How to Gap Spark Plugs (and When Not To)

To gap a spark plug, measure the space between the center electrode and the ground electrode with a wire-type gap gauge, compare it with your engine's spec, and if it's off, bend only the ground electrode in tiny steps until the correct wire slides through with a light drag. On fine-wire iridium and platinum plugs, measure gently and adjust as little as possible. If a new plug is far from spec, the fix is usually the right part number, not a bigger bend.

What you need

• The gap spec for your engine, from the owner's manual, a service manual or the plug maker's catalog
• A wire-type gap gauge (round wires, sometimes called a pin gauge). Most have a small hook or slot for bending the ground electrode.
• Needle-nose pliers (optional, for small adjustments)
• A clean rag and good light
• A spark plug socket to fit the plugs afterwards. The 16 mm magnetic spark plug socket suits the most common plug size and holds the plug so it doesn't drop down the well, but some engines use 14 mm plugs, so check yours first.

What the spark plug gap does

The gap is the space the spark jumps across: from the tip of the center electrode to the ground electrode, the bent metal arm above it (also called the side electrode or ground strap). Your engine maker's spec balances two problems:

• Too wide: the coil needs more voltage to fire the plug. That demand rises with cylinder pressure, so under load the spark may not jump at all. You get misfires and hesitation, and the coil works harder.
• Too narrow: the plug fires easily, but the spark is small and lights the mixture less effectively. That can mean a rougher idle and less complete combustion.

The gap also grows slowly as the electrodes wear, which is one reason old plugs start misfiring under acceleration first.

Where to find the right gap

• Your owner's manual or service manual. Start here. Many give a small range (a minimum and a maximum) rather than one number.
• The plug maker's catalog. Look up your exact engine, not just the model name. The listing gives the plug part number and usually the gap for that application.
• The emissions label under the hood. On some cars it lists the plug gap.
• Not the box. One plug part number can fit many different engines, so the factory preset is a starting point, not necessarily your engine's number. NGK says it's always best to check the gap against the vehicle maker's spec.

Manuals give gaps in inches or millimeters. These are straight conversions, not recommendations:

Inches	Millimeters
0.028 in	0.71 mm
0.032 in	0.81 mm
0.035 in	0.89 mm
0.040 in	1.02 mm
0.044 in	1.12 mm
0.050 in	1.27 mm
0.060 in	1.52 mm

Wire vs coin-style vs feeler gauges

• Wire (pin) gauge: a set of round wires, each a fixed thickness. NGK recommends a round wire-style or pin gauge. Round wires read accurately even on worn, uneven electrodes, and most of these tools include a bending hook. This is the one to buy.
• Coin-style (ramp) gauge: a disc with a tapered edge. You roll the plug around the edge until the ground electrode stops and read the number there. It's cheap and quick on traditional copper-core plugs with a thick center electrode. The catch is the wedge shape: push it in and it pries the electrodes apart, which can bend or snap a fine iridium or platinum tip. Don't use one on fine-wire plugs.
• Flat feeler gauge: flat steel blades. Fine on new plugs with flat electrode faces, but on used plugs the worn faces can make a flat blade read wrong.

How to gap spark plugs step by step

Step 1: Inspect the plug

Check each new plug before you measure. The white insulator should have no cracks or chips, and the ground electrode should sit squarely over the center electrode. A plug dropped on a hard floor can have a closed-up gap or a hairline crack. If the insulator is cracked, don't fit it.

Step 2: Measure the gap

1. Pick the wire that matches your spec. If the spec is in inches and your gauge is metric, convert first.
2. Slide the wire straight through the gap, square to the electrodes. Don't force it or tilt it: both give a false reading.
3. Read the feel. A light, even drag means the gap is right. If the wire falls through without touching, the gap is too wide. If it won't enter without force, it's too narrow.
4. If your spec is a range, the plug is good when the minimum wire passes and the maximum wire is snug or won't go in.
5. If the gap is already in spec, stop. A correct plug doesn't need bending.

