Choice of Spark Plugs

Spark plug brands and part numbers come and go, so for the owner of a classic vehicle the reference book in the spares shop may not always have an answer. There have also been technological changes that impact on choice of plug. Most significant of these is the long-reach plug. The theory is that by placing the spark deeper into the combustion chamber, superior ignition is obtained. Manufacturers generally include a letter or number in the reference number that indicate that a plug is long-reach. There is no problem using these in a Magnette (subject to remarks below re pre-ignition and running-on).

Manufacturers’ recommendations for individual cars are, of course, based on original specifications, so, often, a modified car will need a different plug to suit its state of tune. Spark plugs are specified not only by thread and nose length but also by their “heat” rating. A plug that operates too hot will induce problems like pre-ignition and running on because the metal parts of the plug stay hot enough between sparks to ignite the incoming fuel too early. Plugs that run too cool tend to foul up easily because they do not reach the temperature at which they successfully self-clean.  The plug specified for a car is the one that best steers a course between these two problems. If you are running a high-performance engine, the chances are that the standard plug will no longer be the best choice.

In certain conditions, a long-reach plug will run hotter than the standard equivalent, so the next coolest plug is a better alternative. Each manufacturer’s numbering system will enable you to identify which number specifies the heat rating (see their website). If your plugs regularly foul, the chances are a hotter plug will help.

Another plug development that has occurred since the end of Magnette production is the resistor plug. This is a plug that has a radio suppressor fitted inside it to eliminate interference with radios and engine management systems, making a suppressor cap superfluous. There is no problem using these in a Magnette, but if you already have traditional suppressor caps there is no reason to.

The originally specified Champion NA8 was eventually merged into the N5, which became the normal choice and this is still available as the standard-reach option. The N9Y equivalent long-reach plug has also been used successfully.

NGK plugs are now commonly available and have attracted a following. The long-reach plug suited to our cars is BP6ES. If your car suffers from pinking or running on that can be explained by no other cause (timing, fuel mixture), try a set of B6ES short-reach. The cooler long-reach equivalent is BP7ES.

The Bosch equivalents are:

                 BP6ES = W6D

                B6ES   = W7C

                BP7ES= W5D

 The standard gap setting is 0.025” (“25 thou”). If you have upgraded to an electronic ignition system, a larger gap can generally be used. However, a word of caution: the wider the gap, the faster the erosion of the points, so it is probably enough to go to 0.035” for normal regular use.

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