Honda GX390 Spark Plug Gap, Number & Socket Size Guide

Did you know that an improperly gapped spark plug can decrease your Honda GX390’s performance by up to 50%? It’s shocking, but true. That’s why we’ve put together this handy guide for you.

We’re going to dive into the critical details of the Honda GX390 spark plug gap, number and socket size specifically for you. This isn’t just about ensuring optimal performance; it’s about maximizing efficiency and prolonging the life of your beloved machine.

You don’t need to be an expert mechanic to get this right – with our clear, concise instructions, even a novice can achieve perfect results. So let’s gear up and delve into the nitty-gritty of Honda GX390 spark plugs.

Together, we’ll ensure your engine keeps running smoothly as part of our wider mechanical community.

A Guide To Honda Gx390 Spark Plug Gap, Number & Socket Size

Key Takeaways

  • Spark plug gap must be maintained within the range of 0.028′-0.031′ for reliable ignition system.
  • NGK and DENSO are reputable brands for spark plugs, but equivalent brands can also be used as long as they meet specifications.
  • Improperly gapped spark plugs can significantly decrease engine performance by up to 50%.
  • Using the correct socket size (13/16′ or 21mm) prevents damage to spark plug threads and ensures proper engine performance.
  • Owner´s manual –


You’ll need to know that the spark plug number for your Honda GX390 is either BPR6ES (NGK) 98079-56846 or W20EPR-U (DENSO) 98079-56855, and it should have a gap of 0.028″– 0.031″ – pretty neat, right?

This precision in gap measurement ensures optimal performance from your engine.

Delving deeper into this specification, the importance of maintaining this precise spark plug gap can’t be overstated. It’s not just about meeting manual recommendations; it’s about ensuring a reliable and efficient ignition system for your engine. When you get this right, you’ll feel the smooth operation of your Honda GX390 every time you use it.

The two numbers specified are not random either – they represent two renowned brands in the industry: NGK and DENSO. These manufacturers are known for their superior quality products designed to deliver on performance expectations consistently. When using these parts, remember that ‘equivalent’ means any other brand that meets these exact specifications.

Remember, each part plays a critical role in achieving peak performance from your Honda GX390 engine. By taking care of details like the correct spark plug number and its optimal gap size, you’re contributing towards an enduring machine operation experience without unnecessary interruptions or breakdowns.


To remove or install the ignition component in your engine, you’ll need a wrench that measures 13/16′ (21mm). This is the specific socket size for spark plugs used in Honda GX390 engines. It’s important to note that this size is not arbitrary but carefully defined by engineers to ensure optimal performance and ease of maintenance.

Each part of your Honda GX390 engine was meticulously designed with precision and purpose, including the spark plug socket size. The 13/16′ (21mm) measurement isn’t just a random figure; it’s integral to maintaining the high level of performance expected from your Honda engine.

Using the correct socket size ensures that you can easily remove and install spark plugs without causing damage. Incorrect sizes may strip or damage the spark plug threads leading to potential failure or poor engine performance.

Remember, as a member of our Honda family, we want you to feel confident and equipped when performing maintenance on your engine. Knowing you have the right tools at hand empowers you to take care of your equipment effectively.

Ensure you always use a 13/16″ (21mm) wrench when working with the spark plugs in your Honda GX390 engine, respecting its design specifications while nurturing its longevity and reliability.

Frequently Asked Questions

What are the signs of a bad spark plug in a Honda GX390?

If your Honda GX390’s performance is inconsistent, it’s having trouble starting or the engine idles roughly, these may be signs of a bad spark plug. Misfiring or fuel consumption increase also indicate spark plug issues.

How often should you change the spark plug in a Honda GX390?

You should change the spark plug in your Honda GX390 every 100 hours of use or once per season, whichever comes first. Regular maintenance will keep your engine running smoothly and extend its operational life.

What are the effects of a too wide or too narrow spark plug gap in a Honda GX390?

If your spark plug gap is too wide, it may cause misfires and poor fuel economy. Conversely, a too narrow gap can lead to weak ignition sparks, reducing engine power and performance in your GX390.

Can I use a non-Honda branded spark plug in my Honda GX390?

Yes, you can use a non-Honda branded spark plug in your Honda GX390. However, ensure it’s of the correct specification and is compatible with your engine for optimal performance and to avoid potential damage.

What are the steps to replace a spark plug in a Honda GX390?

First, disconnect the spark plug cap. Then, using a spark plug socket, loosen and remove the old plug. Next, set the new plug’s gap correctly, install it and reattach the cap. Always check your work for safety.


In wrapping up, you’ve got the lowdown on your Honda GX390’s spark plug gap and number. You’ve also learned about its socket size.

So, don’t let a little engine maintenance daunt you; armed with this technical knowledge, you’re more than ready to tackle it head-on.

Remember, with precise information and clear instructions, tinkering with your Honda GX390 becomes less of a chore and more of an accomplishment.

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