Spark Plug Gap Chart

How to use this spark plug gap chart

1. Find your engine. Type a make, model, or engine into the search box — "Silverado", "EcoBoost", "5.3L", "Pentastar". Or tap an application chip (Car/SUV, Truck, Turbo, Classic, Powersports, Small engine) to narrow the list.
2. Tap the row. The spec card pins up top with the factory gap in inches, millimetres, and thou at once, the plug material, the nearest wire-gauge blade, and — the part most charts skip — whether that plug is safe to re-gap at all.
3. Modified? Use the adviser. Enter the horsepower you've added over stock and the chart applies NGK's rule live. Tick the ignition box if you're running an MSD-style system. It warns you the moment the target drifts outside the safe adjustment window.
4. Wondering if yours are worn? Enter the miles on the current plugs. The second card estimates how far the gap has eroded open using NGK's published erosion rate.
5. Save it. "PDF" downloads the whole chart plus the gapping rules for the toolbox. "Copy result" grabs the spec as text. "Share" copies a link with your engine and build settings baked in.

Why this chart is different from other spark plug gap pages

Search "spark plug gap chart" and it's slim pickings. The top result's entire chart is an eleven-cell millimetre-to-inch strip. The biggest plug brand on the page answers the question by telling you to run a part-number search somewhere else and, failing that, to go read your owner's manual. One of the top results is about lawn mower engines. Here's what we built instead:

• An actual chart, searchable by engine. Cars, trucks, boosted engines, classic V8s, powersports, and small engines in one filterable table — not a photo of a laminated wall chart and not a lookup wizard that shows one vehicle at a time.
• Inches, millimetres, and thou together on every row. Spec sheets are metric, gap gauges are imperial, and forum posts are in thou. You shouldn't have to do the division on a greasy phone.
• A re-gap verdict attached to your vehicle. Every source warns "don't gap iridium" once, in the small print, in a paragraph. This chart tells you, on the row you actually looked up, whether you have a fine-wire plug you must never bend or a nickel plug that's designed to be adjusted.
• The performance answer, with a number. "What gap for boost?" is one of the questions Google itself surfaces on this search, and nothing on page one answers it. NGK publishes the rule — reduce about .004 in for every 50 hp you add — so the tool applies it live and stops you when the result exceeds the safe adjustment window.
• A wear estimate. Gaps don't stay where you set them. Enter your mileage and the chart estimates how far yours has eroded open, which is usually the real answer to "why is it misfiring?"
• Free to embed. No competitor on this search offers an embed. Forums, club sites, and shop blogs can drop this chart in with one line of HTML.

How it works — the math behind the numbers

Two things are going on here. The chart itself is a lookup, not a formula: each gap is the manufacturer's published spec for that engine. The adviser on top of it is arithmetic, and every rule it uses is published by the plug makers themselves.

Converting between millimetres and inches

One inch is exactly 25.4 millimetres, so inches = mm ÷ 25.4. A 1.1 mm spec is .043 in; the 0.7 mm you'll see on most turbo engines is .028 in. "Thou" is just thousandths of an inch, so a .040 in gap is 40 thou — same number, no decimal point to lose.

Adjusting for a power adder

More cylinder pressure makes it harder for the spark to jump, so boost, nitrous, and raised compression all call for a tighter gap. NGK's published guidance is to reduce the gap about .004 in for every 50 hp you add. On a stock .044 in engine, a 100 hp power adder puts you at .036 in.

It runs the other way too: a high-output aftermarket ignition system pushes more voltage, so NGK says you can open the gap .002 to .005 in when you fit one.

The limit on how far you can bend a plug

Both Autolite and Brisk cap adjustment at .008 in in either direction from the plug's factory setting. Past that the centre and ground electrodes stop lining up properly, plug life drops, and the ground strap gets weak enough to break. Brisk also notes the gap should never exceed .055 in unless the plug was factory-preset that wide. The adviser watches both limits and tells you to buy a differently-preset plug rather than force this one.

