Hi Rob

I have a 2007 Mazda3 with 120,000 km. It is recommended to change spark plugs at this interval.

If I purchase plugs are they pre-gapped so I may install myself, and how tight do they have to be put in?

Thanks, George.

This used to be a great DIY task, but not any more.

Spark plugs often come pre-gapped, but in the trade, we never trusted that what came out of the box was still accurate. You have to account for bumps during shipping which can alter the gap. Always check the gap before installing plugs.

Installation torque for spark plugs is critical, and if you’re not used to the “feel” of the torque, use a torque wrench. George, in your case, the torque should be eight to 10-lb. ft.

Now the tough stuff.

1. I always remove one high tension lead (spark plug wire), at a time, replacing one spark plug at a time. In this manner, you will not mix up the firing order which can make the engine un-startable.

2. Before removing spark plugs, make sure that the area around the base of the plug is clean. This is especially true if the plugs are recessed into a channel inside the rocker cover; and this is the case with your engine George. If anything falls into the combustion chamber once the spark plug is removed, the debris can damage the cylinder. Clean this area with compressed air and/or a rag wrapped around the end of a screwdriver. You can also use a shop vac.

3. Use a proper spark plug socket on the end of a ratchet extension. Most sockets come with a rubber insert that grips the spark plug. This can make removal and installation easier. I remove the insert because during the “re and re,” (removal and re-installation), if the socket is not supported properly by your free hand, the socket and extension assembly can break the ceramic section of the plug. Two things can happen: the ceramic falls into the combustion chamber and if you happen to break the new plug during installation, you now have to drive to the parts store to buy another one – good luck if you don’t have a second vehicle.

4. Before installing the new plugs, check the gap with a spark plug gauge (.050” to .053”) and coat the threads with a thin film of “anti-seize” compound. You will appreciate this when you replace them next time.

5. To install the new plugs, use a length of small rubber tubing – vacuum hose works well. Use enough length that you can feed the spark plug into the valley of the rocker cover. The hose’s inside diameter should be small enough to fit tightly over the end of the plug. Screw the plug into the head with the hose. This makes the install easy and greatly reduces the risk of cross-threading the plug in the head because you’re only using the hose to begin the threading process.

6. Finish off with the torque wrench, but be careful not to let the socket and extension cock to one side. Even with the rubber insert removed, if twisted over far enough, the internal socket wall can contact the spark plug ceramic and break it.

There’s the long and short of it George, but if this looks too daunting, you always have Plan “B” – take your Mazda to your local shop and have a coffee while the technician sweats the details.

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