How do you read a spark plug gap tool? You read it by matching the thickness of the wire or blade you slide between the center and ground electrodes of the spark plug to the measurement marked on the tool itself. This is key for correct spark plug gap measurement.
Getting the spark plug gap right is vital for your engine to run well. If the gap is too small, the spark might be weak. If it is too big, the spark might not jump at all. This guide will show you exactly how to use different tools to check and set this crucial gap. We will focus on making the spark plug gapping process easy to grasp.
What is a Spark Plug Gap Tool?
A spark plug gap tool is a simple, yet important, device. It helps you measure the distance between the spark plug’s center electrode and its ground electrode. This distance controls how easily electricity can jump across to start combustion in the cylinder.
Engineers design spark plugs with a specific air gap. This gap ensures the spark has the right energy and timing. Using the correct tool makes setting spark plug gap accurate and fast.
Types of Spark Plug Gap Tools
Not all gap tools look the same. Knowing the different types of spark plug gap tools helps you choose the right one for the job.
Coin-Style Gap Tools
These look like a small stack of coins or a flat, multi-edged piece of metal. Each edge has a specific thickness stamped on it.
- How to use: You slide the edge that fits snugly between the electrodes into the gap. The number stamped on that edge is your measurement.
- Pros: They are cheap and easy to carry.
- Cons: They can wear down over time, leading to inaccurate readings.
Feeler Gauge Spark Plug Tools
This is often the most precise tool for spark plug gap measurement. It uses thin, flat blades of metal, each machined to an exact thickness.
- How to use: You find the blade that slides in with slight resistance. This resistance is important; too loose or too tight means you need the next size up or down. This is the main method for spark plug gauge usage.
- Pros: Very accurate, especially when new.
- Cons: Can be fiddly to use, and blades can bend if handled roughly.
Wire-Style Gap Tools
These tools use a set of round wires of varying diameters. They are very common today.
- How to use: You push the wire that fits perfectly between the electrodes. The number etched on the wire handle tells you the size. These are sometimes confusingly called a spark plug wire gap tool, though they measure the plug, not the ignition wire.
- Pros: Very accurate for small gaps and less likely to damage the electrodes than coin-style tools.
- Cons: If the ground electrode is heavily worn, the wire might not sit flat, giving a false reading.
| Tool Type | Primary Feature | Best For | Accuracy Level |
|---|---|---|---|
| Coin-Style | Stamped Edges | Quick checks in the field | Medium |
| Feeler Gauge | Thin Metal Blades | Precision setting spark plug gap | High |
| Wire-Style | Rods of Specific Diameter | General use and quick measurement | High |
Deciphering Your Vehicle’s Specifications
Before you start checking spark plug gap, you must know what gap size your engine needs. This is called the spark plug gap specifications. Using the wrong gap can cause poor fuel economy or engine misfires.
Where to Find Spark Plug Gap Specifications
The correct specification is not universal. It changes based on the engine type, the year it was made, and the type of ignition system used.
- Owner’s Manual: This is the best place to look first. The maintenance section will list the required gap for the original equipment plugs.
- Spark Plug Box: If you buy new plugs, the correct gap is usually printed on the side of the box or packaging.
- Service Manuals: For older or performance vehicles, a factory service manual will have the exact data.
- Online Databases: Many reputable parts suppliers keep databases where you can enter your car’s year, make, and model to find the gap.
Important Note: High-output ignition systems or electronic ignition systems often require a slightly larger gap than older systems. Always defer to the specific requirements for the plugs you are installing.
Common Measurement Units
Spark plug gaps are measured in inches (in) or millimeters (mm). Most tools sold in North America will have markings in thousandths of an inch (e.g., 0.035″). Metric tools use millimeters (e.g., 0.9 mm). Ensure your tool matches the units required for your specifications. For example, a 0.035-inch gap is about 0.89 mm.
Step-by-Step Guide to Checking Spark Plug Gap Measurement
This section details the precise technique for checking spark plug gap using the most common tool: the wire-style gauge. This method is effective for both new plugs and for checking spark plug gap on plugs removed for inspection.
