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Spark Plugs - Removing The Mystery

Original Publication Date:

May, 2010

Spark plugs.

Simple little things, a few dollars each in even the smallest auto parts store.

Problem is - what kind do you get? The original listings in the manual that came with your car are sadly long out of production, and in some cases even the company that made them is long defunct. When was the last time you saw a Lodge spark plug on the shelf at the auto parts store?

Thankfully (and due to standardization) there are newer sparkplugs that can be purchased to fit your Series Rover's engine. The following listing is for the Rover 2.25, in all of its vintages - there is some debate that the very early Series II engines require a different sparkplug but I have never had issues with using these in all of the ones I have owned.

Now, before we proceed, a word about the construction of spark plugs. The measurements that determine the fit of a spark plug to a particular application are pretty simple.

Does it thread in (thread pitch),
Does it reach the combustion chamber but not too long (reach),
Does it seat and seal properly (gasket or tapered seat) and
Is it the right heat range.

The first three of these are pretty simple physical measurements - thread pitch, reach and seating type re simple enough to cross over. The only item here that is a little hard to determine is the heat range.

Heat range is the ability of the plug to shed heat from its structure to the surrounding metal. If the plug does not transfer heat as quickly it's a hot plug (long center electrode structure) - if it is efficient about transfer it is a cold one (shorter center electrode). The balance here is that if too hot, the plug will cause preignition - if too cold it will not clean itself of deposits in normal operation.

If you want to fiddle with heat ranges just realize that the numeric values for plug "heat" are not the same from manufacturer to manufacturer. WHen specifying Japanese-made plugs the plug gets hotter as the number gets lower. For American-made (and Bosch) the number rises as the plug gets hotter.

For example an NGK NGK BK5 series plug is hotter than a BK6 which is hotter than a BK7. Conversely, a Champion 14 or 16 plug is hotter than an 11/12 series which is hotter than a 9/10. Each of these changes represents about a 100C change of temperature at the plug tip in engine operation.

One note: All of the new plugs called out in the list below are resistor-type plugs rather than non-resistor plugs. I have never had an issue using these with both stock and electronic versions of the Rover ignition systems, and definitely experienced much less RF interference from the ignition on my radio equipment. Anyway, just about everything made today is a resistor plug so might as well enjoy the reduced noise on your AM radio. :)

For the Rover 2.25, the required spark plug is:

14 MM plug thread
3/4" reach (length of the threaded portion of the plug)
Gasketed seat
13/16" hex
Resistor (optional - see above)
Copper core
Medium heat range (range from hot to cold)

In looking at this I hear you ask, "What about platinum plugs? Iridium? Multiple electrode?"

In short, don't bother. Not worth the effort for an engine as lightly stressed as the old 2.25. Personally, I don't even like them in my Rover V8s. If the engine isn't designed to use them it's not worth the effort.

For the Rover 2.25:

Champion RN11YC4
AC Delco R44XL
Autolite 64
NGK BPR5ES
NipponDenso W16EPR-U
Bosch WR8DS