How fast a gap erodes open

Every spark burns a little metal off the electrodes. NGK publishes normal gap growth for a four-stroke engine as 0.01 to 0.02 mm per 1,000 km, which works out to roughly .0006 to .0013 in per 1,000 miles. At 60,000 miles a .040 in gap has plausibly opened to somewhere between .078 and .116 in — well past what any coil wants to jump, which is why old plugs cause misfires and hard starts before they cause anything else.

Treat that estimate as directional, not precise. NGK publishes the rate for four-stroke engines as a class, and it describes a conventional nickel plug best. Iridium and platinum were adopted precisely because they erode far more slowly — that resistance is what buys them a 60,000 to 100,000 mile interval in the first place. So on a fine-wire plug the true figure sits toward the bottom of the band or below it, and the tool says so when you've pinned one. The number is there to tell you which direction your gap has drifted and roughly how far, not to replace your service schedule.

One note on our arithmetic, because we'd rather show it than hide it. NGK's own page prints that four-stroke rate as "0.00063~0.000126 inches/1,000 miles". The upper figure has a dropped decimal place — 0.02 mm per 1,000 km is .00127 in per 1,000 miles, not .000126. Their metric column is internally consistent, so that's what we convert from, and we're flagging it rather than quietly propagating the typo.

Where the per-vehicle numbers come from

Each row is the commonly published spec for the most common engine variant of that generation, cross-checked across at least two independent references. Where the manufacturer publishes a window rather than a single number — Honda's 1.0 to 1.1 mm, for instance — we keep the window, because anything inside it is in spec and flattening it to one number invents a precision the manual doesn't claim.

The final authority is the Vehicle Emission Control Information label under your hood. It's to spark plug gap what the door-jamb sticker is to tire pressure: printed for your exact build, and it wins every argument.

Worked examples

A stock 2019 Silverado 5.3L. Pin the row and you get .040 in — 1.02 mm, 40 thou — on an iridium plug flagged pre-gapped. No adder, no ignition box, so the target stays .040 in and the verdict is "check it, don't set it." Worth knowing why: GM ran a .060 in platinum plug on these trucks until a service bulletin moved the whole family to a 1.01 mm iridium plug, and plenty of old .060 in advice is still floating around forums. Both eras are in the chart as separate rows so you can see the changeover.

A 2016 Mustang GT with a 150 hp supercharger. The Gen 1–2 Coyote row pins at .049 to .053 in. Take the midpoint and NGK's rule subtracts .012 in for 150 hp, landing at .039 in. But that's a 12-thou move, past the 8-thou adjustment limit, so the adviser throws a warning: don't bend the factory plug that far, buy one preset near .039 in. That's exactly what supercharger kits ship with, and it's why the warning matters more than the number.

A Honda Civic at 90,000 miles that's started stumbling. The K-series row shows .039 to .043 in. Enter 90,000 miles and the estimate lands in the .098 to .155 in band — two to three times the gap it left the factory with. Nothing to adjust here: that's a set of plugs at the end of its life, and the fix is a fresh set, not a gap tool. Read that estimate as directional rather than precise, though, for the reason in the next paragraph.

Frequently asked questions

What should the gap on my spark plugs be?

It's set by your engine, not by the plug brand. Most modern petrol engines land between .028 in and .044 in, and the single most common factory figure is .040 in (1.0 mm). Turbocharged engines run tighter, usually around .028 in, because boost makes the spark harder to jump. Search your engine in the chart above for the specific number, then confirm it against the under-hood emissions label.

What is the best spark plug gap for performance?

Tighter than stock, if the extra performance comes from cylinder pressure. NGK's guidance is to reduce the gap about .004 in for every 50 hp a turbo, supercharger, nitrous kit, or compression bump adds. Most boosted setups end up under .032 in. If instead you've fitted a high-output ignition system, you can go the other way and open the gap .002 to .005 in. Enter your added power in the adviser above and it does the arithmetic.

