What is a Spark Plug

An internal combustion engine requires three key ingredients to operate: air, fuel and a spark.
A spark plug (also called, very rarely nowadays, a sparking plug) is an electrical device that fits into the cylinder head and provides the spark that ignites the air-fuel mixture that drives an engine.

What does a Spark Plug do?

What does a Spark Plug Do

A spark plug operates by directing electrical current to flow through a centre electrode, forming a spark across an electrode (or air) gap, completing the circuit to a ground electrode.The centre electrode is surrounded by a ceramic insulator which is non-conductive preventing current leakage and ensuring electricity flows in the desired direction.

Spark Plug Anatomy

Although the spark plug is a familiar engine component, spark plug terminology often varies.

The diagram below identifies the major components of a spark plug and their correct terminology.

Spark Plug
Spark Plug Construction


Spark Plug Projection

Projection The projection of a spark plug is measured as the distance from the end of the metal shell to the tip of the centre electrode.A projected spark plug protrudes into the combustion chamber further and provides higher ignitability and improved performance. Simply this can be understood by representing the combustion chamber as a circle. A projected spark plug in effect produces a spark in the middle of the circle (or combustion chamber) allowing for an even flame spread. Conversely, a non-projected spark plug means the flame spread is not even.

Spark Plug Electrode Gap

Electrode The electrode gap of a spark plug dictates the spark magnitude. An incorrect electrode gap may affect engine performance as the spark magnitude may be insufficient to ensure complete combustion of the air-fuel mixture.

The electrode gap is in most cases, set to suit the vehicle that the plugs have been supplied for. In some cases however, a specific electrode gap may be required for some vehicles, for which a plug is not specifically manufactured. As such the electrode gap may need adjustment, and it is always best to quickly consult your owner’s manual

Electrode Gap Adjustment

A variety of techniques are used to adjust electrode gaps, but you should always try and use an appropriate method and tool to adjust the spark plug gap.

When adjusting the electrode gap, the gap may commonly be made too small, as such, the gap may need to be opened slightly. Traditionally people use a lever such as a flat screwdriver to pry open the gap. This is okay on traditional Nickel plugs which have a solid centre electrode of some Ø2.5mm that may withstand such force.

Precious metal spark plugs however have smaller Ø0.6mm fine tips that are laser welded to the centre electrode. These tips are very fragile and not designed to withstand any force.

If it is necessary to widen the gap, use a tool that only pulls back on the ground electrode without touching the center electrode or the porcelain. To close the gap on a plug, gently tap the plug, electrode first on a hard surface.

Electrode Gap Adjustment

Checking your Old Spark Plugs

A visual check of the spark plugs can tell you a great deal about engine conditions. For example, a normal spark plug contains a light beige or gray deposit. It shows a small amount of electrode wear, about .001 inch per 1000-2000 miles of driving. Those clues indicate that the plug is of the correct heat range and that the car has received mixed high- and low-speed driving. Such a spark plug can be cleaned, regapped, and put back in the engine. Even if you are fitting new plugs and not renewing the old ones, it is important to inspect the old plugs as this will tell you how good or bad your engine is running.

Here are just some of the things you can determine from your old Spark Plugs.

Grey-Tan to White Colour.

Spark Plug Tip This is how a good spark plug should look and shows that the engine is running well.

Wet, Oily Deposit

Spark Plug Tip A wet, oily deposit on a plug is caused by crankcase oil leaking past the piston rings and valve guides into the cylinders. This could be the result of worn valve guides, worn cylinder linings or worn piston rings.

Fluffy Black Deposit

Spark Plug Tip A fluffy black deposit indicates carbon- fouling. It comes from too-rich carburetion, over choking or a clogged air filter element. Carbon-fouling also may result from low-speed driving and long periods of idling, which keep plug temperatures so low that normal combustion deposits are not burned off the plugs.
Only one carbon-fouled plug out of an entire engine usually means the compression in that cylinder may be low due to poor piston rings. Ignition system checks may turn up a shorted spark plug cable to that cylinder.

Burned or Blistered Insulator Nose

Spark Plug Tip A burned or blistered insulator nose and badly eroded electrodes indicate plug overheating. That may be caused by improper heat range, over advanced ignition timing, or running on fuel of too low octane. Lean air/fuel mixtures, a blocked-up cooling system, or sticking engine valves can cause that problem too.

Red, Brown, Yellow, or White Coating

Spark Plug Tip A red, brown, yellow, or white coating that forms on the spark plug insulator is a by-product of combustion, resulting from additives in fuel and oils. Usually, scavenger deposits, as they are called, have no ill effect and are quite normal. Sometimes, however, they can cause misfiring at high engine speeds under heavy loads.

Chipped Insulator

Spark Plug Tip This can be caused by heat shock from pre-ignition. Over-advanced ignition timing or too-low octane fuel can also cause the insulator to crack. Another possible cause is improper plug gapping methods in which a side force is put on the insulator tip. Replace the plug and correct the problem during your routine maintenance.

Recommended Spark Plugs

spark plug After all of this blurb, the one question you want the answer to is:
“Which spark plugs shall I use in my Anglia / Prefect”

The best make of spark plugs for the Anglia / Prefect are

Tools Required

The Anglia and Prefect use 14mm spark plugs, that is the thread diameter, not the size of the hexagonal end.

To remove, check and clean the spark plugs, you may need some of the tools shown below.

The minimum being some sort of plug spanner or socket to remove the plug from the cylinder head.

Spark Plug Spanner
Flexi-Plug Spanner
Spark Plug Spanner
Plug Spanner
Spark Plug Tip
Wire Brush
Spark Plug Gauge
Spark Plug Gap Gauge
Feeler Gauge
Feeler Gauge
Spark Plug Socket
Plug Socket for use with a ratchet