Hello Everyone,
opinions about what spark plugs to buy, how to set them, how you can't set them, will they ever seize, and what the gap should be set for varies so much as I have found going through the threads. As an engineer the one authoritative source is the designing application engineer for the product...that should be your first source and possibly the only source.

I will very briefly show below examples without names included how a beginner working on their car (e.g. 1999 XJ8L VDP) needs to understand that everyone means well with no malicious intent providing free advise:

1. Spark Plug Gap: Determined by the output generator voltage, flash point, and internal chamber pressure of vessel. I found answers for the above mentioned car's spark plug gap from .039-.052.

Answer: .035-.039

2. Spark Plug Gap Adjustment: Answers from auto supply counter people were even more surprising. I found answers saying the spark plugs are "pre-gapped", can't be adjusted, use graduated feeler gauges, needle-nose pliers will work. I heard people say use a NGK BKR5EIX-11 with a default gap of .052...whaaaaaaaat????

Answer: No spark plug is "pre-gapped" for any vehicle unless the part has been modified by the vendor for an OEM contract. You may find a spark plug that it's default gap is close to your MANUFACTURERS RECOMMENDED LABORATORY SPECIFICATIONS. But ALL spark plugs should be verified that the gap is set in the range of your application. Select the Spark Plug for your maintenance interval that you feel comfortable with a default gap close to your requirement. DO NOT ADJUST A GAP MORE THAN .008 DIFFERENT THAN THE DEFAULT GAP OUT OF THE BOX. The Ground and Center Electrode could become misaligned much like an Oil Burner electrode causing delayed ignition. Use a wire loop spark plug gap tool with 4-grounding electrode adjustment areas. Make sure the adjustment edges are stronger than the spark plug's grounding electrode, or you will experience the "Superman" effect and the only thing bending will be the metal edges of the tool. Recommended spark plug for 40-50K maintenance interval would be the NGK BKR5EIX Iridium with a out-of-the-box default gap of .031. Adjust the ground electrode as needed. Be careful not to crack the white ceramic dielectric or bend the center electrode's tip.

Last reason I picked this product was due to the lower resistance to electron flow with Iridium VS platinum. This will reduce the load (work) to your electrical generation source so it can deliver a sufficient spark causing the needed and ever welcome "cylinder event".


3. Applying anti-seize compound to the threads: Told that the spark plug would have problems being removed.

Answer: Don't do it....unless you have a steel plug surface being thread. These new plugs are "plated" so this very reason will not occur. A second reason not to use this on your expensive motor asks you the question if you have ever used piping thread sealant and later removed the fitting to see sealant inside the fitting? The sealant compound with excessive heat will not be a welcome foreign body inside your cylinder, and as it breaks down in that tropical environment all types of byproducts will be produced.

I myself have been mentored by many on this forum and understand and acknowledge that these other individuals have much more experience than I will ever have. I only ask that you consider taking time to validate and verify the solution before taking action on these expensive cars.

<<<Mathew>>>

 

I'm about to do a valve cover gasket replacement on my 99VDP with 155k on it, and wanted to replace spark plugs... AFAIK, they're original, and even though I'm not getting any signs of misfire or anything like that, I'd still like the assurance that when I start having problems, at least I know plugs are not the issue. You referenced the NGK Iridium ( BKR5EIX-11) and Advanced Auto Parts got them, but according to their site (link) they have a gap of 0.052. You say the gap should be in the 0.035~039 range. So I guess I'll have to adjust, unless there is another model with smaller gap that's the same fit...?
Either way, great info. I'm glad you shared.
Thanks!

 

Two things gleaned in the past from NGK literature:

1. the allowable gap adjustment is +/- 0.008" from as delivered gap

2. the -11 suffix in your reference means 1.1mm gap, there is a listing available without the -11 suffix

However, the NGK BKR6EGP is listed as having a 0.052" gap in the NGK literature, but every single one out of a set of 8 actually measured 0.034" gap. So, it wasn't that one of them was dropped. This is the way it was manufactured.

 

Platinum plugs do last a long time -- its amazing how long they last and as you stated even with 155k they are doing the job. As plugs age and the gap increases the ignition system works a little harder to provide the spark -- So you can stress the ignition system slightly. If you read the literature from the spark-plug manufacturers they admit about a 2% maximum difference in the engines performance with a new set.


