Yeah okay, this is a very basic procedure and can be done with your eyes closed after you have done it once, but for the novice there is a few tips and tricks that can make the job easier and prevent any potential disasters.

When you go to buy your replacement spark plugs you will see that there are letters and numbers that categorize each different spark plug. We will run through what these mean quickly so you can understand what it all means.

The example I am using is a spark plug manufactured by NGK. Each brand of spark plug has it’s own ‘code’ of numbers and letters however this example will give you a rough idea of what they mean.

The code for our example spark plug is ; BPR5ES-11.

• The first letter stands for thread diameter, of which there are five variants running from 8mm up to 18mm.
• The second letter refers to three different types of plug configuration – Projecting insulator type, Small spark plug and surface gap, semi-surface gap or supplementary gap.
• The third letter dictates whether it is a resistor type plug (R), without resistor (no letter) and Z which indicates a Inductive resistor type plug.
• The number ’5′ refers to the heat rating of the plug. The lower the number the ‘hotter’ the temperature the plug operates at. If you run LPG it is advisable to fit a plug that is one range colder than standard or if your engine is reasonably old and is using oil a one range hotter spark plug can help burn any oil that has found it’s way into the combustion chamber.
• The fifth letter refers to the thread length or reach of the spark plug.
• The sixth letter again refers to the type of spark plug. As an example ‘S’ stands for standard type and ‘P’ stands for platinum type spark plug.
• The last number refers to the plug gap or the gap between the two electrodes.

So how does this information affect which spark plug you should buy for your vehicle? Well, to be honest it doesn’t but it is handy to know why they have all these numbers and letters stamped on the plugs!

When replacing your spark plugs I recommend that you either buy them from an Auto Parts Store that has a listing for your vehicle and can supply you with correct plug or check in your owners manual and the correct spark plug’s to use should be listed in there. Don’t assume that the correct plug was fitted last time and just buy the type of plug that you took out, it pays to check that this is the correct plug for your vehicle as they can greatly affect the running of your vehicle.

Now that I have filled your head with useless information let’s look at what is involved in replacing the spark plugs.

• Number the spark plug leads, either with a Nikko pen or small piece of masking tape wrapped around the lead?with the number written on the tape. If you are unsure of which cylinder is which, don’t stress just number the leads in an order that you can understand or write the number on the cylinder head as well to avoid confusion. If you do get stuck and find you can’t work out what order the leads go back on post your vehicle make and model on the Advice Forum and I’ll do my best to help you out.
• Remove the plug leads using a twisting and pulling movement. Don’t just pull at the lead as they are easily damaged.
• Using a small length of hose or compressed air blow any foreign material away from the spark plug seat.
• The next step is to undo the spark plugs in a anti-clockwise direction. There are two plug sizes used in modern engines, 5/8 and 13/16 and obviously you will need a socket or spark plug spanner of the correct size. Be sure to keep the socket square to the spark plug as any movement sideways in the socket can crack the porcelain stem of the plug and render it useless. If you find that the plug becomes tighter as you undo it try using some sort of lubricating spray (WD-40, RP7 etc) on the spark plug thread and try going in the opposite direction for a turn or two. If you find that it is still tight and getting tighter, STOP and take the car to a workshop and have them remove it for you. Spark plugs can break and if you are left with the thread still in the cylinder head the head will have to be removed and that can be very expensive. If you find a plug that is tighter than the others or is a little difficult to remove I suggest you go down to your parts supplier and buy a ‘Spark Plug Thread Chaser’ which is basically a thread tap and once wound into the thread it will clean the thread back to it’s original condition and make it easier to re-fit the plug. On twin cam engines where the plugs are located deep in the middle of the cylinder head a small length of hose pushed over the end of the spark plug will make installation and removal of the plug’s a piece of cake!

• Lay the spark plugs out in the cylinder order that they came out in so we can check the condition of each cylinder – see Spark Plug Condition Guide.
• The next step is to fit the new plugs. Take each spark plug out of it’s box and visually check the gap. The gap is preset at the factory these days but if for some reason the gap has been closed up or is too big we want to know before fitting them to the vehicle.
• If the gaps look even across all the plugs apply a smear of anti-sieze compound or a couple of drops of engine oil on the thread of each plug. This is particularly important on vehicles fitted with alloy heads as the spark plug thread can ‘bind-up’ in the thread of the cylinder head over time and the spark plug can be difficult to remove without some form of lubrication on the thread.
• Fit the spark plugs to the engine. For the first few turns use minimal force to ensure that they are not cross-threaded. Tighten the spark plug until you feel it contact the seat and them take it another 1/4 of a turn. That will be tight enough.
• Re-fit the ignition leads in their correct order and start the engine. If the engine refuses to start or back-fires you have the leads in the wrong order. Go back and check your numbering.
• That’s it! You’ve done well!

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