Since…you asked: XK120 Service Manual p 49, pt III: Service of plugs: shows Champion N8B for 8-1, and l.10.S for 7-1, both at .022. spark plug gap. Later, NA8, NA10 for sustained hi rev, and for 9-1 sustained hi rev NA12. (N8B became the N5C, again equivalent to NGK B6ES or BP6ES). SB217 pg 2: July 1957 announced that “soon” N 5 eventually was the new supercede for the NA-8 and then became the N5C…quite a cold plug…C being the copper tip…(tuners of the day went to the N9, N9Y, N9YC and the N12YC. but…not factory.) (A lower number being colder in the new number system.) Said another way, the original NA.8/N8-B recommendation for late XKs evolved into the N.5 from July 1957 and was still the recommended plug for an XK150/XK150S (8:1cr) in 1961.
Roger Payne’s work on spark plugs, and “XK140 Explored” is definitive : Service bulletins changed the specified
spark plugs, depending also on compression ratios. An early car
would have been fitted with a certain plug, then a later Service
Bulletin would modify that fitting, and a later still SB would
modify it again. Then, Champion changed the plugs, so later plugs
would have a different number and slightly different construction,
such as copper cores. Service Manual p 49, pt III: Service of
plugs: shows Champion N8B for 8-1, and L.10.S for 7-1, both
at .022. spark plug gap. Later, See SB 95: NA8, NA10 for sustained
hi rev, and for 9-1 sustained hi rev NA12. See SB 99 for a chart
showing N8B for 8-1, NA10 for racing, and NA12 for 9-1 racing. See
Service Bulletin 136 Nov 53 specifies N8B for all 8-1 xk120 engines
(was NA9 prior)(N8B became the N5 and then the N5C, see SB258 Jan
59) and the N5C is equivalent to NGK B6ES or BP6ES). Heat range refers to the ability of the plug to transfer heat from the plug tip to the
engine block. A hot plug transfers heat less, thus the plug stays
hotter. A cold plug transfers heat faster, thus the plug tip in
the combustion chamber runs colder. A projected tip plug provides a
bit broader heat range than the standard tip. Tuners
suggested .025 for smoother idle and general daily driving,
but .022 for sustained hi rev. Tuners in the 50s suggested the KLG,
a boutique racing plug of the day, the KLG 50 for in town, and KLG
60 or 70 for faster driving or if the 50 on inspection showed to be
burning to hot, and the 80 for racing, sustained hi rev. The F
means 14 mm, the E is ¾ reach, a P is projected tip. The KLG FE50
to 70 are equivalent to the Champion N5, now N5C, and equivalent to the NGK B5ES, B6ES. The KLG FE65P, FE85P are equivalent to the NGK BP6ES and to the Champion N8YC, N9YC.
While current Champion are hotter with a higher number, the NGK gets colder as the number goes up. Thus the BP5ES is hot, BP6ES mid, BP7ES colder, BP5 (hotter) equivalent to
the Champion 11 or 12. The NGK 5 to 7 is in the XK use range. NGK has a wider range within any one give plug. Champion gets hotter as the number goes up and .With the Later number system, the 8-12 is in the XK use range. Y is the projected
tip, C is the copper construction. N9Y the older standard plug is
now N9YC with the improved copper construction. N12YC were also
used after market by many owners. Avoid the ‘‘R’’ resistor plugs.
Bosch gets colder as the number gets lower. 5-8 is in the XK use
range. XK series plugs are 14mm thread, ¾ reach, and now could be
standard tip or projected tip. The projected tip was
not in use on the xk120 from the factory. Only in the case of 9-1
pistons on a milled head with a thin head gasket is projected tip
plug to piston interference a possibility to check.
If then to fit as original from factory, it would seem to depend
upon year, month of mfg, according to the SB, for example per
SB136, from engine F1050 the plug fitted was the N8B which became the N5C. See also SB164. The N5C is a very cold plug, too cold for most unless sustained hi rev drives.
The N9yC or N12YC is, for most, a good Champion choice. Equivalent to NGK BP5ES (and just a little hotter than the BP6ES). The Champion N7YC cross refs to the BP7ES…a colder plug. . Err on cold side…if it does not foul…all is fine. If too hot, and If you see a plug burning, and or pink color, that is an imminent and critical danger signal that valves and or piston tops are burning as well and it does not take long for catastrophic damage to occur . The narrow heat range of the older plugs, and especially the older Champion plugs, is why so many different plug numbers are given for each change in tune or driving. Most of these Champion plugs convert to just 2 NGK plugs. Now then, your engine, has some modifications, (condition of any engine and state of tune will affect which plu to use, and how they look upon inspection) so you may need to consider those and do some tests of plug condition immediately after the type of drives you do. Nick