Preparation Before Measuring
Before you begin spark plug gauge usage, follow these critical prep steps:
- Clean the Plug: Wipe off any dirt or oil from the spark plug threads and the area around the electrode. Dirt can interfere with an accurate reading.
- Inspect the Electrodes: Look closely at the center electrode (the tiny metal tip) and the ground electrode (the hook shape over the tip). If the ground electrode is damaged, bent, or covered in heavy carbon buildup, you must clean or replace the plug before measuring.
Executing the Measurement
Now, let’s put the tool to work.
Using a Wire or Feeler Gauge
- Select the Tool Size: Look at your vehicle’s spark plug gap specifications. Start with the wire or blade that is slightly smaller than the required gap.
- Insert the Gauge: Hold the spark plug securely. Gently slide the selected wire or blade between the center electrode and the ground electrode.
- Apply Gentle Pressure: You should feel a slight drag or resistance as the gauge slides through. This is the correct fit.
- Test the Next Size Up: If the wire slides through with no drag at all, it means your gap is too wide. Try the next larger gauge size. If that next size does not fit, then the smaller wire was the correct fit, and the gap is too narrow.
- Confirm the Correct Fit: The correct measurement is the thickness that slides in with a noticeable, but light, drag. If you are checking spark plug gap on a used plug, you might need to try several adjacent sizes to confirm the true space.
Safety Tip: Never force the gauge. Forcing it can bend the ground electrode, immediately ruining the measurement and possibly damaging the plug.
The Process of Adjusting Spark Plug Gap
If your spark plug gap measurement is incorrect, you must adjust the distance. This is where the adjusting spark plug gap technique comes in.
Adjusting a Gap That is Too Small
If the gauge slides through too easily, the gap is too narrow. You need to increase the distance.
- Use the Hook Tool: Many gap tools, especially wire-style ones, have a small hook or lever built into the body of the tool.
- Gently Pry: Place this hook under the curved part of the ground electrode (the hook shape).
- Move Slowly: Gently pull the hook away from the center electrode. Move the electrode just a tiny bit at a time.
- Re-Check: After each small movement, re-insert the correct gauge size. Repeat the adjusting spark plug gap movement until you achieve that slight drag feeling.
Adjusting a Gap That is Too Large
If the gauge does not fit at all, the gap is too wide. You need to decrease the distance.
- Use the Edge Tool: Use the flat edge of the tool (or sometimes a specific tapping point on the body of a wire gauge).
- Tap Gently: Place the tool against the ground electrode. Tap lightly on the side of the electrode with the tool’s body or a small, soft hammer. You are essentially tapping the hook closer to the center electrode.
- Move Carefully: Tapping too hard will close the gap too much. It is always better to tap lightly and check the fit several times. This cautious approach ensures you do not overshoot your target spark plug gap specifications.
Fathoming the Importance of Correct Gapping
Why do mechanics stress the spark plug gapping process so much? The gap dictates the required voltage for the spark to jump.
Voltage Requirements and Coil Load
Your ignition coil creates high voltage. The larger the gap, the higher the voltage needed to force the electricity across the air space.
- Too Wide Gap: Requires very high voltage. If the coil cannot produce enough voltage (common in older cars or when the coil is weak), the spark will fail. This results in a misfire, especially under load (like accelerating hard).
- Too Narrow Gap: Requires low voltage. The spark jumps too easily. This results in a weak, lazy spark that may not fully ignite the fuel mixture, leading to incomplete combustion, carbon buildup, and reduced power.
Effects on Modern Engines
Modern engines use computers (ECU) to time the spark precisely. An incorrect gap throws off this timing.
- Fuel Economy: Incomplete combustion from a weak spark wastes fuel.
- Emissions: Unburnt fuel exits the exhaust, increasing harmful emissions.
- Catalytic Converter Health: Raw fuel entering the hot converter can cause it to overheat and fail prematurely.
By mastering spark plug gauge usage, you protect these expensive components.
Special Considerations for Different Systems
While the basic spark plug gapping process remains the same, some systems have special rules.
Distributorless Ignition Systems (Coil-On-Plug)
These systems deliver very high voltage pulses directly to each plug. Because the voltage is so strong, sometimes the manufacturer recommends a slightly larger gap to take advantage of that power. Always check the spark plug gap specifications for these systems carefully.
