Seasons greetings to all............
When should the af and serpentine belt be replaced ? Be safe.

Rick

 

I thought it was a chain on the 4.2?

 

Your thinking of the timing chain, I'm speaking of the accesory belt for alternator etc.

 

I change my belts on all my vehicles when they get cracks all the way across all the ribs on the underside in the same spot. Probably too soon, but hey, they're cheap(ish).
For the Air Filter (I use two K&Ns even though lots of people say they mess stuff up, but mine have performed flawlessly forever), I clean mine once a year (maybe 8,000 miles), but they are not even CLOSE to dirty by then. The originals were in for 12,000 when I put in the K&Ns, but those were ALSO nearly perfectly clean. While I won't tell you what to do, that has been my experience. Oh yeah; since you have an XK and not the R, you only have the one filter so therefore maybe it would get just a tad bit dirtier a tad sooner.