Your Harley Spark Plug Gap Chart For Maximum Performance

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Mr. Chase Manhattan

Maintaining the proper spark plug gap on your Harley-Davidson motorcycle is key to ensuring your engine runs smoothly and efficiently. The gap is the distance between the center and side electrodes of the spark plug, crucial for the right ignition of the fuel-air mixture in your engine’s cylinders. A correct spark plug gap ensures that your Harley’s engine fires correctly and maintains optimal performance. But you know that, which is why you’re here, right?

It’s important to note that spark plug gap specifications can vary between different Harley models. For instance, the Shovelhead models from 1975 to 1981 traditionally require a gap of 0.025 to 0.030 inches, while the same model from 1982 to 1984 typically uses a wider gap of 0.040 to 0.045 inches. These are general guidelines, and for the exact specifications, you should always refer to your motorcycle’s owner manual or a spark plug gap chart (like the one below!) specifically for Harley-Davidson motorcycles. Keeping these measurements precise helps prevent various engine issues and contributes to a more enjoyable and reliable ride on your beloved motorcycle – and keep reading so I can show you what you need to know first!

Related resources:
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Important Torque Specs for Harley Davidson Motorcycles

Custom infographic containing a harley davidson spark plug, introducing the basic concepts behind how a plug functions

Spark plugs are crucial for your motorcycle’s ignition, and choosing the right type with the correct gap is essential for optimal performance.

Types of Spark Plugs

Your Harley-Davidson’s engine performance largely depends on the type of spark plug you use. The three most common materials used for spark plugs are:

  • Iridium: These spark plugs have a fine wire center electrode, are durable, and provide excellent longevity and performance. They generally offer better ignition and require less voltage.
  • Platinum: With a platinum disc welded to the tip, these spark plugs offer a solid performance and are typically more durable than copper spark plugs.
  • Copper: The traditional choice, copper spark plugs have a solid copper core and offer good conductivity; however, they may not last as long as iridium or platinum plugs.

Spark Plug Anatomy

Understanding the construction of a spark plug can help you better maintain and replace them:

  • Central Electrode: This is the part of the spark plug where the electrical charge accumulates before it jumps the gap and creates a spark to ignite the fuel-air mixture.
  • Insulator: Usually made of a ceramic material, it prevents current from leaking and ensures it flows to the intended path.
  • Metal Case: This houses the insulator and the central electrode, often with a hex shape that allows a spark plug wrench to grip it for installation or removal.

When installing spark plugs, the gap between the central electrode and the ground electrode is crucial. It’s important to refer to your Harley-Davidson’s specific spark plug gap chart for the precise measurements, as the gap impacts the effectiveness of the spark igniting the fuel-air mixture in the engine.

The Complete Chart for Harley Spark Plug Gaps

Harley Spark Plug Gap Chart

Model Recommended Spark Plug Gap Notes
Twin Cam 0.038 – 0.043 inches Applies to various Twin Cam variations since late 1990s.
Sportster 0.040 inches
Shovelhead (1975-1981) 0.025 – 0.030 inches
Shovelhead (1982-1984) 0.040 – 0.045 inches Wider gap for later models.
V-Rod 0.040 inches New standard for Harley performance.
XR 1000 0.040 inches Performance-oriented variant.
EVO 1340 0.032 – 0.036 inches Replaced Shovelhead in the ’80s and ’90s.

When setting up your Harley-Davidson for a smooth ride, getting the spark plug gap right is crucial. Each model of the Harley family has its own specifications, and ensuring accuracy will bolster both performance and engine health.

Twin Cam

For Twin Cam engines, you should maintain a spark plug gap ranging between 0.038 and 0.043 inches. This applies to the numerous variations of the Twin Cam from its introduction in the late 1990s to its evolution in recent years.


If you’ve got a Sportster model, keep your spark plug gap within 0.040 inches. Doing so will help your Sportster’s engine fire up successfully and operate at its best.


Shovelhead engines, which were prevalent from 1966 to 1985, require a closer gap. For models from 1975-1981, set your gap between 0.025 and 0.030 inches, and for 1982-1984 models, between 0.040 and 0.045 inches.


Introduced in the early 2000s, the V-Rod set a new standard for Harley performance. Your V-Rod will typically require a spark plug gap of 0.040 inches, ensuring high performance and efficiency.

XR 1000

The XR 1000, a performance-oriented variant of the Sportster, demands a precise spark plug gap to perform at its peak. Aim for a gap of 0.040 inches.

EVO 1340

Lastly, your EVO 1340 engine, which took the baton from Shovelhead and ran through the ’80s and ’90s, benefits from a gap setting between 0.032 and 0.036 inches. This ensures a clean burn and optimal power from your EVO.

When tuning your Harley, selecting the right spark plug is pivotal for optimal engine performance. The correct spark can improve efficiency, and ensure a smooth ride.

Choosing the Right Plug

Type: There are various types of spark plugs including standard, platinum, iridium, and more.

  • Standard plugs are often used for stock Harley engines and are a solid choice for regular maintenance.
  • Iridium and platinum plugs offer improved longevity and performance, especially under high-compression or high-RPM conditions.

Gap: Proper gapping of the spark plug is crucial for the best engine performance.

  • For example, Shovelhead models from 1975-1981 should have a gap of 0.025 – 0.030 inches, while 1982-1984 models typically require 0.040 – 0.045 inches.
  • Incorrect gapping can lead to spark misfire, which reduces efficiency and can harm your engine.

Compatibility with Performance Upgrades

When upgrading your Harley with aftermarket performance parts, consider upgrading your spark plugs to match. Performance upgrades often change the demands on your engine, which can alter the ideal spark plug choice.

