99 XJ8, I have seen both .39 and .52 as the spark plug gaps. I am trying to quell some hopefully upstream problems.
Which is it, enquiring minds want to know ?

 

According to JTIS the gap specifications for a '99 are:

Normally Aspirated 1.0 - 1.1mm (0.039 - 0.043 inches)

Supercharged 1.2 - 1.3mm (0.047 - 0.051 inches)

 

Looking back,it was .43 not .39. Many source are quoting .52 but I will go with what the fine folks at the Jag forum.
Thanks

 

TSB 303-29

DATE
9/98

V8 XJ Series/XK8

MODEL
1997 MY-ON XK8 Range
1998 MY-ON V8 XJ Series

VIN
001001-ON
812256-ON

Idle Quality - Spark Plugs - Revised Gaps

Issue:

The specification for spark plug gaps on all V8 engines has been revised to improve engine idling quality.

Action:

When installing a set of spark plugs check, and if necessary, reset the gaps to 1.3 mm (0.052 inch) before installation.

This policy applies only when a set of spark plugs is replaced at a routine service interval, or as the result of a customer complaint. Do not remove or replace existing spark plugs just to increase the gap.

SERVICE INTERVAL - V8 ENGINES

Normally aspirated 100,000 miles
Supercharged 60,000 miles
Warranty Information:

This Technical Bulletin is issued for information only.

 

That TSB would apply to the AJ26 engine. Does it apply to the '99 AJ27 in question?

 

yes