Harley-davidson Spark Plug Chart

Choosing the right spark plug is crucial for your Harley-Davidson’s performance, fuel efficiency, and longevity. This guide breaks down the official Harley-Davidson spark plug chart, helping you match the correct plug to your bike’s engine, year, and riding style—whether you’re cruising downtown or hitting the open highway.

Key Takeaways

• Correct spark plugs ensure optimal combustion: Using the right plug prevents misfires, hard starts, and poor fuel economy.
• Harley-Davidson recommends specific plugs per model: Always refer to your owner’s manual or the official spark plug chart for accuracy.
• Heat range matters: A plug that’s too hot or too cold can damage your engine over time.
• Platinum and iridium plugs last longer: Though more expensive, they offer better performance and durability than standard copper plugs.
• Regular inspection prevents issues: Check spark plugs every 5,000–10,000 miles for wear, fouling, or damage.
• Installation torque is critical: Over-tightening can crack the cylinder head; under-tightening causes poor heat transfer.
• Aftermarket plugs can work—but choose wisely: Not all third-party plugs meet Harley’s specifications, so verify compatibility.

Why the Right Spark Plug Matters for Your Harley-Davidson

If you own a Harley-Davidson, you know it’s more than just a motorcycle—it’s a lifestyle. The deep rumble of the engine, the feel of the open road, and the pride of riding a legendary machine all come together because of one tiny but mighty component: the spark plug. Yes, that small threaded piece of metal plays a huge role in how your bike performs, sounds, and lasts.

Think of the spark plug as the heartbeat of your engine. Every time it fires, it ignites the air-fuel mixture in the combustion chamber, creating the power that moves you forward. If that spark is weak, delayed, or inconsistent, your ride suffers. You might notice sluggish acceleration, rough idling, or even stalling at stoplights. Worse, using the wrong spark plug can lead to engine damage over time, costing you far more than a simple replacement.

That’s why understanding the Harley-Davidson spark plug chart isn’t just helpful—it’s essential. Whether you’re a weekend rider or a full-time road warrior, knowing which spark plug fits your specific model ensures your Harley runs smoothly, efficiently, and reliably. This guide will walk you through everything you need to know, from decoding the chart to choosing upgrades and performing maintenance like a pro.

Understanding the Harley-Davidson Spark Plug Chart

The Harley-Davidson spark plug chart is a reference tool that lists the recommended spark plugs for each motorcycle model, engine type, and production year. It’s typically found in your owner’s manual, on Harley’s official website, or in service manuals. But what does it actually tell you?

At its core, the chart matches your bike’s engine specifications—like displacement, compression ratio, and ignition system—to the correct spark plug type. For example, a 2020 Harley-Davidson Road King with a Milwaukee-Eight 107 engine uses a different plug than a 2005 Sportster 883. The chart accounts for these differences so you don’t have to guess.

Each entry in the chart includes:
– The spark plug model number (e.g., Harley part number 6134 or 6135)
– The plug’s heat range
– The electrode material (copper, platinum, iridium)
– The recommended gap (if adjustable)

For instance, many modern Harley touring models use the Harley-Davidson 6135 spark plug, which is a platinum-tipped plug designed for long life and consistent performance. Older Evolution engines might use the 6134, a copper-core plug that’s more affordable but requires more frequent replacement.

It’s important to note that Harley updates its spark plug recommendations as engine technology evolves. The Milwaukee-Eight engines introduced in 2017, for example, were designed to run hotter and more efficiently, requiring plugs with better heat dissipation and durability. That’s why relying on an outdated chart or guessing based on a friend’s bike can lead to problems.

How to Read the Chart Like a Pro

Reading the Harley-Davidson spark plug chart isn’t complicated, but it does require attention to detail. Start by identifying your motorcycle’s model, year, and engine size. You can usually find this info on the VIN plate near the front of the frame or in your owner’s manual.

Once you have that, locate your model in the chart. Harley often groups models by engine family—like Twin Cam, Evolution, or Milwaukee-Eight—so you’ll see sections for each. Within those sections, you’ll find the recommended spark plug part number.

For example:
– 2018 Harley-Davidson Street Glide (Milwaukee-Eight 107): Plug 6135
– 2008 Harley-Davidson Dyna Super Glide (Twin Cam 88): Plug 6134
– 2002 Harley-Davidson Sportster 1200: Plug 6134

Some charts also include notes about special conditions. For instance, if you ride in extreme heat or cold, or if you’ve modified your engine (like adding a high-performance cam or exhaust), you might need a different heat range plug. Always check for these notes—they can make a big difference.

Why You Shouldn’t Ignore the Chart

It might be tempting to grab any spark plug that fits, especially if you’re in a hurry or working on a budget. But skipping the chart can lead to serious issues. Using a plug with the wrong heat range, for example, can cause pre-ignition (knocking) or fouling. A plug that’s too hot might melt the electrodes; one that’s too cold won’t burn off carbon deposits, leading to misfires.

Even small differences in thread length or reach can cause problems. A plug that’s too long might protrude into the combustion chamber and damage the piston. One that’s too short won’t seat properly, leading to poor heat transfer and potential overheating.

