What size are the threads on spark plugs?
In short, spark plug threads come in several diameters. The most common modern size is 14 mm with a 1.25 mm pitch, but engines use 10 mm, 12 mm, 16 mm, 18 mm and even 20 mm thread sizes depending on the design.
Thread size is dictated by the engine’s cylinder head design, the seating style (tapered vs gasketed), and the plug’s reach. Using the wrong size can cross-thread, leak, or damage threads, so always verify against the vehicle’s service manual or the plug you are installing. When replacing a plug, match the exact size, pitch, and seat type of the original part to ensure a proper seal and reliable operation.
Common thread sizes you’ll encounter
These sizes cover the majority of passenger cars, trucks and small engines. The exact size for a given vehicle is specified in its owner’s manual or on the plug itself.
- 10 mm diameter — found on some small engines and older motorcycles; pitch often 1.0 mm or 1.25 mm.
- 12 mm diameter — used on a range of small cars and engines; pitch commonly around 1.25 mm (varies by model).
- 14 mm diameter — by far the most common in modern gasoline engines; pitch typically 1.25 mm; available in tapered or gasketed seat variants.
- 16 mm diameter — appears on some European and higher-displacement engines; pitch commonly 1.5 mm.
- 18 mm diameter — used on certain larger engines and some diesel applications; pitch commonly 1.5 mm.
- 20 mm diameter — rare in passenger cars but found on some large engines and specialty equipment; pitch commonly 1.5 mm.
Note: In practice, many engines use 14 mm plugs with a 1.25 mm pitch, but always confirm the exact size, pitch and seat type for your vehicle to avoid damage.
How to identify the correct thread size for your spark plug
Use these steps to confirm the right thread size before buying replacements or removing components.
- Check the vehicle’s service manual or the spark plug specification listed by the manufacturer.
- Inspect the existing spark plug: read the markings on the shell or compare with your current plug model.
- Measure the thread diameter with a caliper or compare against a spark plug thread gauge.
- Note the seat type (tapered vs gasketed) because that affects sealing and head compatibility.
- Verify the plug reach (how far the plug threads extend into the bore) and torque specification for installation.
Following these steps helps prevent cross-threading, leaks and improper seating, which can all compromise engine performance and longevity.
Seat type and reach
The two main seat styles are tapered (conical) seats and gasketed seats. Most modern engines specify a gasketed or tapered seat depending on the cylinder head design, while reach describes how far the plug extends into the combustion chamber. Using the wrong seat or reach can prevent a proper seal and affect ignition performance. Always match both the thread size and the seat/reach requirements to the original specification.
Summary
Spark plug thread sizes are not universal. The most common size today is 14 × 1.25, but you may encounter 10, 12, 16, 18 or even 20 mm diameters depending on the engine. Always verify the exact size, pitch, seat type and reach from the vehicle’s manual or the plug you’re replacing, and torque the plug to the manufacturer’s specification to maintain engine integrity and reliability.
Why are spark plugs 14mm?
Additionally, 14mm thread size spark plugs have a higher copper core electrode, which provides better conductivity and a more efficient spark. This feature enhances the performance of the engine and helps save fuel. It also reduces emissions, making these spark plugs more environmentally friendly.
What size are spark plug threads?
Most spark plugs in vehicles today are metric M14x1. 25. The 14mm is the diameter and the thread pitch is 1.25mm. Thread pitch is the measurement between each thread (measuring from peak to peak) there's 1.25 millimeters.
What size thread is a 5 8 spark plug?
A spark plug with a 5/8" hex size (16mm) typically has a 14mm thread size. While 5/8" is the measurement for the socket needed to install the plug, the internal thread that screws into the engine is usually 14mm for this size of spark plug.
- Hex Size: 5/8" (16mm)
- Thread Size: 14mm
What size tap for a spark plug?
Spark plug tap sizes vary, but common metric sizes include M18x1.5, M14x1.25, M12x1.25, and M10x1.0. The "M" indicates a metric thread, the first number is the diameter in millimeters (e.g., 14mm), and the second number is the thread pitch (distance between threads) in millimeters (e.g., 1.25mm). You must determine the correct size for your engine, as it depends on the specific spark plug and cylinder head.
You can watch this video to learn how to identify different spark plug sizes: 56sMatt's ShopYouTube · Nov 21, 2021
Common spark plug tap sizes
- M18x1.5: Common on some engines. Diameter is 18mm, pitch is 1.5mm.
- M14x1.25: A very common size, especially in many automotive and small engine applications. Diameter is 14mm, pitch is 1.25mm.
- M12x1.25: Frequently used on larger dirt bikes and other engines. Diameter is 12mm, pitch is 1.25mm.
- M10x1.0: Often reserved for smaller, lower-powered engines. Diameter is 10mm, pitch is 1.0mm.
You can watch this video to learn how to rethread a spark plug hole: 57sDIY with Michael BordersYouTube · Jul 5, 2023
How to choose the right tap size
- Check the spark plug: The thread size is printed on the spark plug itself or its packaging.
- Measure the diameter and pitch: If not listed, you can measure the diameter (first number) and pitch (second number) using a caliper and thread gauge.
- Match the tap to the spark plug: Ensure the tap size you purchase matches the spark plug's thread size. For example, if your spark plug is an M14x1.25, you will need an M14x1.25 tap.
You can also watch this video to learn how to create your own spark plug chaser: 54sPro Tool ReviewsYouTube · Sep 9, 2019
