Bill,
The rc12yc would be a much colder plug I think.
Do they realy use them in the 4.0?
Do you know why they would put such a cold plug in?
Brett
…
Organization: Montclair Jaguar
Sender: owner-modern@jag-lovers.org
Hmm…
I thought Id mention… “whilst” Jaguar originally screwed these
together with Champion RC9YCs… Jaguar now says to use RC12YCs…
BillDate: Wed, 11 Nov 1998 21:52:02 -0800
From: Bill Weismann weismann@ix.netcom.com
To: modern@jag-lovers.org
Subject: Plugs that Jaguar Reccomends…
Hmm…
I thought Id mention… “whilst” Jaguar originally screwed these
together with Champion RC9YCs… Jaguar now says to use RC12YCs…
Bill
Bill,
Can you clarify this? What years?–
–Pete Crosby (pac@atl.hp.com a.k.a. @Pete_Crosby3)
1988 BRG XJ40 in the sunny south (Atlanta, GA, USA)
Hmm…
I thought Id mention… “whilst” Jaguar originally screwed these
together with Champion RC9YCs… Jaguar now says to use RC12YCs…
Bill
Brett Gazdzinski wrote:
Bill,
The rc12yc would be a much colder plug I think.
Do they realy use them in the 4.0?
Do you know why they would put such a cold plug in?
Brett
…
Date: Wed, 11 Nov 1998 21:52:02 -0800
From: Bill Weismann <@Bill_Weismann1>
Organization: Montclair Jaguar
To: modern@jag-lovers.org
Subject: Plugs that Jaguar Reccomends…
Sender: owner-modern@jag-lovers.org
Hmm…
I thought Id mention… “whilst” Jaguar originally screwed these
together with Champion RC9YCs… Jaguar now says to use RC12YCs…
Bill
Brett,
From what I had been told, there were preignition and idle problems with
the cars (And yup… it includes the 4.0s specifically) that the 12s
helped to cure.
I also think its some kind of emissions thing.
bear in mind, this applies to AMERICAN-SPEC Cars! (In case someone in
the UK is reading this!)
Pete Crosby (pac@atl.hp.com) wrote:
Bill,
Can you clarify this? What years?
–
–Pete Crosby (pac@atl.hp.com a.k.a. pac@i3106aa.atl.hp.com)
1988 BRG XJ40 in the sunny south (Atlanta, GA, USA)
Hmm…
I thought Id mention… “whilst” Jaguar originally screwed these
together with Champion RC9YCs… Jaguar now says to use RC12YCs…
Bill
Pete,
Im not sure exactly what years… but I know it covers at LEAST till the
1993MY, and for all XJ40s before that…
I use NGKs anyway tho 
Bill