What thread size is my spark plug?

For most cars, the spark plug thread diameter is 14 mm with a pitch of 1.25 mm; however, engines vary and some use 12 mm, 16 mm, or 18 mm threads. Check your owner's manual or inspect the old plug to confirm the exact size.

Common thread sizes you’ll encounter

Here’s a quick guide to the sizes you’re most likely to run into, along with where they’re commonly found.

• 14 mm diameter, 1.25 mm pitch (14x1.25): The prevailing size for modern automotive engines.


• 14 mm diameter, 1.5 mm pitch (14x1.5): Used on some older or specific-engine designs.


• 12 mm diameter, 1.25 mm pitch (12x1.25): Common on smaller engines, some motorcycles, and many garden tools.


• 16 mm diameter, 1.5 mm pitch (16x1.5): Found on certain motorcycles and niche engines.


• 18 mm diameter, 1.5 mm pitch (18x1.5): Present on a subset of engines in trucks, cars, and heavy equipment.

Keep in mind that diameter alone does not guarantee compatibility—the thread pitch and reach (how far the thread goes into the cylinder head) matter as well. This list highlights frequent sizes but is not exhaustive.

How to determine your spark plug thread size

Use these steps to identify the correct size for your engine without guessing. When in doubt, bring the old plug or its packaging to a parts store for a precise match.

1. Consult the vehicle’s owner manual or service manual for the exact spark plug specification, including thread size and reach.


2. Carefully remove the spark plug and look for any markings on the plug itself that indicate size or heat range.


3. Measure the thread diameter with calipers (or a precise ruler across the threaded portion). If you’re unsure, use the old plug as a reference.


4. Determine the thread pitch by counting threads per unit length or using a thread pitch gauge; many 14 mm plugs use either 1.25 or 1.5 pitch.


5. Cross-check the plug’s part number with your engine model/year or engine code to ensure you’re ordering the correct replacement.

In practice, many people rely on the packaging or a dealership/parts retailer cross-reference to confirm the exact size, reach, and heat range needed for their engine.

What to do if you’re unsure or replacing multiple plugs

If you’re uncertain or servicing multiple cylinders, consider having a professional handle the change. When you do replace plugs, torque them to the manufacturer’s specification. For modern plugs, use minimal or no anti-seize on the threads unless the manufacturer explicitly recommends it, as improper use can affect torque and sealing.

Summary

The default expectation for most modern gasoline engines is a 14 mm thread diameter, typically with a 1.25 mm pitch. However, you may encounter 12 mm, 16 mm, or 18 mm sizes depending on the engine design. Always verify with the owner’s manual, the old plug, or the retailer’s cross-reference to ensure you get the correct thread size, reach, and heat range. Correct sizing prevents cross-threading and helps maintain optimal engine performance.

What is the thread size of a standard spark plug?

The SAE Standard includes the following sizes: 7⁄8- inch nominal diameter with 18 threads per inch: 18-millimeter nominal diameter with a 18- millimeter nominal diameter with 1.5-millimeter pitch; 14-millimeter nominal diameter with a 1.25-millimeter pitch; 10-millimeter nominal diameter with a 1.0 millimeter pitch; 3⁄ ...

What is the spark plug thread?

Spark plug thread sizes are specified by a two-part metric number (e.g., M14 x 1.25), where the first number is the thread diameter in millimeters and the second number is the pitch (distance between threads) in millimeters. Common sizes for vehicles include M14 x 1.25 and M18 x 1.5, while M10 x 1.0 and M12 x 1.25 are also used, particularly in smaller engines, motorcycles, and ATVs. 
This video explains the common spark plug thread sizes and their applications: 59sMatt's ShopYouTube · Nov 21, 2021

• Common Thread Sizes:
  • M14 x 1.25: The most common size for modern vehicles. 
  • M18 x 1.5: Often found in larger engines, tractors, and some industrial applications. 
  • M12 x 1.25: Frequently used in smaller engines and motorcycles. 
  • M10 x 1.0: Typically reserved for small or low-powered engines. 
• How to Read the Size:
  • The "M" stands for metric. 
  • The first number is the diameter of the threads in millimeters. 
  • The second number is the thread pitch, which is the distance between each thread crest in millimeters. 
• Example: An "M14 x 1.25" spark plug has a thread diameter of 14mm and a thread pitch of 1.25mm. 

You can watch this video to learn how to use spark plug thread chasers to clean and realign the threads: 57sdonyboy73YouTube · Nov 22, 2019

How to find spark plug thread size?

How is it measured? The diameter can be measured using a set of Calipers. The pitch can be measured using a Thread Gauge.

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

1. Check the spark plug: The thread size is printed on the spark plug itself or its packaging. 
2. 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. 
3. 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