How to change your sparkplugs

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What am I dealing with?

Spark plugs are fed voltage from a bike’s ignition system, which creates the vital spark to ignite the air/fuel mix in the engine. If your spark plugs aren’t fulfilling this basic function within the motor, you’ll get misfires and an engine which runs rougher than a tramp’s arse.

Why should I bother?

Engines rev a lot higher than a car, so bike plugs wear a lot quicker. Every time a plug sparks, a high voltage charge jumps from the centre projector to the outer ground electrode, and minute particles are literally burned away from the ground electrode. Most bike plugs have a life of 10-20,000 miles, or less if the bike is thrashed. Because of their location (on top of the engine near the combustion chamber) they get extremely hot and the cap material breaks down, affecting the quality of spark. If a cap has broken down then moisture in the air will affect the quality of the spark, as it will always try to jump the nearest metallic earthing point (like the cylinder head).

Stuff you’ll need

A half decent set of tools to remove bodywork or chassis parts that obstruct your access to the spark plugs. A spark plug spanner of the correct width and length, and a torque wrench to tighten up the new plug.

Can I really do it?

If you can take a bike’s bodywork off then you’re halfway to doing the job. The rest is no harder than changing a rear wheel.

What if my plug caps have wiring connectors?

Then the bike has the new type of caps called coil caps – each has its own coil, which converts low voltage into high voltage to create a spark. These have to be replaced like-for-like, which means paying for the official manufacturer’s part. These caps are generally longer lasting than traditional plug caps.

1. Always replace the old plug with exactly the same type, or the equivalent if buying a different make of plug. The owner’s manual will give the correct plug details – heat range, cap size etc. Most shops that sell plugs will have charts stating which model should have which plug. It’s best to change all the bike’s plugs in one go.

2. It’s the same like-for-like rule for plug caps as it is for plugs themselves. If they are of a resistor type (denoted with the letter R in the plug’s or cap’s symbol code) then they must be replaced with the same type. The in-built resistor suppresses electrical activity. Without the resistor the bike’s ECU could be affected and cause poor running.

3. Spark plugs are screwed into the cylinder head and project into the combustion chamber. This means they can be difficult to get at because the camshafts and valves sit above the combustion chamber. If that’s not bad enough, a lot of bikes today need the fairing, fuel tank and airbox removed just to get at the caps and plugs.

4. A good plug spanner socket makes the job easier. The gripping part (the socket wall) will be thick and strong to prevent rounding off the plug’s gripping faces. It’ll also have a rubber grommet in the upper half to grip the plug when lifting it out after it’s been unscrewed, and hold it as you lower a new plug into position.

5. Sometimes the plug spanner won’t be long enough to clear the space where the plug lives. If this is the case you’ll have to attach extension bars between the spanner and ratchet. Now you can undo the spark plug as you would a nut (clockwise from the top).

6. With the plug cracked loose, unwind it out of its thread by hand, holding on to the spanner or extension bar(s). This way you can feel when the plug is free from its threads and lift the spanner and plug clear. It’s not a problem if you have to undo it completely with the ratchet, just be light-handed as you don’t want to rip a thread.

7. There’s a good chance the new plug will have a metal cap-like part on its very top section. This is an adaptor to allow non-bike plug caps to fit on. Remove it with pliers and throw it away if the old plugs do not use the same adaptor. If the old plug has threads on the top (as above) then that’s what you’ll find under the adaptor.

8. On all new plugs there will be a gasket/sealing ring to seal the plug against the cylinder head. As with any plugs or bolts with seal rings (oil and coolant drain bolts) the plug has to be tightened to the correct torque figure. A workshop manual or dealer can give you this figure.

9. Place the new plug into the spanner and lower it carefully into place. Using the plug spanner/extension bar(s), screw the plug in until finger tight. Now attach a torque wrench to the spanner and tighten to the correct torque figure. Never try to tighten with just a ratchet. Stripping plug threads in the cylinder head is expensive to fix.

10. Plug caps are attached to what is called an HT (High Tension) lead where the high voltage charge is carried to the plug. There’s a coarse thread inside the cap and the HT lead screws on to this. Firstly pull back the covering rubber seal and then unwind the plug cap from the HT lead.

11. There’s always the chance that somehow water has worked back past the HT lead seal and corrosion has set in within the lead. What you need to do is cut off at least an eighth of an inch of lead to ensure a good contact is made with the inner material (the voltage carrying section) of the lead and the screw of the new plug cap.

12. Replace the old cap’s water seals (top and bottom) with new ones supplied with the new plug cap. To prevent any water ingress smear a little silcone sealing compound (the same stuff used to seal baths, showers and sinks against tiles – but use the clear, see-through type) around the seals for a perfect fit. Job done.

MCN Technical Staff

By MCN Technical Staff