A spark plug holds a small gap between two electrodes inside the combustion chamber, seals the chamber against pressure and heat, and insulates the high-voltage lead from the engine. Set the gap, which is commonly about 0.6 to 1.0 mm, by bending the ground electrode only, keep the plug clean and the right heat range for the engine, and read its tip to see whether the mixture is rich, lean or burning oil.
Part of Internal Combustion
The parts of a spark plug, how its gap and heat range are chosen, how to read a used plug, and why a settlement should salvage rather than make them.
A Plug Is An Insulator, A Seal And A Gap In One
The plug has a metal shell that screws into the head, a ceramic insulator inside it, a centre electrode through the insulator and a ground electrode bent over from the shell to leave the gap. The ceramic must stay a good insulator at high temperature and under pressure, and the seal between centre electrode and insulator must hold gas-tight. That makes it the hardest part of the ignition system to make. The corpus describes the material in Porcelain: fired at 1,260 to 1,400 °C it vitrifies into a dense, waterproof insulating body. A spark plug asks more of that body than a dish does, and the practical advice is to salvage plugs, clean them and keep a stock.
Gap, Heat Range And What The Tip Tells You
Gap. The gap is commonly about 0.6 to 1.0 mm, which is 0.024 to 0.039 inch (a recalled range, flagged; the maker’s figure for the engine governs). The voltage needed to jump it rises with the gap and with the pressure in the cylinder, so a wide gap on a high-compression engine can misfire under load, and a very narrow gap gives a weak spark that deposits can bridge. Set it by bending the ground electrode against a feeler or plug gauge; never lever the centre electrode, which can crack the insulator.
Heat range. A plug has to run hot enough to burn off deposits and cool enough not to light the mixture by itself. A plug whose insulator tip stays too hot causes pre-ignition and can melt; one that stays too cool fouls with soot. The right range belongs to the engine, and a plug from a different engine is a guess.
Reading the tip. The corpus carburetor article uses plug colour: pale plugs with a lean engine, wet plugs with a rich one. In general (recalled practice, flagged), a dry sooty black tip points to a rich mixture or a cold plug, a wet oily tip to oil getting past rings or valve guides, and a white or blistered tip to a lean mixture or overheating. Read it after a steady run, then stop the engine without idling it.
What You Need
- A plug gauge or feeler strips, and a wire brush.
- Spare plugs of the right type.
- A socket that fits the plug and a way to avoid cross-threading.
- Copper or soft-metal washers as the seat needs, replaced when crushed.
Method: Clean, Set And Fit A Plug
Step 1: Remove the plug from a cool engine. Pulling it hot from an aluminium head risks stripping the thread.
Step 2: Inspect. Cracks in the porcelain, loose centre electrode, rounded or burned electrodes, or heavy deposits mean the plug is finished.
Step 3: Clean the electrodes and the nose with a brush and a file; a dirty plug leaks the spark to ground along its surface.
Step 4: Set the gap by bending the ground electrode, and check it is parallel to the centre electrode.
Step 5: Start the thread by hand, then seat it with the socket using a crushed washer. Over-tightening distorts the shell and loosens the seal.
Worked Example: How Many Times A Plug Fires
For a single-cylinder four-stroke at 3,000 rpm, with one spark per two revolutions:
- Sparks per minute: 3,000 ÷ 2 = 1,500.
- Sparks per hour: 1,500 × 60 = 90,000.
- Over 100 hours: 90,000 × 100 = 9 million.
Each spark erodes a tiny amount of metal from the electrodes, so the gap opens slowly and the plug needs more voltage. That is why a plug that fires well on the bench can fail on a worn engine, and why gap and condition are checked on a schedule. The arithmetic gives the count, not the wear per spark, which is not sourced.
Common Mistakes
- Bending the centre electrode. It cracks the insulator.
- Setting the gap from memory. A wrong gap changes the spark and the timing feel.
- Using a plug of the wrong thread length. A long thread can hit the piston; a short one leaves the electrodes in a pocket.
- Not fitting the washer. The plug leaks compression and overheats.
- Chasing a fouled plug without finding the cause. Oil or a rich mixture will foul the next one.
Safety
The plug lead carries a shock and the spark ignites fuel vapour
Do not hold the lead while the engine turns. When you test for a spark, ground the plug body on bare metal away from spilt fuel, ethanol vapour or producer gas, which all ignite from a spark. Plugs are hot after running; let them cool before handling. See Magneto Ignition for the voltage side.
What’s Next
- Learn next: Ignition Timing — when the spark must fire
- Combine with: Carburetor Design — the mixture the plug reads