The RN14YC plugs that I bought in response to this thread - to smooth out my rough hot idle - do not fit my '01 XJ. They require the large size socket and I cannot physically install them in the XJ as the large size socket will not slip down inside the recess in the motor.
Is this particular to the late model XJs?
I'm glad that the autolite 3924 are working good for those who have gone to them, and work well for overall well running 4.0 motors that have a hot idle "miss" due to reasons i've run in the thread.
as for the RN14YC, they are spec for 4.2 and early 4.0 amc/renix motors.
tmk, the 3924 autolites will fit all the heads, and am not too shure about the champion diameter size of socket needed.
the 3924 are the old school skinny socket, the champion truck "fatties" use a bigger socket. (same thread pitch and thread total length, of course)
are you shure you have the right thread size and length? compare both the RN14 and the 3924 autolites to see what i'm talking about.
Here's another tip: IF the autolites 3924 are running well but when you pull them they are super clean....go down another heat range step. in the autolites, it's the last two numbers of the 4 digit plugs that as you go down one number, it's one step "colder" in how the plug runs. that is, it moves heat quicker from the tip of the plug to the "colder" cylinder head.
in that case, you could go to 3923, but this will depend on how much air in/ exhaust out upgrades you've done, such as a high flow cat/ muffler / cold air intake/ bored throttle body where this can be done.