Wrong spark plugs or incorrect gaps cause misfires, hard starting, and poor fuel economy. Getting the plugs right is simple maintenance that affects every aspect of engine performance.
This guide covers the specifications you need and the correct replacement procedure.
Table of Contents
- Why Spark Plugs Matter
- Finding the Right Spark Plug
- Spark Plug Specifications by Brand
- Gap Specifications
- Replacement Procedure
- FAQ
Why Spark Plugs Matter
Spark plugs ignite the air-fuel mixture in each cylinder. A weak or inconsistent spark means incomplete combustion, which shows up as:
- Hard starting
- Rough idle
- Misfires under load
- Poor acceleration
- Decreased fuel economy
- Increased emissions
Marine spark plugs work harder than automotive plugs. Outboards often run at sustained high RPM, deal with salt air corrosion, and sit unused for weeks between uses. These conditions accelerate plug wear.
Finding the Right Spark Plug
OEM Part Numbers
The surest way to get the correct plug is to use the manufacturer's part number from your service manual. Cross-reference charts exist, but OEM numbers eliminate guesswork.
Key Specifications
If you're cross-referencing, match these specifications:
Thread size: Most outboards use 14mm x 19mm reach. Some smaller engines use 10mm or 12mm threads.
Heat range: This determines how quickly the plug dissipates heat. Too cold and the plug fouls. Too hot and the electrode burns or causes pre-ignition. Match the OEM recommendation exactly.
Electrode design: Standard, fine-wire, or projected tip. Match what the manufacturer specifies.
Resistor type: Most modern outboards require resistor plugs (R designation) to prevent radio interference with electronics.
Spark Plug Specifications by Brand
Yamaha Outboards
| Model | Spark Plug | Gap |
|---|---|---|
| F25-F30 | NGK DPR6EA-9 | 0.035" (0.9mm) |
| F40-F60 | NGK LFR5A-11 | 0.043" (1.1mm) |
| F70-F90 | NGK LFR5A-11 | 0.043" (1.1mm) |
| F115-F150 | NGK LFR6A-11 | 0.043" (1.1mm) |
| F200-F300 | NGK LZFR5C-11 | 0.043" (1.1mm) |
Yamaha outboard parts are available for all models.
Mercury Outboards
| Model | Spark Plug | Gap |
|---|---|---|
| 25-60 HP 4-Stroke | NGK BPR6ES | 0.040" (1.0mm) |
| 75-115 HP 4-Stroke | NGK IZFR6K-11S | 0.043" (1.1mm) |
| 150-300 HP Verado | NGK IZFR5G | 0.028" (0.7mm) |
| 200-300 HP 4-Stroke | NGK ILZFR5B-11 | 0.040" (1.0mm) |
| OptiMax (all) | NGK PZFR5F-11 | 0.040" (1.0mm) |
Mercury outboard parts ship same-day for most orders.
Johnson/Evinrude
| Model | Spark Plug | Gap |
|---|---|---|
| 40-60 HP E-TEC | NGK PZFR5F-11 | 0.030" (0.8mm) |
| 75-90 HP E-TEC | NGK PZFR5F-11 | 0.030" (0.8mm) |
| 115-300 HP E-TEC | NGK PZFR5F-11 | 0.030" (0.8mm) |
| Older 2-Stroke | Champion QL77JC4 | 0.030" (0.8mm) |
Find Johnson/Evinrude parts including spark plugs and ignition components.
Suzuki Outboards
| Model | Spark Plug | Gap |
|---|---|---|
| DF25-DF60 | NGK BPR6ES | 0.028-0.031" |
| DF70-DF90 | NGK DCPR6E | 0.028-0.031" |
| DF100-DF140 | NGK ILZFR6D | 0.040" (1.0mm) |
| DF150-DF300 | NGK ILZFR6D | 0.040" (1.0mm) |
Honda Outboards
| Model | Spark Plug | Gap |
|---|---|---|
| BF25-BF50 | NGK BPR6ES | 0.028-0.031" |
| BF60-BF90 | NGK IZFR6K-11S | 0.043" (1.1mm) |
| BF115-BF250 | NGK IZFR6K-11S | 0.043" (1.1mm) |
Gap Specifications
The gap—the distance between the center electrode and ground electrode—must match specifications exactly.
