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Common Tread

How to gap a spark plug

Aug 22, 2018

One of the most important parts of your spark plugs isn’t even part of the plug.

It’s the gap between the center electrode and the ground electrode. It’s an air gap, and one that needs to be set very precisely in order for your engine to function correctly.

How to gap a spark plug

Understand how the gap affects engine performance

Check the gap on all spark plugs, even ones that claim to be pre-gapped

Use a ramp-style gauge when necessary

Utilize feeler gauges to measure gaps most accurately

Utilize gap-closing tools when gaps are too large

Why gap matters

On one hand, a large gap allows for a big, fat spark with which to ignite the air and fuel mixture. On the other hand, high cylinder pressures (caused in part by a high compression ratio) can make it difficult for the electricity to bridge a larger spark plug gap. The larger the gap, the harder the ignition components need to work to get the spark to jump it. It also should be noted that your gap increases as the spark plug is used — the constant arcing wears away at the metal of both electrodes, opening the gap up over time.

Measuring and adjusting your plug gap is mission-critical. RevZilla photo.

Which brings us to a spark plug’s composition. For a long time, center electrodes were made of copper, and plugs were gapped in a specific manner. However, precious metals began being used for spark plug construction, including platinum and iridium, generally applied as a coating to the electrodes. In an effort to create more durable plugs, those metals were incorporated into the electrode base material, because they are good conductors, can withstand extreme temperatures, and are hard and not very corrosive. But they’re also expensive! So center electrode wire diameter grew finer and finer as narrower wires were used to curtail costs.

This is important for gapping because it means that many modern spark plug electrodes either have a fine coating of precious metal on them, or are made of a very narrow wire, both of which can be damaged when using a traditional spark plug gap tool. So in this how-to article, I’ll show you how to gap a copper-core plug, as well as a newer precious-metal plug.

Hey Lem, I heard I don’t have to gap plugs because they come from the factory pre-gapped

Yes, you need to check the gap at bare minimum. The gap may be correct, but it should be checked. A spark plug may have all sorts of applications — industrial stationary motors, snowmobiles, go-karts, medium-duty trucks, and maybe a motorcycle or two. Pretty clearly, the gap for a spark plug can’t be correct for every possible engine in which it may be used. Check the gap, then adjust it for your application if necessary.

Coin-style gap tool.
The ubiquitous coin-style gap tool. Perfect for copper-core spark plugs. Not so hot for modern plugs, though. Read on. RevZilla photo.

How to gap a plain copper-core spark plug with the most common tool available

First, you’d best know your spark plug gap by heart, or consult your manual. If your bike is of fairly recent vintage, you can also find this information on the emissions sticker, which is often under the seat, on the frame, or on the swingarm.

Swingarm sticker
Note the plug gap given on this swingarm sticker. RevZilla photo.

If you’re on the side of the road and need to swap plugs and you ask for a gap gauge, three or four grizzled grease monkeys will likely produce ones plucked from tool rolls or keychains. About 99 percent of the time, the tool that’s handed to you is what’s known as the “coin-style” gap tool. You can find them with seemingly everyone’s name or logo on them. They’re nearly all made by an outfit in Connecticut called Ullman Devices, and they call this style gap tool their SG-2.

Gap tool.
See the ramp? RevZilla photo.

This gappin’ device features a ramp of increasing thickness that traverses the circumference of the tool. The idea here is to slip the tool between the spark plug’s electrodes and rotate the tool until light drag is felt between them, and the tool will indicate the measurement of the gap. Should the gap be too wide, it can be closed by tapping the electrode on a convenient surface to close it suitably, taking care not to crack the porcelain or alumina insulator. (That’s the white part.)

And if the gap is too narrow? Well, here’s where many people get hung up. As tempting as it might be to yank that spark plug and use the ramp to open up the distance between the electrodes, don’t! You’ll gouge the ramp. That will lead to inaccuracy as the wear builds, and there’s also a good chance of forming burrs. Instead, look at that center hole. That’s not just so you can slip one of these on your keychain. (Though it works well for that, too.)

Just like openin' a beer. No wonder I'm so good at this. RevZilla photo.

Instead, pop the ground electrode through that hole and use the gapper like a beer bottle opener. Lift the electrode (gently!) and open the gap. Be judicious; a little goes a long way. Don’t open it too much and then close it up over and over homing in on the right gap. That little electrode will weaken as you bend it, and they can break free in the intense pressure and heat of a combustion chamber. You really don’t want to put that thing into the top of your piston or blow it out past a valve. If you find yourself opening and closing too many times, toss the plug and start over. Spark plugs are (sort of) cheap. Top-end jobs are not. Then, install your spark plugs. (Anti-seize is generally not required. AC Delco, Autolite, NGK, and Champion all recommend dry installation of their sparkin’ plugs.)

