Spark plugs are one of the most critical components for motorcycle engines. They transfer electrical current from the ignition to the combustion chamber to ignite the air and fuel mixture, which gets your bike running.
Spark plugs can suffer wear and tear over time due to their consistent use, so it’s essential to replace them as needed. But you need to make sure the spark plug you buy is well suited to your bike.
Harley-Davidson motorcycles are big, premium machines that require the best components available, so you’ll want to make sure the replacement spark plug is the best option for your Harley.
Here are our top spark plugs for Harley-Davidson motorcycles.
Reviews of the Best Spark Plugs for Harleys
NGK Iridium IX Spark Plugs
NGK is known for producing quality spark plugs for a whole range of motorcycles, and the Iridium IX spark plugs are some of the best units they produce. The fine wire center iridium electrode is smaller than both platinum and copper electrodes. Thanks to its durability, it ensures a consistently stable spark with a long lifespan.
The small-diameter center electrode requires less voltage, which extends the system’s life. On top of this, the metal shell plating has anti-corrosion qualities. There’s a copper core at the center of the plug that promotes active heat removal, although the iridium alloy does also have an extremely high melting point.
Iridium IX plugs produce an efficient spark for a quick start every time you want to go for a ride, even if your bike has been sitting for a while or it’s cold out.
- Compatible with a wide selection of Harleys
- Iridium-center electrode
- Improved performance over OE (original equipment) plugs
NGK Spark Plugs Standard
NGK’s standard spark plugs are built to a very high standard. The plugs use a high-alumina ceramic construction, which, along with good electrical insulation properties, is said to aid heat dissipation.
The spark plugs use a nickel alloy tip with a deep-seated copper core for a wide heat range. A wide-operating heat range can prevent issues, including the bike running both too cool and too hot.
There are many different types, so you’ll want to be sure you get the right plug for your model. Check your current plug information or your owner’s manual.
- Great value
- Well made
- Wide-operating heat ranges
- Standard nickel alloy tip with copper core
Accel U-Groove Spark Plugs for HD
Accel U-Groove spark plugs are specifically built for Harley Big Twins and Sportsters between the years 1986–2022. Be sure to check your model and year against the compatibility chart on Revzilla, but the plugs will fit most bikes within these specifications.
There is a special U-shape cut into the ground electrode that produces a double-wide hot spark suitable to ignite both standard and high-compression engines. The U-groove plugs are particularly good for high-compression motors, so will suit those that are performance focused, with performance-based upgrades.
Accel uses a copper/nickel alloy electrode for a broad heat range and quick, effective heat dissipation. Accel claims its U-groove plugs are proven to deliver high performance for both racing and street use.
- Harley-Davidson specific
- U-shape cut for double-wide electrode
- Suitable for high-compression engines
- Broad heat range
- Might be too hot for some standard engines
Accel Platinum Spark Plugs for HD
Accel’s platinum spark plugs improve on their standard plugs by using a platinum tip that is said to hold a precise gap for longer than a nickel/copper tip. Along with the platinum tip, a chromium-nickel alloy side wire provides a bigger, better spark using less voltage.
Many Harley riders consider platinum plugs to be an upgrade from OE Harley plugs.
One issue with these plugs, though, is that they’re not pre-gapped, so you need to set the gap yourself — be sure to set it the same as your current plugs to avoid issues.
- Harley specific
- Platinum tips
- Upgrade to OE plugs
- Not pre-gapped
E3 E3.36 Spark Plugs for Powersports
The E3.36 spark plug is a great-value favorite that plenty of Harley riders choose when it’s time to replace their plugs.
It comes with the DiamondFIRE ground electrode, which is open at the tip, equating to a more complete, efficient combustion. E3 states that this technology results in an increase in power output, fuel economy, and reduced emissions.
These spark plugs are also very durable. In fact, E3 offers a limited lifetime warranty with its plugs, good for five years or 100,000 miles, according to the website.
- Suitable for most later-model V-twin engines
- Patented DiamondFire ground electrode
- Good value
- Not suitable for older models
Care, Considerations and Cost
Spark plugs can make or break engine performance, so it’s important you know the details. That way, you can make an informed purchase and protect your motorcycle.
Here are some factors to note when buying a spark plug for your Harley.
When to Change Your Spark Plug
Changing your spark plugs isn’t normally on your regular maintenance schedule for your motorcycle. It is quite easy to forget about them altogether.
