Spark Plug Choice

I’d be interested to hear suggestions as to suitable plugs for a standard XK120 3.4 litre (+.040") 8:1 engine. I don’t want the type with built-in resistor, as I have the Lucas 78113A suppressor caps which connect on to the plugs. I finally acquired a full set of these, which were an option originally, as I prefer the way they attach to the leads by acorn nuts as on the distributor end of the leads. I really dislike the idea of screwing plug caps into the ends of copper-core leads! Not a very elegant engineering solution, it seems to me.
Any suggestions re suitable plugs welcome, also any thoughts about relative merits of plug makes and types.

Ive had excellent luck with NGKs: IIRC, the number is BP5ES


Had the same idea and I also found 6 Lucas suppressors 78113A (at last). I’m now in the process of fitting these to replace the Champion spark plug caps (see photo), At the same time removing the single suppressor Lucas 78120A in the centre of the distributor cap that so far did the job.
But I opted to use original Champion N5 plugs (not the N5Cu plugs: too modern…) which are still available on shows, autojumbles, etc. My XK 140 runs perfect on these old (new) plugs. So why not stay original as long you can??

Bob K.

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Ill bite: why is having a hidden copper conductor in the plug, “too modern?”

NGK BP5ES in both my 150 & 120. perfect.

Thanks, guys - strong vote for NGK BP5ES, then. I believe I have Champion N5 at the moment.
Ignition leads looking good, Bob! I will be soldering the brass split washers under the acorn nuts for added security. Had to have the plug threads in the head helicoiled, as a couple of plugs were a really sloppy fit.

Got you there, Paul!!!
But in all honesty: for me this hobby is also to experience the “contemporary” technology from when the car was designed and built. If I want to drive a perfect car, I take my Bimmer and not my Jaguar XK. So I try to stay original “as far as possible” which expression at the same time opens a whole new discussion, which I fully accept. And yes, I confess that I’ve used 1950’s “Terry’s Security” hose clamps instead of original “Cheney” clamps on my XK, because I’m not prepared to pay “fancy prices” for a hose clamp. Mechanically they are very close; only the numbering on the Terry’s is different from the Cheney’s. But if I see a new Champion N5 for the same price as a Champion N5-Cu or an NGK, you know what I will take…

Bob K.

If you REALLY want to get ‘fancy’ on occasion I am able to source a SET OF 6 black body NA-8 Champion plugs. NOS. the Healy guys usually vacuum them up faster than I can post them… last set sold in a heartbeat for $600. USD… next set will be more. THIS Is why ‘concours’ cars sell for the ‘big bucks’ if you MUST have original mail me off line

Fair enough: no judgment from me!

Hobby on!!!

Thanks for the offer but in line with my earlier reply to Wiggles, I think the change-over from Champion NA8 to N5 in early 1957 is in my perception still within the “lifetime” of the XK 140 range.
Bob K…

Don’t forget that Champion only made black body plugs until about 1962 and they should read : Champion MADE IN ENGLAND in red. G

NGK catalogue recommends B6ES for road use XK120, rather than BP6ES. Anyone have any thoughts why projected electrode would be preferable (or not)?

Possibly the 7:1 flat top pistons vs the 8:1 dome tops?
Given your original engine number ending in -7, can you see which you have?

FWIW, I’m running Autolite XP64 iridium plugs.

I’m running 8:1 dome tops, Rob. +.040" overbore.
How are you finding the iridium plugs?

BP5ES…the 6 is one range colder for maintained hi speed. gap at …025 depending on your coil/distributor. by the way do not use the platinum or iridium that are specifically made for modern hi voltage coil plugs etc. You can read details on the spark plug web places. Now to the thread side road…to be and experience Original driving…find some 80 octane fuel.

Thanks, Nick. I guess NGK has to cover themselves in case people do use their cars hard. I will certainly only be cruising (boy racer days are over!) so will go with the 5. But… B5ES or BP5ES…??
BTW, I’m only using a modern Lucas HA12 coil, or maybe my 1950 Lucas HS12 Sports coil.

Hi Chris:

If you check the NGK U.K. website on one of the early pages it shows a breakdown of what the product numbers translate into: B (14mm thread), P (projected centre electrode), 6 (heat range, lower numbers are ‘softer’ or hotter), E (thread reach, 19mm), S (standard super copper cored centre electrode).


Hi Chris,

yes, I’ve seen that, too. My question is whether there is any advantage (or disadvantage) to the projected centre electrode?


You know, thinking about it, I have noticed an occasional misfire at idle recently.
Back to the Champion N7YC’s as of this morning.

BTW here is the Champion web site recommendation list for '51 XK120, and an iridium plug is top of the list, followed by double platinums, platinums and copper pluses.

Of course it doesn’t mean Champion has actually tried each of these in a 120.
BTW a couple of them are L=1/2" short reach for early 7:1 heads.
And a couple of them are for the XK120C, including a resistor in case you want to listen to the CD player in your C-Type.

Here is the Champion part numbering system.

The original spec in the manual is NA8 for the 8:1 and L10S for the 7:1 ratios, but they don’t come up on websites linked from Champion, they cross reference to N5C and L82C.

Generally, longer electrode = a hotter running plug. No advantage, except at a rich, WOT condition. On the street?