Propane or Natural Gas Conversion Kits
Engines converted to run on LPG (liquefied petroleum gas) often run hotter and require more voltage. Mechanics frequently recommend installing colder heat-range plugs and slightly adjusting spark plug gap to a smaller size than gasoline engines to ensure a reliable spark under high cylinder pressures.
Inspecting Used Plugs During Checking Spark Plug Gap
When you pull old plugs out to inspect them, the way the gap has changed tells a story about your engine’s health.
| Appearance of Gap Area | Possible Cause | Action Required |
|---|---|---|
| Ground electrode is severely eroded/rounded off. | Normal wear, or gap was set too wide initially. | Replace plugs and set new gap correctly. |
| Center electrode is burned down very fast. | Ignition timing is too advanced, or the plug heat range is too hot. | Check timing; use a colder plug. |
| Gap is clearly wider than specification, but electrodes look okay. | The ground strap has stretched due to vibration or improper tool use. | Correct the spark plug gap measurement. |
Maintaining Your Gap Tools
The accuracy of your spark plug gap measurement depends entirely on the condition of your tool. A worn-out gauge is worse than no gauge at all.
Cleaning and Storage
- Keep it Clean: After every use, wipe off any oil, carbon dust, or moisture from the blades or wires.
- Avoid Dropping: Dropping a feeler gauge set can bend the thin blades, instantly making them inaccurate.
- Lubrication (For Feeler Gaues): Lightly apply a drop of general-purpose oil to the blades occasionally to prevent rust, especially if you live in a humid area.
When setting spark plug gap, always verify your tool’s accuracy periodically by checking it against a known standard gauge if you have access to one, or against a new, factory-gapped plug.
Advanced Techniques: Measuring Wear
Sometimes you need to determine how much wear is on an old plug without replacing it. This requires careful spark plug gauge usage.
When checking spark plug gap on an old plug, use a gauge that is slightly larger than the specified gap. If that gauge slides in easily, the plug is worn out, even if the visual appearance seems okay.
If you are trying to measure an existing gap on a plug installed in the engine (though this is not recommended), you must be extremely careful not to touch the tool to any metal engine parts, which could cause a short or a shock.
FAQ Section
Q: Can I use a ruler to measure the spark plug gap?
A: No, you should never use a ruler. Rulers are not precise enough. Spark plug gaps are measured in thousandths of an inch (or hundredths of a millimeter). A standard ruler cannot give you the accuracy needed for proper spark plug gap measurement. Only use a proper gap tool like a feeler gauge or wire gauge.
Q: Do I need to gap iridium or platinum spark plugs?
A: Yes, generally you do. While many premium plugs come pre-gapped, you should always verify the spark plug gap specifications upon opening the box. Because the center electrode on these plugs is very fine, you must be extra careful when adjusting spark plug gap. Use the appropriate tool and only use the designated gap adjustment point on the tool to avoid chipping the fine electrode material.
Q: What happens if I use the wrong type of spark plug wire gap tool?
A: If you use a tool that is too large, you will bend the ground electrode too far out, resulting in a gap that is too wide. This forces the ignition coil to work too hard, leading to misfires or coil failure. If the tool is too small, the gap will be too tight, leading to a weak spark and poor combustion.
Q: How often should I be checking spark plug gap?
A: You should check the gap any time you remove the plugs for inspection or before installing new plugs. For standard copper plugs, manufacturers often recommend checking the gap every 5,000 to 7,500 miles. For long-life iridium or platinum plugs, checking every 15,000 to 30,000 miles, or at the replacement interval specified by your manual, is usually sufficient.
Q: Is there a difference between checking the gap and setting spark plug gap?
A: Yes. Checking spark plug gap measurement is simply measuring the distance. Setting spark plug gap involves making the necessary adjustments (widening or closing the distance) using the adjusting spark plug gap techniques described above until the measurement matches the required specification.
Q: Can I use a standard set of feeler gauges for this job?
A: While a standard set of feeler gauges can work, a dedicated feeler gauge spark plug tool is better. Dedicated tools usually have blades designed to curve slightly, which matches the shape of the ground electrode better. They also often incorporate the prying and tapping mechanisms needed for easy adjusting spark plug gap.