Feeler Gauge Set vs Spark Plug Gap Tool: Which One Should You Buy?

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Below: what each tool does, how the two compare side by side, the four gap tools from the listings ranked by what they publish, and answers to the questions that come up at the workbench.

Feeler gauge set vs spark plug gap tool: what each one does

A feeler gauge set is a stack of thin blades, each stamped with a thickness, used to measure a gap by feel: the blade that slides through with a slight drag tells you the size. A spark plug gap tool is built for one job: it measures the plug gap and gives you a way to change it. The Toolwiz listing is the clearest example, pairing a 32-blade feeler gauge with a red gapping tool that closes the electrode with a knob.

So the honest answer: for setting plug gaps, buy the gap tool. For a toolbox that also does valves and other clearances, the feeler gauge set earns its shelf space.

The gear: four gap tools from the listings

Ranked by what the listings publish, most complete first.

Toolwiz Spark Plug Gap Tool with 32-Blade Feeler Gauge. The most complete kit here. The gapping tool fits 10mm, 12mm, 14mm and 16mm spark plugs (the listing says it does not fit 18mm thread, long reach or short reach plugs), and a knob closes the electrode down to the gap you want; the listing is explicit that it only reduces the gap, it cannot enlarge it. The 32-blade feeler gauge spans 0.001 to 0.040 in (0.03 to 1.0 mm), dual-marked, in hardened and polished 65 manganese steel. The listing’s own tip: electrodes can spring back, so use a slightly smaller blade than the target, like 0.028 in for a 0.030 in gap. One caution: the spec table disagrees with the title in several places, listing 18mm fitments, a package of a plug checker and a power adapter, and a gauge range of without feeler gauge. Go by the title and bullets: 10 to 16mm plugs, one gap tool and one 32-blade gauge, model THT0003RD, 12.7 ounces.

OEMTOOLS 25442 9-Wire Spark Plug Gap Gauge. The classic wire gauge: 9 wire sizes from .025 to .060 in, with inch and millimeter scales on a compact gauge that fits a pocket or tool chest. It measures; it does not adjust. Listed weight 1.41 ounces, model 25442.

Harrier Hardware 3-Pack Coin Gap Gauge. Three coin-style gap gauges in one listing: 0.02 to 0.1 in on the SAE side, 0.5 to 2.5 mm on the metric side, engraved markings, and a punched hole for a keychain or workshop hook. One for the toolbox, one for the glove box, one to lend. Listed weight 2.08 ounces.

BBTO 2 Pcs Spark Plug Gap Gauge. Two coin-style gauges with an adjuster slot on both sides: slide the slot onto the plug’s hook and pull slightly to set the gap. The scale runs 0.040, 0.044, 0.045, 0.050, 0.060 and 0.080 in, with metric engraved on the back, packed in a plastic storage box. About 1.9 in (4.8 cm) across. Listed weight 1.13 ounces.

Choosing between them

Buy the spark plug gap tool if plug gaps are the job: the Toolwiz knob tool is the only one here that both measures and adjusts with precision. Buy the feeler gauge set if the garage also does valve adjustments or other clearance work: blades from 0.001 to 0.040 in cover far more than plug gaps. For the glove box or the trail kit, a coin gauge on a keychain or a pocket wire gauge is the one that is there when you need it. See our best spark plug gap tool and best feeler gauge set picks for more options.

Frequently Asked Questions

Can I gap spark plugs with just a feeler gauge set?

You can measure with it, but you still need a way to move the electrode. A coin gauge with an adjuster slot, like the BBTO, or a dedicated gap tool, like the Toolwiz, does the adjusting.

What is the difference between a wire gauge and a coin gauge?

A wire gauge, like the OEMTOOLS 25442, measures with round wires in fixed steps from .025 to .060 in. A coin gauge has a tapered edge you slide under the electrode until it stops. Both measure; neither adjusts as precisely as a knob tool.

Can a spark plug gap tool make the gap bigger?

The Toolwiz listing is explicit: its tool only reduces the gap, it cannot enlarge it. If the gap measures too small, the Toolwiz tool cannot open it, and the listing notes that iridium and platinum plugs usually do not require adjustment; check the plug maker’s guidance before moving the electrode.

Do pre-gapped plugs need checking?

The Toolwiz listing notes that iridium and platinum plugs usually do not require adjustment. Even so, a quick check with a wire gauge catches a plug whose gap shifted in handling.

Why does the exact gap matter?

The gap is the distance the spark jumps. Too wide or too narrow and the engine can misfire, idle rough or lose efficiency; every listing here ties accurate gapping to engine performance and smooth running.

What do the two scales on a gauge mean?

SAE inches and metric millimeters. The Harrier reads 0.02 to 0.1 in and 0.5 to 2.5 mm; the Toolwiz blades read 0.001 to 0.040 in and 0.03 to 1.0 mm. Use the scale your plug’s spec is written in.

Related: best spark plug gap tool, best feeler gauge set, best spark plug socket set, best torque wrench for spark plugs and magnetic vs rubber spark plug sockets.