I'm changing the plugs in my 1999 XK8, 4.0L. non-Super Charged Coupe. I'm replacing the plugs with NGK Iridium BKR5EIX-11 plugs. After searching for the proper plug gap, I found recommended plug gaps of .040, .043, .045, and even .050. I called NGK and the NGK tech said the BKR5EIX-11 has a factory gap of .043 but they recommend that it be closed to ,040 in the 4.0L engine. They told me that the -11 at the end of the plug number means a 1.1mm gap which is the same as .043. So, according to NGK, the plug has a factory gap of .043 which should be closed up to .040 for a 1999 Jaguar XK8 non-Super Charged application.

 

The last update I have is the Vehicle Specification Booklet 2004.

 

The correct NGK Iridium spark plugs for your 1999 XK8 are the IFR5N10 - which come with the correct gap already set.

 

^^^^^ this ^^^^^

there are the newer : NGK 96457 FR5BHX Ruthenium HX Plugs

More pricey, and I haven’t tried them yet.

Nothing wrong with using the IFR5N10 plugs. They are quite long lasting and very reliable.

OP, I don’t believe you will find any functional difference between a gap of 0.040” and 0.043”.

The center electrodes on these plugs is very hard and thereby extremely brittle.

if you do monkey around with the plug gap be sure to not put any pressure on it. Your tool should only contact the side electrode.

Z

 

I believe the original spark plugs Jaguar specified for the 1999 X100 were NGK double platinum, Jaguar part number NCA3850EA 1, but they were subsequently updated to the iridium IFR5N10.

The four-digit part number for the NGK IFR5N10 is 7866, which is how some sellers list them.

According to the July 2004 Vehicle Specification Booklet that Bob posted, the gap should be 0.9 - 1.0 mm / 0.035 - 0.039 in.

NGK pre-gaps their iridium plugs, but you should always check them. I find that about 1 in every 8 plugs is pre-gapped a little off.

NGK also states that no anti-seize compound should be applied to its modern plugs with zinc, nickel or trivalent plating on the plug threads, because anti-seize is not required and will lead to over-torquing the plugs.

Cheers,

Don

 

I followed the attached Jaguar TSB with my plugs on my 1997 XK8 when I replaced them many years ago and increased the plug gap from 1.1mm to 1.3mm. I can't say that it made any difference to the idle quality but it's always idled well anyway.

Richard

 

I have followed that TSB on some cars as well, but the TSB is dated 9/1998, so, presumably, it applies to the original specification double-platinum spark plugs.

The Vehicle Specification Booklet that Bob posted is dated 7/2004, so, presumably, the gaps listed apply to the supersession iridium plugs.

 

I installed a new set of the proper NGK IR IFR5N10 spark plugs in my wife's 2006 XK8 in February 2017 at 108,000 miles. By the way, the factory plugs still looked almost brand new....

I checked all 8 gaps on the new plugs. They were right at .040 so I left the gaps alone. The car now has just over 127,000 miles and I have had no issues with these plugs....

Denso and NGK iridium plugs will routinely do 150,000 to 200,000 miles with no issues. In retrospect, I should have left the factory plugs in place....

 

Probably doesn't add anything but here's a newer VSB