Spark Plug Replacement Guide for 2004-2007 Harley Davidson FLHRS Road King Custom
Keep your Twin Cam 88 firing on all cylinders with the right spark plugs for your Road King Custom.
- The correct spark plug gap for the Twin Cam 88 engine is 0.038"-0.043". This is critical for performance.
- High-quality aftermarket brands like Champion (RA8HC) and NGK (DCPR7E) are excellent, affordable alternatives to OEM plugs.
- Symptoms of failure include rough idle, poor acceleration, hard starting, and decreased gas mileage.
- Replacement is an easy DIY job that requires a 5/8" spark plug socket, a gapping tool, and a torque wrench.
Signs of Failing Spark Plugs
Spark plugs are a regular maintenance item and will wear out over time. When they start to fail on your Road King, you'll notice it in the ride. The engine might become hard to start, especially when cold. You may feel a stumble or hesitation when you twist the throttle, or the idle might become rough and uneven. A noticeable drop in fuel economy and a general feeling of sluggish performance are also common symptoms. If a plug is badly fouled or has failed, it can cause a persistent engine misfire, which may trigger your check engine light.
Choosing the Right Aftermarket Spark Plug
When it's time for new plugs, you have great options beyond the dealership. The original Harley-Davidson 6R12 plugs are reliable, but aftermarket brands like Champion and NGK offer the same or better quality, often for less money. In fact, many believe that Champion or NGK actually manufacture the plugs that Harley puts in their boxes.
Brand Quality Tiers
| Tier | Brands | Description |
|---|---|---|
| Premium Aftermarket | NGK (Iridium) | NGK's Iridium IX plugs, like the DCPR7EIX, are a top-tier choice. They have a fine-wire iridium tip that provides a very stable spark and is extremely durable, leading to a longer service life. They are an excellent upgrade for riders who want the best performance and longest interval between changes. |
| Standard Aftermarket / OE-Equivalent | Champion, NGK (Standard) | This is the sweet spot for most owners. The Champion RA8HC is considered a direct OE-replacement for the stock 6R12 plug. The standard NGK DCPR7E is also a very popular and trusted equivalent. Both offer excellent performance and reliability for a stock Twin Cam 88 engine at a great price. |
Check the Spark Plug Gap!
The spark plug gap is critical on a Twin Cam 88 engine. The factory manual specifies a gap between 0.038 and 0.043 inches. Many mechanics set them right in the middle at 0.040 inches. Unlike newer engines, the Twin Cam's computer uses voltage feedback across this gap to detect engine knock, so an incorrect gap can lead to poor performance. Always check and adjust the gap on new plugs before installation, even if they claim to be pre-gapped.
Cost of New Spark Plugs for a 2004-2007 Road King Custom
Spark plugs are one of the most affordable maintenance parts for your motorcycle. Prices depend on the material, with standard copper plugs being the most budget-friendly and iridium plugs costing more.
| Part Type | Our Price Range |
|---|---|
| New Aftermarket Spark Plugs | $3.91 - $14.32 |
Pro Tip: Reading Your Old Plugs
Before you toss your old plugs, take a look at the firing end. The color and condition can tell you a lot about your engine's health. A light tan or grayish color means everything is running well. A black, sooty plug indicates a rich fuel mixture, while a shiny or wet black plug points to oil getting into the cylinder. This information can help you diagnose other potential issues before they become serious problems.
Frequently Asked Questions
How often should I change the spark plugs on my Road King?
Harley-Davidson's official recommendation has varied, but a common interval is every 10,000 to 20,000 miles. However, plugs can often last longer. A good rule of thumb is to inspect them every 5,000 miles during other service and replace them when they show signs of wear, such as a rounded electrode, or when you notice performance issues.
Do I need to use anti-seize on the new spark plug threads?
This is a debated topic. Some major brands like NGK advise against it, as their plugs have a special trivalent plating that provides anti-seize properties and prevents corrosion. Adding lubricant can cause you to over-torque the plug, potentially damaging the threads in the cylinder head. It's best to follow the recommendation of the specific plug manufacturer. If you do use it, apply only a very small amount to the threads, avoiding the electrode.
What is the difference between standard and iridium spark plugs?
The main difference is the material of the center electrode. Standard plugs typically use a copper core with a nickel-alloy tip. Iridium plugs use a much harder and more durable precious metal. This allows for a finer electrode tip, which creates a more focused and consistent spark, and they last significantly longer than standard plugs.
Can I replace the spark plugs myself?
Yes, this is a very straightforward job for a DIY mechanic. You will need a 5/8-inch spark plug socket, a ratchet with an extension, a torque wrench, and a spark plug gapping tool. Always thread the new plugs in by hand first to avoid cross-threading the aluminum cylinder heads.
Helpful Videos
Technical Specifications
OEM Part Numbers: 32362-04A 6R12
Engine: Twin Cam 88 Spark Plug Gap: 0.038" - 0.043" (0.96mm - 1.09mm) Socket Size: 5/8 inch Torque Spec: 12-18 ft-lbs (Check service manual for exact spec)
We Have This Part in Stock
The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.
- Harley Davidson FLHRS Road King Custom: