The AJ26 (and AJ26.5) engines have the 1.2/1.3 gap specification. The newer AJ27 (and 4.2) have a different coil pack setup, and for these with the iridium plugs the specification is 0.9/1.0, especially for the supercharger cars I would say. A bigger gap can be harder on the coils as it takes more energy for them to fire, but am no expert on this area....
Do you know what is the correct gap for a 2004 Jag XJ8 ?
Hi rf69,
I have moved your post from the forum for the X308 to the forum for the X350, which is the Jaguar factory project code for your 2004 XJ8.
The correct spark plug for both the normally aspirated AJ33 and AJ34 and supercharged AJ33S and AJ34S 4.2L engines is Jaguar part number AJ8 4575, which is made by NGK and is their part number IFR5N10 (7866). This is an iridium electrode plug and the gap specified by Jaguar is 0.9mm-1.0mm (0.035-0.039 in.).
Iridium electrode plugs are gaped by the mfg, no attempt should be made to adjust the gap, as damage to the thin wire electrode could occur.
I have found that the factory gaps are not always correct, sometimes out of range by as much as 0.3mm. So I always check them carefully with a wire-style gauge and adjust the ground electrode if necessary. As long as you don't put any stress on the iridium tip of the center electrode there is no risk of damaging the plug.
While NGK recommends against adjusting the gaps of precious metal plugs, they concede that it may be necessary and is possible. See the last paragraph of this snip from their website:
I highly recommend that you check the gap of every single-ground-electrode* spark plug prior to installation. Benny Hill taught us what happens when we assume.
*(it is not feasible to measure the gap on some multi-ground-electrode plugs, such as those used in some BMW engines, but as far as I can recall, most if not all Jaguar engines call for single-ground-electrode plugs).
