after searching the forums thoroughly, checking the 09-2009 vehicle specifications book and, drum roll, reading the vehicle care manual that came with the car a sacrifice to the Jaguar gods is in order. add in a Rockauto search.

blew a coil and decided to be thorough and replace its plug just in case. it was a new coil and some shorted wires did it in. all plugs and coils were changed by a trusted indie 10k miles ago.
vehicle care book recommended NGK PFR5G-11 double platinum 2647, gap .040-.043
i find installed the Iridium 1FR5N10 which is also rec'ed on Rockauto chart and here in the forum. the measured gap is .032.
09-2009 vehicle spec book recommends the Iridium gap .035-.039

i have also seen the TSB from 1998 increasing gap to .052, however this was during the AJ26 engine run and is not guaranteed to apply to an AJ27. the 09-2009 vehicle specification book is later and should supersede this earlier tech bulletin.
there is of course the little matter of the vehicle care manual that came with the car saying another thing.
pricing is very similar at Rockauto and interestingly the Iridium is listed as the OEM with a gap of .040 recommended.

I am considering replacing all of the plugs and re gaping them, however there are many different specs around.

on other cars gap can have a major effect on idle quality and misfires. i was getting a little miss at idle, but have the confounding variable of failing ignition coil wires which are now fixed. would simply like to get things right and see what happens.

 

Do not disturb the gap on ANY iridium plug. Purchase the correct iridium plugs and install them right out of the box. Do not attempt to re-gap them - you will do more harm than good....

 

as long as one stays away from the center electrode and checks gap very carefully, there should be no issue. the key with any modern plug is to only move the outer part.

 

Agree on being careful in gapping iridium plugs!

The AJ26 has a different ignition setup compared to the AJ27, so that can't be compared.

For a stock boost XKR with new coils I would say the upper 0.39 ish is indeed a max gap.

If you would have increased the boost, then you may want to consider to go smaller in gap size.

 

On the Denso Website:

2005 Jaguar XKR 4.2 uses Iridium Plug IT20TT: which is Gapped @: 1mm
They call it: The Iridium TT

I was told not to re-Gap it.....

Billy Clyde in Houston

 

Just to add a little something to this thread. I'm doing my plugs now and ordered NGK Iridium IX 5464 plugs, gapped at 0.040. This is what NGK says is correct for my 2005 4.2L XK8. I have seen different gap measures for this engine and model year, from both official and unofficial sources.

The vehicle specs book in the How To section is for MY2004 on, should be applicable. But it says a different plug (4575) which is not on NGK and 0.035 to 0.039 gap.
Gary VanR's reference guide shows NGK IFR5N-10, which is also shown as an option on the NGK website. But Gary says gap 0.046" and again NGK says 0.040".

Not sure what to think, but I guess I will go with NGK. Anyone have a source on a more "authoratative" answer?

 

2006 XK8 currently with 120,650 miles....

I installed a set of NGK IR IFR5N10 iridium plugs in February 2017 at just over 108,000 miles. As is typically recommended for iridium plug installation, I did not attempt to change the gap. Absolutely no issues with these plugs since I installed them....

I ordered them from rockauto.com....

 

I've the same engine and also use IFR5-N10. As they're iridium do not regap or even measure without understanding how easily they're ruined.

 

yes, one must be extremely delicate when making measurements. only use the wire gauge and barely touch the center electrode.
Update, the car runs very well at the .040 gap that i set.