Personally -- I would look in the owners manual and get that plug. I have noticed that some web sites have the wrong plug listed for the later models - so just make sure you get the correct plug.

 

I would suggest using the JTIS available on this site. It is quite specific as to what plug to use, what gap to use and what anti-seize compound to use. Read and follow the Jaguar instructions.

 

Here is some old Jaguar info

bob gauff

 

I appreciate your kindness and asking this question. I would recommend using the following spark plug in place of the -11...NGK BKR5EIX Iridium because the spark gap set in the manufacturing process will allow you to adjust the electrode in the allowable range of movement.

This recommendation is coming from a person that owns two XJ-8 Jaguars and I have spoken extensively with application engineers about this selection. The car owners manual written in 1999 does not have the capability of self-creating addendums notifying each owner of new technology advancements and part replacement improvements.

It only matters if it can produce the proper spark "event" reliably with the voltage/amperage provided, in the designed location of the cylinder for combustion. How many times do we improve technology in our cars without utilizing the original legacy technology of the past? Someday the original make and model parts will be obsolete that are listed in the owners manual.

I recommend that you challenge any recommendation (e.g. using anti-seize thread compound). Really, if you understand the recent developments in technology with metallurgy or speak with anyone technically knowledgeable you will be spoken this word......."plated". That simple engineering technique was not commonly used on spark plugs when the owner's manual was written. Do we ignore technology and revert to legacy instructions based on legacy knowledge, standards, and manufacturing processes?

<<<Mathew>>>

 

Then you should read the attachments provided by Motorcarman.

Your "recommendation" for sparkplug gap is not in agreement with the Jaguar recommendation in the TSB. The use of the -11 suffixed BKR5EIX stems from the gap recommended in the TSB.

As for the use of anti-seize compounds, that remains an open question. Certainly plated plugs have been in circulation well before the model year of the cars in question. The only caveat is that the installer understand the need to reduce the torque figure when anti-seize is used.

 

Quick update...
Just replaced all 8 spark plugs with the NGK BKR5EIX-11 kind. All of them out of the box have a gap of 0.040. The spark plugs I pulled out were original NGK's (didnt get the model #), and the gap on those was ranging from 0.060 to 0.065 (!!!). If the original spark plugs came with a 0.052 gap (Jaguar recomended), than it's a lot of burn-off... Either way, I went ahead and installed new plugs with the 0.040 gap. Less stress on the coils, and after a nice test drive, I see/feel no ill effects...
All good..

 

Most of the plugs I thought came from the plug manufacturer with a rotational instruction after contact, 1/4-1/2 turn as I recall. I presume this is due to the compression washer that is installed and also why they should not be reused.

Sounds like reasonable instructions to me, but would like to know the thoughts of others regarding fitment to a torque value versus a rotational value.

Is the torque value used in the case the plugs are removed and replaced, and in that case, the plug manufacturers instruction for a rotational amount would no longer be valid?

 

Dont have a torque wrench (probably should invest in one based on how much work I've done on the car by myself) but you can definitely feel when you "hit" the compression washer. Three 1/4 turns compress it fully (you feel it too), and then I applied about 75% of a 1/4 turn (damn math) to tighten, and it felt very secure at that point.. Will see...

 

Hello Everyone,
I see more opportunity to add content clarity about Anti-Seize compounds and plating comments.

Well to answer the comment about spark plug "plating"... No, it has never been the normal mode of manufacturing since dinosaurs roamed the earth to have plating done on spark plugs.

In fact, NGK ONLY STARTED PLATING IN DECEMBER OF 1999. Well past the printing of the 1999 Jaguar Vandan Plas XJ8 manual. So no, the plating never was available at the time the manual was written as other have previously written in the thread.


Hopefully that thread has now been put aside.

Next comment to clarify: ANTI-SEIZE COMPOUND USE

The NGK plating used is Trivalent Chromate not being a heavy metal and more environmentally friendly that Hexavalent Chromate. You never use any anti-seize compounds on these coatings....Why you ask? Well simply when used they actually present false torque readings. These coatings make it so anti-seize compounds are not needed, nor should ever be used.