However, there are a few times when you should prioritize replacing your spark plugs.
- Change your Harley’s spark plugs every 15,000 miles. Even if you don’t think there’s anything wrong with your current plugs, at that point, they will have suffered enough wear and tear and be ready to be replaced.
- If your motorcycle is running rough, sounds rough like it is sputtering or losing power, your spark plugs are the first thing you should check and likely replace.
- If you buy a used bike, ask the previous owner when the spark plugs were last changed. If they don’t know, it’s worth changing them just to be safe.
- Changing your spark plugs can be a simple way to upgrade your bike’s performance.
Make Sure the Spark Plug Is Compatible
Spark plugs are not universal; they are relative to engine type and capacity.
Be sure to check that the plugs you choose are suitable for your Harley specifically. You can use Revzilla’s vehicle checker to select the right components for your model.
Know the Difference Between Spark Plug Materials
There are generally three types of metal used in spark plugs: copper, platinum, and iridium.
Copper plugs are more budget friendly, while single-platinum and double-platinum plugs fall somewhere in the middle, followed by iridium plugs, which are the most sought after and premium types of plugs.
Copper plugs are the most common standard spark plugs. Copper is an excellent conductor of electricity and rarely overheats, which means copper plugs are durable and respond well to high temperatures.
Platinum is also an excellent electricity conductor, although not as good as copper. The big difference is that platinum is more durable than copper, so you can expect a longer service range, especially if a plug is double-platinum tipped.
Iridium is more expensive than both copper and platinum, but it is both harder and stronger, making it the ideal material for spark plugs. Iridium plugs are considered to be the best on the market, but they can be hard to find because the material is rare, and they are more expensive to buy.
Understand the Terms Gap, Reach, Heat Range, and Ground Electrodes
When shopping around for spark plugs there are some common terms that will come up that may leave you scratching your head in confusion.
Gap refers to the gap between the ground and center electrodes. Getting a plug with the right gap for your bike is crucial to make sure there are no engine misfires.
Some bikes require big gaps, big sparks, and therefore big voltage and others are the complete opposite. Most spark plugs will already be set up with the right gap, so it isn’t something to worry about, just something to be aware of.
Reach is how far the plug reaches into the engine cylinder. Check the thread on the original plug so you get one with a similar number.
A plug that reaches too far into the cylinder can lead to issues, such as pre-ignition or a jam in the cylinder. Equally, a plug with not enough thread will not be as effective.
Plugs are either hot or cold. Hot plugs work with lower RPM bikes and cold plugs work with higher RPM bikes.
The heat range is usually stated on the plug itself. You will want to match this with the manufacturer’s guide and recommendations for your model. The lower the number, the hotter the spark plug; the higher the number, the colder it is.
The more electrodes in a plug, the more durable it will be. Multiple electrodes are good for engines with high compression.
Having a single-ground electrode plug is more fuel efficient, but a double will have a longer lifespan, so that’s something to consider.
Electrode size is also a consideration. Large electrodes are good for low-end power and smaller electrodes are more suited to higher rpm, higher speed, and more riding power.
Do Iridium Spark Plugs Improve Motorcycle Performance?
Iridium spark plugs are both stronger and harder than copper or platinum spark plugs. The result of this is improved engine performance in terms of stronger acceleration, more engine power, improved fuel efficiency, and more efficient starting power.
To be clear, the performance gains from iridium spark plugs alone is not going to blow your mind. The difference will be in the efficiency of your ignition, noticeable power when riding uphill, and smoothness when riding at slow speeds or in idle.
Swapping to an iridium plug mainly translates to a longer service life. For performance gains, the swap should be done in conjunction with other modifications, such as performance exhausts and air filters.
How Many Spark Plugs Do Harleys Have?
Harley-Davidson motorcycles usually have four spark plugs, two on the left-hand side of the engine and two on the right.
Can You Change Spark Plugs Yourself?
Yes, you can change spark plugs yourself relatively easily. There are plenty of YouTube videos to take you through the process, and even manufacturer guides will come with your new spark plugs to help you through the task.
NGK Iridium IX Spark Plugs are easily the best on the market, for both performance and efficiency, with the only downside being they are more expensive than other options.
If you’re looking for a budget-friendly alternative, the NGK Standard Spark Plugs are also a great option, providing quality replacements for your original plugs at a good price.