Iridium spark plugs are a common type of combustion engine ignition device found in industrial engines worldwide. They’re known for their strength, reliability, conductivity, and strong resistance to extreme internal temperatures.

They’re fitted into the engine and connected via a HT lead to the ignition coil. The coil temporarily powers the plug with a fixed electrical voltage. When the engine is started, iridium spark plugs convert this electrical charge into the spark needed to light the air-fuel mixture.

How Do They Work? 

1. The stroke of the piston begins at top dead centre (T.D.C.) and ends at bottom dead centre (B.D.C.). In this stroke, the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing vacuum pressure in the cylinder through its downward motion. The piston is moving down as air is being sucked in by the downward motion against the piston.

2. Compression: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke, the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.

3. Combustion: Also known as power or ignition. This is the start of the second revolution of the four-stroke cycle. At this point the crankshaft has completed a full 360-degree revolution. While the piston is at T.D.C. (the end of the compression stroke) the compressed air-fuel mixture is ignited by a spark plug, forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to turn the crankshaft.

4. Exhaust: Also known as outlet. During the exhaust stroke, the piston, once again, returns from B.D.C. to T.D.C. while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust valve.

These four strokes can be remembered by the colloquial phrase, “Suck, Squeeze, Bang, Blow”.

It’s therefore important to keep a close eye on the condition of your iridium spark plugs when operating any kind of combustion engine. Despite their relative material strength and solidity, spark plugs will wear down, degrade in quality, and eventually fail completely if left unchecked. If the plug breaks down completely, your engine may not start at all.

Here’s our quick guide to keeping on top of spark plug care and replacement.

What’s The Lifespan Of An Iridium Spark Plug?

The average, modern iridium spark plug should last for 3-4,000 hours of engine uptime, depending on the gas quality. Each make and brand of spark plug should come with its own estimated rating for hours of use without degradation or failure. Your spark plug may well live on past the advertised limits of use (depending on your luck, of course).

Spark plugs used in high-intensity or continually running engines in industrial settings, will need more frequent changes.

Older models, with non-precious metal-tips will degrade and fail much faster.

Some manufacturers recommend that spark plugs are changed at fixed intervals or after a certain number of hours’ runtime, regardless of the plug’s operating quality. It’s often better to use your instincts, manufacturer and engineer recommendations for the engine and plug, and current handling and engine performance as a baseline, rather than relying on an arbitrary, ‘one-size-fits-all’ figure.

How Do I Know If My Spark Plug Needs Replacing?

Spark plugs tend to fail due to a natural build-up of residue between the electrode and the plug, as well as excessive heat widening the gap between the electrodes.

If you see a build-up of residue around the top of the spark plug during maintenance, the spark plug may need replacing, cleaning or re-gapping.
How much will an iridium spark plug cost to change?

A industrial double iridium 14mm spark plug will cost you from around £20, and a 18mm double iridium spark plug from around £65 (as of the time of writing).

Expect to pay anything from £80-£1500 for an engine refit, depending on engine type/number of cylinders in the engine.

Find Out More

R & M Walsh specialises in spark plug fittings, replacements, and system upgrades for all types of gas-powered combustion engine. If you’re having any issues at all with your spark plugs, call or email us today for advice.

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