I know this has been debated over and over here, but I have discovered something that might be rather crucial information..
Basically there are 2 different spark plugs type called JIS and ISO. The difference is 2,5mm in height (on the top end-towards the coil)


On ebay some vendors are selling JIS sparkplugs on ebay for my car. I have used their compatibility search engines and ended up with a JIS plug, EVEN though I have now noticed that in the Jag manual the standard plug is an ISO sized plug. ( champion 9C9YCC) Even Britishparts.co.uk are selling JIS sized NGK plugs for my car. I have given my vin number to britishparts and they confirmed that the NGK BCPR6ES ( JIS sized) is correct. How is this possible??


First I ordered these NGK Iridium plugs from Ebay:





According to NGK.com these are JIS sized and NOT correct for my 4.0 X300


This is btw the specifications for the Champion plug ( notice they are ISO) :





I then ordered standard NGK plugs from britishparts assuming they would be ISO.
Turns out they are also JIS height :



Now as I mentioned Britishparts confirmed that these should be correct, but when using the compatibility search engine on NGK.com they are listed as wrong type.
All the alternatives (iridium or standard etc) on NGK.com are all ISO type height plugs.
for instance this Iridium plug is supposed to be the correct ISO plug for my car when using the compatibility search engine on NGK.com:






I am no spark plug expert, so maybe there is an easy explanation for this.
But I have to say I find it strange to read about so many members here having issues with NGK plugs. Could it simply be that they have installed JIS sized plugs instead of ISO?? I guess that would cause some coil issues in the long run..

 

Interesting thread, however can you share with me the threads that you have seen about people having problems with NGK spark plugs? I believe it is quite the opposite. There are many people who have had an issue with Champion and love the NGK plugs.


I believe people have had problems when using iridium plugs, Bosch branded plugs, and perhaps some other make, but not NGK. I think people also seem to think Autolites are ok to use with these engines.


I am using NGKs and have been quite happy with them. When doing my research, I did find that most shops try to pawn off the BR"6" plug for the AJ16 engine, however I found that that plug is a direct replacement for the originally spec'ed RC9YCC plug. The direct replacement for the later spec'ed RC12YCC plug is the NGK BR"5" plugs. Champions run hotter plugs as the number goes "up", NGK is the opposite, the hotter the plug, the "lower" the number.

 

Jaguar issued an AMENDED TSB in 12/98 for the spark plug heat range specs.
The RC9YC were replaced with the hotter RC12YC for ALL AJ16 engines as VEE stated.

bob

 

Yes, I know that. It was basically the purpose of my post. If you insert your car into any website to auto select NGK spark plugs, it will direct you to the NGK "6" series, which is a direct substitute for the originally specified Champion "9" series. You want the "5" series if you're going NGK, to substitute the revised specified Champion "12" series.

For some reason, websites know to direct you to 12 series Champions, but not the proper NGK, or perhaps other brands. (I've never checked the other brands)

 

Thanks Vee
I am also aware of the RC12YC recommendation.
And that is a so called ISO plug.
So WHY the #### are britishparts and a lot of other ebay guys selling a JIS sized plug for the X300? That's the question..

 

I don't see a problem. The coil spring can certainly make up the 2.5mm, or 1/10th of an inch difference in size.

 

Yes.. I think I agree with you. But it still puzzles me a bit. I found this information about it though ;

While the small difference in height will not affect most vehicles (ISO and JIS types can often be interchanged), some vehicles (particularly with direct fire ignition or specially fitted plug caps) MUST use the correct plug standard or a bad contact between plug and cap may be result.