Step 3: Decide whether to adjust at all

Work out how far off the plug is. NGK recommends adjusting no more than 0.008 inch (about 0.2 mm) either way from the factory preset. Say a plug comes set at 0.044 inch and your engine calls for 0.040 inch: that 0.004-inch change is within the limit. If your engine calls for 0.032 inch, the plug is 0.012 inch away, which suggests it's the wrong part number for your engine. Check the catalog before you bend anything.

Step 4: Adjust the ground electrode only

1. To widen the gap: hook the gauge's bending slot under the end of the ground electrode, or grip the electrode near its bend with needle-nose pliers, and lift it a tiny amount.
2. To close the gap: bend the ground electrode down toward the center electrode a tiny amount with the same tool.
3. Keep every tool off the center electrode and the white insulator. Never pry between the electrodes, and never try to bend the center electrode: it's set in the ceramic, and forcing it can crack the insulator.
4. Work in small steps, in one direction, and re-measure after each. Bending the electrode back and forth weakens the metal, and a piece that breaks off inside a cylinder can do serious engine damage.
5. Keep the ground electrode square and centered over the center tip. If it has a small precious-metal pad, keep the pad lined up with the tip.

Step 5: Recheck, then fit the plug

Measure once more with a light touch. When the correct wire slides through with a light drag, the plug is ready. Start it by hand and tighten it to your manual's torque spec. Our guide on how to change spark plugs covers removal, threading and tightening, with or without a torque wrench.

Iridium and platinum plugs: extra care

Fine-wire iridium and platinum plugs have a very thin center electrode, well under a millimeter on many designs. It resists spark wear but bends or snaps easily. Plug makers differ on adjusting them:

• NGK says not to put any pressure or prying force on the fine-wire center electrode or insulator, to use a tool that only moves the ground electrode, and to stay within the 0.008-inch limit.
• Bosch says its precious-metal plugs come pre-gapped, don't need adjusting, and that adjusting the gap can damage the electrodes.

So measure with a round wire and a light touch, adjust only if the plug is actually out of spec, keep the change small, and follow any "do not adjust" note on the box. If a fine-wire plug is well off, exchange it for the correct part.

When not to gap a spark plug

• It's already in spec. Leave it alone.
• It has more than one ground electrode. Multi-ground plugs are set at the factory and aren't designed to be adjusted.
• The packaging says not to. The plug maker's instructions win.
• It needs a big change. More than about 0.2 mm (0.008 in) usually means the wrong plug.
• It's worn out. Closing the gap on an old plug with rounded, eroded electrodes doesn't make it new. If your plugs are due, replace them.
• It's damaged. A cracked insulator or bent center electrode means replacing the plug.

Mistakes to avoid

• Forcing a coin-style gauge into a fine-wire plug to "check" it
• Prying the gap open with a screwdriver blade or the edge of a gauge
• Trusting the gap on the box instead of your engine's spec
• Measuring at an angle, which makes the gap read wrong
• Bending the ground electrode back and forth until it's "close enough"

FAQ

Do iridium spark plugs need to be gapped?

They need to be checked, not automatically adjusted. Most come pre-gapped, but the preset may not match your engine. Measure gently with a round wire gauge. If it's out of spec and the box allows adjustment, move only the ground electrode and stay within about 0.008 inch (0.2 mm) of the preset.

Do spark plugs come pre-gapped?

Most do, but one part number can fit many engines with different gap specs, so the preset is only a starting point. A dropped plug can also be knocked out of spec.

What happens if the spark plug gap is too big or too small?

Too big, and the coil may not have enough voltage to fire the plug under load, causing misfires and hesitation. Too small, and the spark is weak, which can cause a rough idle and incomplete combustion. Either way, set it to your engine's spec.

How do you gap spark plugs without a tool?

You can't measure a gap accurately by eye, and the differences involved are fractions of a millimeter. A flat feeler gauge from your toolbox works on new plugs. Otherwise, a wire-type gap gauge is inexpensive and also gives you the right tool for bending. Don't pry with a screwdriver.

Can you gap used spark plugs?

You can reset the gap on a used standard plug you plan to refit, but it won't fix worn electrodes. If the plugs are near their replacement interval or the electrodes look rounded, fit new ones instead.

Not sure your plugs are the problem in the first place? Read bad spark plug symptoms: 8 signs it's time to replace them.

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