ALT-4 How to Replace Spark Plugs
This has to be the easiest of all maintenance tasks to perform on an engine.
The spark plug is basically an insulated pair of terminals, across which a spark is created which is used to explode the petrol/air mix in the cylinder of the engine. Spark plugs come in a bewildering variety of sizes and it is important to make sure that you consult the engine manual to ensure you buy the correct ones.
The tools you will need are:
A spark plug spanner or socket wrenchA feeler gaugeA gap setting tool
The manual will tell you the correct gap to set between the electrodes, in Europe this will be expressed in fractions of a millimetre, in the USA in Thousandths of an inch.
The feeler gauge set is a set of flat metal plates each marked with its thickness, they are usually in a bunch, arranged a bit like a multi-bladed pen knife. It is important to wipe them clean before use, as even a layer of dust can affect the thickness, especially if you need to use more than one blade.
Getting the gap set correctly is important as it will ensure the optimum spark size.The electrodes are quite fragile, as is the porcelain insulator so you ned to use the gapping tool correctly.
Select the correct sized gauge and slide it in between the centre electrode and the outer. It should glide in smoothly with a silky drag. If any force is required you need to open the gap, if you can see daylight you need to close it. To adjust the gap, hook the gap tool onto the outer electrode and gently lever it open or closed. To close the gap a little you can tap the end of the plug gently on a clean workbench. You should never try to adjust the centre electrode, as this will damage the insulation and ruin the plug. If you have a small piece of fine emery paper use this to clean the tip of the electrodes, especially if the engine is a two-stroke, such as the cheaper motorcycle engines, small outboard motors or a chain saw.
To remove the old plug, gently ease off the plug cap, holding the plug, not pulling on the high tension lead. Tuck the lead out of the way and place the plug spanner over the head of the plug. proprietary plug spanners usually have a rubber insert that will grip the plug as you remove it, to save dropping it down inside the engine compartment.
With the spanner in place turn it anti-clockwise to remove it. Wipe any debris away from the hole to avoid it falling into the cylinder then take your new plug and start to screw it in. It should go in easily, if it does not never use force, but take it out and try again. You should be able to screw it in finger tight, then use the plug spanner to give no more than a quarter turn to lock it. NEVER use any force.
When it is in place snap the lead back onto the top of the plug and repeat the process for each of the other cylinders.
Use a clean kitchen towel to wipe any grease away from the leads and close the bonnet (or Hood, if you live the wrong side of the Atlantic) start the engine and enjoy a crisper sounding engine.