  • Aftermarket ignition systems may require a different spark plug type for optimal performance.
  • Higher performance engines may benefit from plugs that can handle increased heat and pressure, like NGK Iridium IX Spark Plugs for the Sportster or Bosch Platinum Plus Spark Plugs for the Softail.

Always ensure that the plugs you select are compatible with your Harley model and any performance upgrades you’ve installed to maintain a well-running motorcycle. Remember, each model might have specific requirements, so cross-reference your bike’s specifications with manufacturer recommendations.

Proper installation and regular maintenance of your spark plugs are crucial for the smooth running of your Harley-Davidson motorcycle. Pay attention to the torque settings and the spark plug gap to ensure optimal performance.

Changing Spark Plugs

When it’s time to change your spark plugs, start by selecting the correct type for your Harley model. You’ll want to gently remove the old spark plugs using a spark plug socket—avoid applying too much force to prevent damage. Next, check the spark plug gap of the new plugs with a gap gauge; they should match your motorcycle’s specifications. For example, a common gap range for certain Shovelhead models from 1975-1981 is 0.025 – 0.030 inches.

Torquing and Spark Plug Gap

Once you’ve verified the plug gap, you can install the new spark plugs. Carefully thread them by hand at first to prevent cross-threading. Then, use a torque wrench to tighten the plugs to your manufacturer’s specs—over-tightening can damage the plug or the threads. A typical torque range for spark plugs can be around 12-18 ft-lbs, but always verify with your service manual. Remember, maintaining the correct plug gap and torque is key; the gap allows for the appropriate spark to ignite the fuel and air mixture, while the right torque ensures a proper seal and heat transfer.

In maintaining the performance of your Harley-Davidson motorcycle, understanding how to identify and address common spark plug issues is essential. If you encounter engine misfires, it could be a sign that your spark plugs aren’t gapped properly, leading to issues in the combustion process.

Common Spark Plug Issues

  • Incorrect Spark Plug Gap: A gap that is too wide or too narrow can cause your engine to perform poorly or not at all. Ensure you follow the correct specifications for your model.
  • Fouled Spark Plugs: Black, sooty deposits on the spark plug can indicate a rich fuel mixture, while a white, blistered plug may suggest a lean condition. Clean or replace plugs as needed.
  • Damaged Electrodes: A worn or damaged electrode can result in an inadequate spark. Inspect your spark plugs during regular maintenance intervals.

Diagnosing Engine Misfires

  1. Listen: A misfiring engine will often produce irregular or stuttering sounds from the exhaust.
  2. Inspect: Check for visible signs of wear or damage on each spark plug.
  3. Test: Use a spark plug tester or multimeter to assess the plugs’ electrical performance.

By diagnosing these issues promptly, you can maintain the smooth running of your engine and extend the life of your spark plugs.

When selecting the right spark plug for your Harley-Davidson motorcycle, it is essential to refer to a spark plug specifications chart to ensure compatibility and optimum engine performance. Here’s a concise reference to help you identify the correct spark plug gap and part variations for some popular Harley models and spark plugs.

Spark Plug Gap:

  • Shovelhead (1975-1981): 0.025 – 0.030 inches
  • Shovelhead (1982-1984): 0.040 – 0.045 inches
ModelStandard PlugIridium PlugGap
BreakoutNGK DCPR7ENGK DCPR7EIXCheck Manual
Sportster6R12NGK DCPR7EIX0.040 inches
SoftailRA8HCDENSO Iridium IXU220.040 inches
Touring5R6AAutolite Platinum0.038 inches

Spark Plug Brand References: (Do not treat this list as a cross-reference chart.)

  • Champion: RN12YC, RL82YC
  • NGK: DCPR7E, BPR5ES-11, Iridium DCPR7EIX
  • Autolite: AP4265DP2, Autolite Platinum
  • Denso: Iridium IXU22, IW16
  • Bosch: WR8DP, WR8DC

NGK Platinum or Iridium?
When you’re choosing between NGK Platinum or Iridium plugs like the BPR5EIX-11 or DCPR7EIX, iridium versions typically offer improved performance and longevity but check compatibility with your specific bike model.

When it comes to maintaining your Harley-Davidson’s performance, setting the right spark plug gap is essential. Here, you’ll find answers to common questions regarding the spark plug gaps for various Harley engines.

How do I set the spark plug gap for a Twin Cam 88 engine?

For a Twin Cam 88 engine, you should set the spark plug gap between 0.038 and 0.043 inches. This helps ensure the engine runs smoothly.

What should the spark plug gap be on a Harley 103 engine?

Your Harley 103 engine’s spark plug gap should be set to 0.040 inches to maintain optimal combustion.

Can you tell me the ideal spark plug gap for a Harley Evo?

The ideal spark plug gap for a Harley Evo engine is typically between 0.035 and 0.040 inches.

What is the recommended spark plug gap for the 114 M8 motor?

For the 114 M8 motor, Harley-Davidson recommends a spark plug gap of 0.040 inches.

How do I use a spark plug gap tool for my Harley Davidson?

To use a spark plug gap tool on your Harley Davidson, insert the edge of the tool between the spark plug gap and move it until the gauge matches the recommended gap size for your engine. Adjust as necessary by bending the electrode.

What is the suggested spark plug gap for NGK Iridium plugs in a Harley?

For NGK Iridium plugs in a Harley, a common gap setting is 0.040 inches, but always check your owner’s manual or the specific plug manufacturer’s recommendation to confirm.

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Hey y’all! It’s Chase Manhattan, a life-long gearhead, tinkerer, and adrenaline junky. I like to write about all things technical in the Harley Davidson and motorcycling space.