MkV Spark Plug specs

Can anyone advise on the correct spark plugs for the 3.5L Mk5 engine?

The service manual specifies L10, and the modern equivalent is the L86C which is a short reach relatively “hot” plug used in many motorcycles.

Does the 3.5L really need a short-reach plug? the cylinder head thread seems much deeper, adequate for a normal plug?

I am currently just using some spare plugs I had in stock from another car, Bosch Super 4 R6, but apparently they are too cold, and I have also been advised they cause problems with old ignition systems, and I should stay with a normal single electrode plug.

According to the Champion Plug guide L90C is the correct plug for the cars.

Multi-electrode plugs improve performance by lightening your wallet.


I also have L90C’s.
I did once fit Bosch NGK BPR6HS but then began to have running issues. Once I changed back to L90C’s issue was gone.

Champion list the cross reference for the L10 as part number 896 which is the Champion L90C Racing Plug, at and at at . I use L10 originals and have collected a personal stash of NOS. Presumably a little patient searching and waiting may find more. If you need any dimensional measurements on an L10, let me know.

I’ve heard numerous fellows in the SS Register say the same thing, Only Champions. It’s a mystery? Yet in a TR6 if you put in Champions it runs poorly until you put in NGK’s! :crazy_face:

I measured the thread depth in the head, and its 22mm - so the 3.5L should therefore be using normal length plugs? According to the plug specs at the spares shop, the L10 and equivalent are short-reach plugs with 13mm threads. That means the plugs are recessed, and part of the head plug thread is exposed to combustion?
I noted Graham Jordan’s comments, and also the comments that some plugs work better than others.
I have ordered NGK BP-4ES with are the same heat range, but 22mm thread. I will fit them today and see.

Any observations on why Jaguar specs a short-thread plug?

Within a proper heat range, Ive never observed a difference in performance between plug manufacturers, in any given engine.

Longevity and reliability? Yes, but that’s a different kettle.