Harley-Davidson engineers spend countless hours testing spark plugs under real-world conditions to ensure they meet the brand’s performance and durability standards. When you follow the chart, you’re benefiting from that expertise. It’s like using the right oil or tires—small choices that add up to a better ride.

Types of Spark Plugs Used in Harley-Davidson Motorcycles

Not all spark plugs are created equal, and Harley-Davidson uses several types depending on the engine and model year. Understanding the differences helps you make informed decisions, especially if you’re considering an upgrade.

Copper Core Spark Plugs

Copper core plugs are the most basic and affordable option. They feature a solid copper center electrode with a nickel alloy coating. These plugs offer excellent conductivity and are great for older Harley models, especially those with Evolution or early Twin Cam engines.

However, copper plugs have a shorter lifespan—typically 10,000 to 20,000 miles—because the nickel coating wears down over time. They also require more frequent gap adjustments. If you ride an older Harley and stick to stock performance, copper plugs are a solid, budget-friendly choice.

Platinum Spark Plugs

Platinum plugs are a step up in durability and performance. They use a platinum disc welded to the center electrode, which resists wear and maintains a sharp edge longer than copper. This means better spark consistency and longer life—often 60,000 miles or more.

Harley began using platinum plugs in many of its Twin Cam and early Milwaukee-Eight engines. The Harley 6135 plug, for example, is a platinum-tipped plug commonly found in touring models. It’s designed to handle higher combustion temperatures and provide reliable performance under heavy loads, like long highway rides or two-up touring.

Iridium Spark Plugs

Iridium is the premium choice for spark plugs. It’s one of the hardest metals used in automotive applications, offering exceptional wear resistance and a fine wire center electrode that requires less voltage to fire. This results in a stronger, more efficient spark.

While Harley doesn’t factory-install iridium plugs on most models, many riders upgrade to them for better performance and longevity. Brands like NGK and Denso offer iridium plugs that are direct replacements for Harley part numbers. For example, the NGK Iridium IX DCPR7EIX can replace the 6134 in many applications.

Iridium plugs can last up to 100,000 miles, making them ideal for riders who want minimal maintenance. They’re especially beneficial if you’ve modified your engine for more power, as they handle higher temperatures and pressures better than copper or platinum.

Double Platinum and Double Iridium

Some high-end plugs feature dual electrodes—platinum or iridium on both the center and ground electrodes. These are designed for waste-spark ignition systems, where the plug fires on both the compression and exhaust strokes. While not common on Harleys, they’re worth noting if you’re exploring aftermarket options.

How to Choose the Right Spark Plug for Your Harley

Choosing the right spark plug starts with your owner’s manual and the Harley-Davidson spark plug chart. But there are other factors to consider, especially if you’ve modified your bike or ride in extreme conditions.

Match the Plug to Your Engine

Every Harley engine has specific requirements. The Milwaukee-Eight, for example, runs hotter and has a higher compression ratio than older engines. Using a plug designed for a Twin Cam in a Milwaukee-Eight could lead to overheating or pre-ignition.

Always check the chart for your exact model and year. If you’re unsure, visit a Harley dealership or consult a certified mechanic. They can cross-reference your VIN to confirm the correct plug.

Consider Your Riding Style

How you ride affects spark plug performance. If you frequently ride short distances or in stop-and-go traffic, your plugs may foul more easily due to incomplete combustion. In this case, a hotter plug (one that dissipates heat more slowly) might help burn off deposits.

On the other hand, if you ride long highway miles at high speeds, your engine runs hotter. A colder plug (one that transfers heat away faster) can prevent overheating and electrode damage.

Most stock Harleys are tuned for a balance of city and highway riding, so the factory-recommended plug is usually ideal. But if your riding habits are extreme, consider consulting a tuner or performance shop.

Upgrades and Modifications

If you’ve added performance parts—like a high-flow air filter, aftermarket exhaust, or a performance cam—your engine may run leaner or hotter. This can affect spark plug performance.

In these cases, you might need a different heat range plug. For example, a hotter plug can help prevent fouling in a lean-running engine, while a colder plug can protect against detonation in a high-compression setup.

Always re-tune your fuel management system when making modifications. A professional dyno tune ensures your air-fuel ratio is correct, which directly impacts spark plug health.

Aftermarket vs. OEM Plugs

Harley-Davidson sells OEM (original equipment manufacturer) spark plugs, which are guaranteed to meet factory specs. They’re reliable and widely available at dealerships.

But aftermarket brands like NGK, Denso, and Autolite also make high-quality plugs that meet or exceed Harley’s standards. In many cases, they’re less expensive and offer better performance, especially with iridium or platinum designs.

The key is to choose a plug with the same specifications: thread size, reach, heat range, and gap. For example, if your chart calls for a 6134 plug, look for an aftermarket equivalent with the same dimensions and heat rating.

Avoid cheap, generic plugs. They may fit, but they often lack the materials and precision needed for Harley engines. Poor-quality plugs can cause misfires, reduced fuel economy, and even engine damage.

