Replacing Spark Plugs

Replacing Spark Plugs

Before replacing spark plugs in your car or motorcycle first check the owner’s manual or service manual to determine the correct spark plugs for your vehicle. Unless you have a heavily modified engine or are running custom tuning, you should not deviate from the manufacturer’s specifications. For Ford vehicles, I recommend using Motorcraft spark plugs because that's what the engineers designed the engine to use. For motorcycles, the recommended brand depends on the manufacturer, but generally Japanese manufacturers' use either NGK or Denso spark plugs.

Buy your spark plugs from a reputable seller. There are fake spark plugs on the market so just be aware of that when you are looking for new ones. To help consumers spot fakes, NGK has the following recommendations:

  1. Inspect font type and signs of smudges on the spark plug
  2. Look out for a machine cutting mark on the caulking (could indicate a counterfeit)
  3. Check the hexagon for a LOT number and the correct font type
  4. The C-groove portion show machine cutting marks (could indicate a counterfeit)
  5. Have a close look at the electrode. Compare to a genuine product's electrode

NGK Spark Plug

None of the brands referred to here requires the application of anti-seize to the threads of the spark plugs. Spark plugs come pre-gapped, but double check the plug gap with a wire type feeler gauge, do not use a coin type gauge. Coin gauges will not give you an accurate measurement because they have a sloped surface.

Be extra careful when checking the gap on Iridium plugs, they have a very narrow electrode that can be easily damaged. If you need to reduce the gap on a spark plug, do not bang it on the work bench, use a tool like the one pictured below that will set an accurate gap. Do not insert a feeler gauge between the electrode and the ground strap and crank down on the gapping tool because you will crush the electrode and ruin the plug.

When installing new spark plugs in the cylinder head, be careful not to over tighten them. Again, do not use anti-seize since it is a lubricant and will cause you to overtighten the spark plugs. The installation procedure in most vehicle service manuals is to 1) hand tighten the plugs, then 2) tighten them to a specific torque using a torque wrench. If you do not have a torque wrench, get one (see link below). The proper torque is measured in lb.in or Nm (sometimes lb.ft) and is specified in some owner's guides and in all service manuals.

Tools:

  • Torque Wrench 20 - 240 ft in (20 ft lb max)
  • Magnetic Spark Plug Socket Set
  • Socket Extension
  • Wire-type Feeler Gauge
  • Spark Plug Gapping Tool

Sources:

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