What happens if the spark plug gap is wrong?

Too wide and the coil has to generate more voltage than it was built for — you get misfires under load, a P0300-series code, and coils that die early. Too narrow and the spark is weak, which shows up as a rough idle, poor economy, and carbon fouling. Either way a modern engine will usually tell you: the crank sensor catches the misfire long before you feel it, so plug in an OBD2 scanner and read the cylinder number before you start pulling coils.

Should I re-gap iridium spark plugs?

Generally no. Fine-wire iridium and platinum plugs ship pre-gapped and their centre electrodes are fragile — Briggs & Stratton's guidance is blunt about it, and NGK warns never to put prying force on the fine wire or the insulator. Measure with a wire gauge to confirm what you were sent, and if the number's wrong, order the right part number instead of bending it. If you must adjust one, move the ground electrode only, never use a coin-style gapper, and never touch a multi-ground plug at all.

How do I convert a spark plug gap from mm to inches?

Divide by 25.4. So 0.9 mm ÷ 25.4 = .035 in, and 1.1 mm = .043 in. Going the other way, multiply inches by 25.4. The mm-to-inch table in the tool above has the common values already worked out, and every row of the main chart shows both units side by side so you never have to do it at all.

Related tools

• Bolt & Lug-Nut Torque Calculator — plugs thread into aluminium heads at low torque; this runs the T = K × D × F formula for any fastener.
• Lug Nut Torque Chart by Make & Model — the same searchable-spec treatment for wheel torque.
• Ft-Lbs to Nm Torque Converter — flip a spec across ft-lb, N·m, and in-lb.
• Metric Bolt Torque Chart — M6–M24 in Class 8.8 / 10.9 / 12.9, live for dry / oiled / anti-seize.
• Browse all free tools by Mike Reeves →

Mike's recommendations for a plug change

Getting the gap right is half the job. The other half is buying the right plug and putting it in without stripping an aluminium head:

• Best Spark Plugs — buy the part number that ships pre-gapped for your engine. On a fine-wire plug that's cheaper and safer than trying to correct one by hand.
• Best Mechanic Tool Sets — a plug socket with the rubber retainer, a wire-style gap gauge, and a short extension. Skip the coin-style gapper entirely.
• Best OBD2 Scanners — a wrong or worn gap announces itself as a P0301–P0308 misfire. Read the code, fix the cylinder that's actually failing.

Choosing between brands for the same engine? Bosch vs AC Delco spark plugs goes through where each one actually earns its money, and if the light is already on, our check-engine-light guide decodes what the scanner hands back.

Sources & methodology

• NGK Spark Plugs — Do I need to set the "gap" when installing a new set of plugs? — the −.004 in per 50 hp power-adder rule, the +.002–.005 in high-output-ignition allowance, and the fine-wire handling warning.
• NGK Spark Plugs — How do I "read" a spark plug? — normal gap-growth rates (0.01–0.02 mm per 1,000 km, four-stroke).
• Autolite — Proper Gapping Instructions — the .008 in adjustment limit and the ban on gapping multi-ground plugs.
• Brisk USA — Spark Plug Gap Chart — mm-to-inch reference, the .055 in ceiling, and the boosted-engine guidance.
• Briggs & Stratton — The right spark plug and gap for my small engine — small-engine gaps and the platinum/iridium/twin-tip warning.
• Champion Auto Parts — How To Gap Spark Plugs — the .028–.060 in range across vehicles.
• GM TSB 03-06-04-060 (ACDelco 41-985 / GM 12571164) — the Gen III/IV V8 changeover from .060 in platinum to 1.01 mm iridium.
• OEM owner's-manual and service-data gap values for the listed engine families.

The application table and every gapping rule above are tested by fixtures.test.mjs alongside the component source, asserting unit conversions, the NGK adder and erosion arithmetic, the adjustment-limit guard, and table-wide invariants — including that no fine-wire plug is ever labelled adjustable. About Mike Reeves · Last reviewed August 14, 2026.

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