How to gap a spark plug with a better tool (and why you should, if possible)

The above method works just fine if you’ve got plain-Jane copper-core spark plugs that get changed frequently. However, since many modern motorcycles have fairly inaccessible spark plugs and very long maintenance intervals, plugs are often constructed using the precious metals in very small quantities we covered earlier.

Here’s the rub: the coin-style gapping tool can scrape off those precious metal coatings you paid so much for unless handled very gingerly, and the “lifting” action of the center electrode adjuster can scrape and wear the ground electrode. Depending on the plug, the gap tool may also “push” off the center electrode, which is obviously very problematic for that poor narrow-diameter wire. It’s all too easy to crush or bend the center electrode. If you should happen to do this, discard the spark plug and start over… and use a different gap method!

The other problem with the coin-style gap tools is that they naturally measures only one edge of the electrode’s gap. If the electrodes aren’t square to one another, you won’t really know. This is almost a certainty for those parties who just wedge the ramp into the gap and sliiiiiiide the plug along for the ride. It’s a ramp! The flat electrode is going to be gapped with one side higher than the other.

Wire-type gauge
A very ancient Bosch wire-style gap tool that's been in my toolbox since I was a rookie wrench. RevZilla photo.

Instead, you should use a wire-type gap gauge or a feeler gauge to measure the gap, so the most stressful event the electrodes will deal with is light scraping of the measuring tool. These will generally have wires of different thicknesses (or metal strips) to slip between the electrodes and ascertain the width of the gap.

That tool also has a nice electrode-bender. I love this tool. It's still available, and it's one of the best ones I own. RevZilla photo.

Most tools will also feature a provision to open the gap. The better tools will grab the ground electrode from non-critical areas in order to adjust them. Some tools allow opening only, but a few can close the gap as well as open it. Once you adjust, re-measure, and install your spark plugs.

Wire gap tool
If you've got one of these tools, you may find yourself eyeballing your gap to some degree. On a separate note, I hate this stupid tool. You try to bend the electrode, and the tool itself bends half the time. Avoid with extreme prejudice. RevZilla photo.

Lem, my gauge doesn’t have the wire for the gap I need

One thing that’s not so great about those wire-type gap gauge is that the wires generally come in only a few common thicknesses. If you’re shooting for a specific thickness in a wire you don’t have, you’ll have to use the two closest wire sizes as rough go/no-go gauges. That sort of sucks for consistency, though; at that point you may as well just eyeball the gap and call it good.

Feeler gauges
My favorite tools to measure plug gap. RevZilla photo.

And to close the gap, I just tap it shut, right?

That’s what most people do. There is a style of gap-closing tool that works by tension, rather than impact, removing the detrimental effects of both over-striking and closing the gap too much as well as cracking the insulator from impacting a hard surface. Oh yeah… that alumina can crack internally, too, and so can the resistor within. If you want a fun engine performance diagnostic exercise, try and find the source of the brand-new misfire you’ve introduced to your engine that stems from a spark plug with a miniscule internal crack. You’ll be a crack mechanic in no time.

Plug gap closer
No more broken resistors. No more kicking on the side of the road pointlessly and chasing hard-to-find ignition problems (caused by the owner!). RevZilla photo.

Instead, if your gap is over spec, use a plug gap closing tool. They cost a mint, relatively speaking, but I like them because they minimize the amount of back-and-forth bending (and thus work-hardening) the ground electrodes. Again, you do not want the electrode from a $1.49 spark plug to come free and bury itself in your nice soft aluminum alloy pistons, nor do you want to crack an insulator. I’ve done this. It sucks. And the plugs are often the last place you’ll think to check… because you just replaced them!

Best of all, this tool only acts on the exterior edge of the ground electrode, so there is no prayer of damaging either electrode’s firing surface.

Lem, this sounds damn near impossible to do on the side of the road.

It's not impossible. A few things to think about: If your bike does require either precious metal and/or fine-wire type plugs, you very well may have hard-to-access spark plugs, so the job is gonna be a bit of a bear anyway. If that’s the case, this will not add an appreciable amount of time to the job. If you’re in this camp, it can often help to simply gap your plugs that you carry with you before you’d need to swap them. Remember too, that those plugs are likely very long-lived, so simply performing your regular maintenance should prevent the need to swap plugs outside the shop.

This sounds like you are trying to sell me three different tools, rather than that $1.99 coin-style gapper, you charlatan

You’re correct. When you think about it, gapping a modern spark plug boils down to at least one and probably two of three potential jobs: measuring the gap in a non-destructive way, opening the gap, and closing it. Those three tasks are pretty different; your tools should reflect that. And do I have to remind you how much some spark plugs can cost?

Plug tools
Overkill? Maybe. Maybe not. How much did your fancy-dan spark plugs cost again? RevZilla photo.

You’ll see me on the side of the road, burning my hands on the fuel-soiled plugs I’ve pulled out of a stupid old Whateverhead, with my humble Ullman, prying on some copper-core spark plug, yes. When it comes to my modern junk, though? I’ve got the tools to handle the job at hand. I recommend you equip yourself similarly.