Too wide: Ignition system struggles to generate spark. Hard starting, misfires at high RPM.
Too narrow: Weak spark, incomplete combustion. Poor performance, fouling.
How to Set the Gap
- Use a wire-type gap gauge (not flat gauges—they're inaccurate for fine-wire plugs)
- Measure the gap on the new plug before installation
- If adjustment needed, bend the ground electrode only—never the center electrode
- Recheck after bending
Pre-gapped plugs: Many modern plugs come pre-gapped. Still verify before installation—shipping and handling can change the gap.
Replacement Procedure
Tools Needed
- Spark plug socket (5/8" or 13/16" depending on plug)
- Ratchet with extension
- Gap gauge
- Torque wrench
- Anti-seize compound (optional, check manufacturer recommendation)
- Dielectric grease
Step 1: Access the Plugs
Remove the engine cowling. Locate the spark plugs—typically on the side of the cylinder head(s). On V6 engines, you'll need to access both banks.
Step 2: Remove Plug Wires or Coil Packs
On older engines, pull the plug wire boot straight off—don't pull the wire itself. On newer engines with coil-on-plug ignition, remove the mounting bolts and pull the coil assembly.
Step 3: Clean Around the Plug
Blow out debris around the plug hole with compressed air. You don't want dirt falling into the cylinder when you remove the plug.
Step 4: Remove Old Plugs
Use the spark plug socket with extension. Turn counterclockwise. If a plug is seized, apply penetrating oil and wait before forcing it.
Step 5: Inspect Old Plugs
The condition tells you about engine health:
- Tan/light gray: Normal, healthy combustion
- Black/sooty: Rich mixture or oil fouling
- White/blistered: Lean mixture or overheating
- Wet: Flooded or not firing
(See our guide on reading spark plug colors for detailed diagnosis.)
Step 6: Gap New Plugs
Verify the gap on each new plug. Adjust if needed.
Step 7: Install New Plugs
Hand-thread first. Cross-threading aluminum heads is expensive. Thread by hand until finger-tight.
Torque specifications:
| Thread Size | Torque (Gasket Seat) | Torque (Tapered Seat) |
|---|---|---|
| 10mm | 8-11 ft-lb | 7-10 ft-lb |
| 12mm | 11-15 ft-lb | 11-15 ft-lb |
| 14mm | 18-22 ft-lb | 15-20 ft-lb |
If you don't have a torque wrench: tighten finger-tight, then 1/2 turn for new plugs with a gasket, 1/16 turn for tapered seat plugs.
Step 8: Reconnect Wires/Coils
Apply a small amount of dielectric grease to the inside of plug wire boots—this prevents corrosion and makes future removal easier.
Push boots firmly onto plugs until they click. Reinstall coil packs if applicable.
Replacement Interval
Conventional plugs: Every 100 hours or annually, whichever comes first.
Iridium/platinum plugs: Every 200-300 hours, but inspect annually.
Replace sooner if you notice performance issues or the plugs show abnormal wear.
FAQ
Can I use automotive spark plugs in my outboard?
Only if they match specifications exactly—thread size, reach, heat range, and electrode design. Marine-specific plugs often have better corrosion resistance. When in doubt, use the manufacturer's recommended plug.
Should I use anti-seize on spark plug threads?
Opinions vary. Some manufacturers say yes, others warn against it (can affect torque readings and cause over-tightening). Check your service manual. If using anti-seize, reduce torque by 20%.
My plugs keep fouling. What's wrong?
Common causes: running too rich, excessive idling, wrong heat range (too cold), oil getting past rings or valve seals, or bad ignition components causing weak spark.
Are iridium plugs worth the extra cost?
For outboards that see regular use, yes. They last 2-3 times longer than standard plugs and maintain consistent performance. For boats that sit most of the year, standard plugs are fine.
One plug looks different from the others. Why?
One cylinder running differently—could be an injector, compression, or ignition issue isolated to that cylinder. Investigate before just changing the plug.
Bottom Line
Match the manufacturer's specifications for plug type and gap. Use a torque wrench or at least the turn method to avoid stripping threads. Inspect old plugs for clues about engine condition. A $15 set of plugs and 30 minutes of work keeps your ignition system firing properly.
Find spark plugs and ignition components for your outboard at SeaSierra.
