good seal. So for me, I re-use just once. Same with oil plug, I can usually re-use, but they are so cheap, I just buy several at a time now.

And that’s very wise advice, Gregmatic. Usually how I do it, but with the plugs on the 4.0s so down deep in the wells I don’t think that method can be effectively carried out. ? All I could do was pretty much carefully drop the new plugs down into the wells and wiggle the socket a bit on them to (hopefully) get them started correctly by feel. :pray: At first I tried inserting the new plug into the socket and then quickly pushing it down the well, but when I tried that the plug would always fall out of the socket before making it to the hole. :angry: One feature I noticed that Jag left off the V-12 extended socket was the usual rubber insert to grip the plug until it’s in place. :slightly_frowning_face:

I wondered whether NGK makes a copper plug for the 4.0, but didn’t go down that path since I was intent on using the OEM Champion coppers. If they do, I might give them a try on the next change, depending on how things work out with the Champions. :thinking:

That’s why when I do plugs on my V12, I just go ahead and remove the throttle pedestal/coil. (I already have the A/C compressor removed, so mine is much easier). Putting it back on you have to readjust throttle levers, but that’s not too bad. Then it’s so easy to get to all the plugs by hand. It’s also miles easier getting the actual wires on the plugs by hand. It’s too easy to ruin a wire trying to muscle it off with pliers, which I have done. And it’s so much easier feeling the ‘click’ when pushing them on by hand.

the NGK plug (2238 tr5) we use on our V12s is actually nickel. And 5 coppers make a nickel, so it must be better! :wink:

You read my mind on that one, Gregmatic. After a bit of paranoia over whether I might have “misinstalled” one or two of the plugs, I thought about pulling them out and reinstalling them, but then wondered whether the washers would be good for a second go. :grimacing: I know that’s a big no no with copper washers, but I think the (steel?) ones don’t have that “deformity” issue. Anyone here know?

I think you’re ok pulling them out and reinstalling them. Just make sure you put the required torque on the plug, even if you think you’re turning too hard. The washer is taking that extra force, not the threads. If you go over the required torque, then your threads are in trouble.

My Volvo with an aluminum block requires 18 ft lbs of torque for plugs. With anti-seize on them, I drop that down to 14 ftlbs.

LOL. During all this I happened to run across NAPA’s website (I have never bought parts from them before, btw) and they have a chart on their spark plug page that claims longevity for the different types (supposedly) … IIRC, platinums last 2x longer than coppers, double platinums last 3x longer than coppers, and iridiums last 4x longer than coppers. Or something like that . :crazy_face: I think that’s a bit misleading tor someone not in the know who goes there to buy plugs, in that they may end up putting, e.g. iridiums in an older Jag like ours (which is a no no), thinking only of the saving of time, trouble and $$ in the future. btw, NAPA also had another category of plugs on there, that I have never heard of. Supposedly they last even longer than iridiums, but of course cost accordingly :money_with_wings: . Must be the latest plug technology.

Yeah, I’ve given up on trying longer lasting plugs. Some engines just need a specific spark plug.

My 97 Volvo Turbo, it ONLY runs well on Volvo OEM plugs (I think they’re platinum). I tried iridium, and within 6 weeks, it started misfiring very badly. I once tried copper, and it ran fine for about 6 months, but then I noticed it idled a bit rough. So back to Volvo OEM, car runs perfect.

Another issue I had on the Volvo with non-OEM plugs, if I had to move the car only running it for 15 seconds, next time I started it, would idle TERRIBLE for 20 seconds. The non-oem plugs apparently got sooted up a bit on a cold start, and need about a minute or two to burn off. The OEM plugs may be better and not getting sooted up when cold?

Same with the Jaguar. Reading the archives, it seemed that messing with ‘better’ plugs caused issues for a lot of people. You’ll probably be fine with your Champion coppers, they’ll last fairly long, but the NGK nickels have such a good rep running on our V12s, having read through 10 years of archives. They’re also cheap ($2 each). Win Win.