I appreciate the opportunity to clarify this thread and hope the engineering misstatements are done.

Sincerely,

<<<Mathew>>>

 

I appreciate your comments and have detailed my response below. The Spark Plug Gap used was a center baseline adjustment based on the Jaguar CD and Jaguar application engineers that I spoke with. The Anti-seize reply is listed below as is the plating response, Sir.

Thank you

<<<Mathew>>>

 

Just a note based on my experience........

Iridium plugs, wonderful invention. Put 'em in and forget 'em. I have "fixed" so many Jag V8's by doing nothing more than removing platinum, surface gap or "split fire" spark plugs.


Just my opinion, everybody's got one.

Cheers,

 

ouch ...

should be about 1/2 turn after seating at finger tight on a new plug, 1/6 turn on a used plug

 

No.

It simply means that the operator must understand the need to adjust the torque spec.

Torque wrenches are inferior substitutes for reading bolt stretch used when it is impossible to use a micrometer.

Torque angle is a reliable second alternative to reading stretch because for a given pitch the fastener elastic yield can be calculated with certainty.

The gasket is the wildcard.

Given the above, the "alternative" spec (NGK) of 1/2 to 3/4 turn after finger tight seems to be more reliable because it is a torque angle spec.

Lubricated threads make it easier to locate a consistent "finger tight" starting point on used threads.

It so happens that for a set of lubed new NGK plugs with the proper allowance for lube, using a *freshly* calibrated torque wrench, placed *all* the plugs at 5/8 of a turn from finger tight turned only by the ratchet extension.

With that knowledge, an operator can dispense with a torque wrench altogether.

So ...

-- proper torque can be achieved in a number of ways

-- proper lubrication aids damage free future removal

Furthermore ...

FORD has circulated a TSB regarding sparkplug removal on their aluminum head V8 engines. It specifies a maximum removal torque of 39 ft/lb. It recommends unfastening by 1/4 turn, spraying a penetrant such as brake cleaner down the sparkplug wells, and letting it sit at least 30 minutes. Repeat as required overnight. Work plug back and forth. Cold engine.

cause? multiple incidents of pulled threads requiring head replacement. it is so common that there are engine specific thread repair kits on the market.

So ...

The use of lube seems like a good *practical* idea. This is especially true given the extended service interval.

Don't expect NGK to be stepping up to the plate and paying for stripped thread repair anytime soon.

 

I'm little confused... Everywhere I find information that it's better to put iridium instead of platinum spark plugs on XJ8 engines.

Ok... but I'm with 3.2 V8 engine (AJ26), manufacture year - 1998. And all the online specification says - USE NGK PLATINUM...

So... after all - what is the best choice for my engine?! And is the NGK BKR5EIX-11 the same for 3.2 liters engines?

 

I guess we have to interpolate here. Your car originally had, and specifies what are NGK Double Platinum plugs; all the AJ26's did. All the tech info appears to support the platinum as the only option. However, here in the states, if I go to order the specified AJ26 part, Jaguar supercedes it to the iridium plug we have always used in the AJ27's. Experience shows the iridium is a far better spark plug, even worthy of it's higher cost. The iridium plug is P/N AJ84575 or C2A1535. That would also mean narrowing the gap from 1.2-1.3mm to 0.9-1.0mm.

That's not official, but I'd be comfortable if it were mine.

Cheers,

 

Steve -- very nice information.

What does Jaguar sell for the later V8's ? -- mine will be due for some work next spring and I want to have them on hand.

I don't stray from stock on modern engines - it surprising how the small changes can effect the running.

 

Thank you very much for this information xjrguy! I ordered a set of 8 iridium NGK and as soon as they arrive I'll let you know how my jag is working.

Quote:

Originally Posted by xjrguy

The iridium plug is P/N AJ84575 or C2A1535. That would also mean narrowing the gap from 1.2-1.3mm to 0.9-1.0mm

As much as I understand this part № are the official from Jaguar... I have looked around for a cross-reference and, correct me if I'm wrong, but the NGK part number for the same spark plugs is the mentioned BKR5EIX11 No. 5464. Is that right?

And about the gap... as soon as the spark plugs are here I'll check and refit the gap as you said - to 0.9 - 1.0mm.

Thank you once again.