How Many Spark Plugs Does a Harley Have?

When most Harley riders go to look at power specs, it’s safe to say that spark plugs aren’t at the top of their list. They’re almost never mentioned in engine-related material, and it may even be said that spark plugs are basically taken for granted.

Until they fail or start to work improperly, that is. If the spark plugs in your Harley don’t transfer voltage and create a spark in the combustion chamber effectively, the best power specs in the world won’t help you get the most out of your motorcycle.

So, specifically, how many spark plugs does a Harley have? Different Harley models have different numbers of plugs, and there are some very specific reasons why. It’s important to know what spark plugs do, and why they’re so important for great performance.

The Basics of Harley Spark Plug Design

There are two basic design principles when it comes to spark plugs and engine design for Harley Davidson motorcycles:

(1) single spark plugs for each cylinder.

(2) twin spark technology, which relies on a pair of spark plugs for each cylinder.

In a single-spark Harley Davidson engine, the job of the spark plug is basically the same as it is in any other motorcycle engine. It’s a ceramic element tasked with the job of transferring a spark into the engine to ignite the gas in the cylinder.

The spark plug is separated from the cylinder, which is basically a metal shell, by a combination of water and air vapor. When the spark is generated by the plug, the gas is ionized and the piston movement begins.

In traditional Harley engine design, single-cylinder plugs were a staple, but one of the design advances HD came up with was to use two spark plugs per cylinder. It’s a slightly different design that offers certain performance advantages, so let’s examine the mechanisms involved and see how that advantage works.

The Advantage of Twin Spark Plugs

In a twin-spark engine, each plug creates a different flame front within the cylinder. This helps the gas in the combustion chamber burn more efficiently, which in turn creates more horsepower and faster acceleration. It also helps generate faster ignition because the load on each plug is reduced.

This performance advantage can be applied in different ways to a variety of Harley models. When you go to evaluate HD performance and match it up with your needs, it’s important to look at the model specs to see if you’re dealing with a bike based on single- or twin-plug technology.

Harley Davidson Plug Types

Iridium Spark Plugs

But twin-plug technology isn’t the only way to upgrade the performance of your Harley. One of the best things you can do to upgrade your ignition system is to use an iridium spark plug.

Why? Because iridium plugs are performance components that work by using a welded iridium electron. This material helps prevent misfires, and it also improves the quality of the spark.

That means better cold starting and better acceleration once you’re out on the road. Iridium plugs also improve the entire fuel combustion process, which means they can help reduce carbon deposits within the combustion chamber.

In addition, iridium spark plugs last longer. Most manufacturers, including HD, recommend changing the plugs every two years, but iridium plugs tend to have a longer service life.

They cost a little extra, but for those riders looking for added performance benefits, they’re well worth the extra money, and they can also pay off on the back end by extending engine life.

Plasma-Assisted Plugs

Now let’s talk about plasma plugs. The name describes how they work to some extent—in a plasma-based plug, an internal capacitor creates an electrical pulse that saturates the fuel in the combustion chamber with plasma.

The result is faster ignition along with a faster burn. That increases the torque applied to the crankshaft, which translates to extra power transferred to the wheels and lower fuel consumption.

When they first came out, plasma-assisted plugs, which were pioneered by a company called Pulstar, were considered a breakthrough in motorcycle plug technology. The electrode in these plugs is six times stronger than the copper in conventional plugs, and their construction is more compact, which means extra performance in low-voltage situations.

The Performance Factor

While it’s appealing to try to get increased performance with these two types of spark plugs, simply switching plugs doesn’t guarantee improvement.

Specific plug types are carefully matched up with different Harley models according to the engine design and the power requirements.

The specs tend to vary slightly from one model to another, so it’s important to know which plugs match up well with your Harley if you want to switch plugs to gain a performance advantage.

Replacing Harley Plugs

The basic Harley Davidson maintenance recommendation for Harley plugs is to replace them every two years or 30,000 miles, whichever comes first, but these numbers also change from model to model.

Many DIYers with older motorcycles consider plug replacement a job they can handle on their own, but the intricacies of Harley design can make this problematic.

In the M8, for instance, there are two plugs per cylinder, and the plugs on one side are easily accessible. But the two plugs on the right side of these bikes are harder to reach, and in order to change them, the entire fuel tank has to be removed from the bike.

It’s not a hard job, but it’s definitely one that should be done by either a professional mechanic or someone with extensive experience doing DIY motorcycle repairs.

Gapping the plugs precisely is very important as well. We’ll use the M8 as an example again—the required gap is between 0.031-0.035 inches, and while gapping tools make this a relatively simple job, it does require experience and a fairly high skill level.

Get the Best Plugs and Performance for Your Harley

If you’re at all confused about spark plugs and HD engine performance, you should pay a visit to your dealership. The techs will talk to you about your specific model and make the appropriate recommendations, and they’ll make sure you get the spark plugs that will give you the best performance, with no compromises.

If you’re in the Houston area, you can do all this at the Kingwood dealership. Simply visit our website and we’ll make sure you have the right plugs for your specific Harley when you consult with one of our techs, or you can call us at (281) 358-0457.