Installing and Maintaining Your Harley Spark Plugs

Even the best spark plug won’t perform well if it’s installed incorrectly or neglected. Proper installation and regular maintenance are key to getting the most out of your plugs.

Step-by-Step Installation Guide

1. **Let the engine cool.** Never change spark plugs on a hot engine. Heat can warp components and make removal difficult.
2. **Remove the spark plug wires or coils.** Gently pull the boot, not the wire. If it’s stuck, twist it slightly to break the seal.
3. **Clean the area.** Use compressed air to blow out debris around the plug holes. Dirt falling into the cylinder can cause serious damage.
4. **Remove the old plugs.** Use a spark plug socket and ratchet. Turn counterclockwise. If a plug is stuck, apply a penetrating oil and let it sit before trying again.
5. **Check the gap.** If your new plug isn’t pre-gapped, use a gap tool to set it to the manufacturer’s specification (usually 0.038–0.043 inches for Harleys).
6. **Install the new plugs.** Hand-tighten first to avoid cross-threading. Then use a torque wrench to tighten to spec—typically 12–18 ft-lbs for most Harley plugs. Over-tightening can crack the cylinder head.
7. **Reattach wires or coils.** Push them on firmly until you hear a click.

When to Replace Spark Plugs

Harley recommends replacing spark plugs every 10,000 to 20,000 miles for copper plugs, and up to 60,000 miles for platinum or iridium. But you should inspect them more frequently—every 5,000 miles is a good rule of thumb.

Signs it’s time to replace your plugs include:
– Hard starting
– Rough idle
– Poor acceleration
– Decreased fuel economy
– Check engine light (especially if related to misfires)

During inspection, look for:
– **Normal wear:** Light gray or tan deposits, minimal electrode erosion.
– **Fouling:** Black, sooty deposits (rich mixture or short trips).
– **Overheating:** White or blistered insulator, melted electrodes.
– **Oil fouling:** Wet, oily deposits (possible valve seal or piston ring issue).

If you see abnormal wear, diagnose the root cause before replacing the plugs. A fouled plug might be a symptom of a larger problem.

Tips for Longer Plug Life

– Use high-quality fuel and oil.
– Avoid excessive idling.
– Warm up your engine gradually.
– Keep your air filter clean.
– Follow your maintenance schedule.

Regular care goes a long way in protecting your spark plugs—and your engine.

Common Mistakes to Avoid

Even experienced riders make spark plug mistakes. Here are the most common ones—and how to avoid them.

Using the Wrong Plug

This is the #1 mistake. Always double-check the chart. Don’t assume all Harleys use the same plug.

Over-Tightening

Spark plugs are threaded into aluminum cylinder heads, which are soft. Over-tightening can strip threads or crack the head. Always use a torque wrench.

Ignoring the Gap

A incorrect gap affects spark strength. Too wide, and the spark may not jump; too narrow, and it may not ignite the mixture efficiently.

Reusing Old Wires or Boots

If your spark plug wires are cracked or brittle, replace them. Weak wires reduce spark energy.

Not Cleaning the Area

Dirt in the cylinder can score the walls or damage the piston. Always clean before removing plugs.

Conclusion

The Harley-Davidson spark plug chart is more than just a list of part numbers—it’s a roadmap to better performance, reliability, and longevity for your motorcycle. By choosing the right plug, installing it correctly, and maintaining it regularly, you ensure your Harley runs the way it was meant to: smooth, powerful, and unmistakably American.

Whether you’re riding a vintage Sportster or a modern CVO Limited, the principles remain the same. Respect the engineering, follow the recommendations, and listen to your bike. A healthy spark plug means a happy engine—and more miles of open-road freedom.

So next time you’re due for a tune-up, don’t guess. Consult the chart, pick the right plug, and ride with confidence. Your Harley deserves nothing less.

Frequently Asked Questions

What spark plug does a 2020 Harley-Davidson Road King use?

A 2020 Harley-Davidson Road King with the Milwaukee-Eight 107 engine uses the Harley-Davidson 6135 spark plug, which is a platinum-tipped plug designed for long life and consistent performance under heavy loads.

Can I use an aftermarket spark plug in my Harley?

Yes, you can use aftermarket plugs from brands like NGK or Denso, as long as they match the specifications (thread size, reach, heat range, and gap) of the OEM plug recommended in the Harley-Davidson spark plug chart.

How often should I replace my Harley spark plugs?

Copper plugs should be replaced every 10,000 to 20,000 miles, while platinum or iridium plugs can last up to 60,000 miles or more. Inspect them every 5,000 miles for wear or fouling.

What happens if I use the wrong spark plug?

Using the wrong plug can cause misfires, hard starting, poor fuel economy, or even engine damage due to incorrect heat range or physical incompatibility with the combustion chamber.

Do I need to gap my Harley spark plugs?

Most modern Harley spark plugs come pre-gapped, but it’s always a good idea to check with a gap tool. The typical gap for Harley plugs is between 0.038 and 0.043 inches.

Can a bad spark plug cause my Harley to stall?

Yes, a worn or fouled spark plug can cause weak or inconsistent sparks, leading to misfires, rough idling, and stalling—especially at low speeds or when the engine is under load.