Are all spark plugs the same size? A detailed answer

Spark plugs and glow plugs are used in petrol engines and diesel engines to ignite the gasoline or petrol–air mixture so that it powers up.

Average petrol or diesel engine has at least 4 gas cylinders with one plug per cylinder. Of course, there are also engines with 6 or 8 cylinders still with one plug per cylinder.

Now, these simple plugs aren’t designed to last your engine’s lifetime. At some point, they’ll run out of life, needing a replacement. Most plugs will last 20k-40k miles before needing a replacement, of course depending on your car model or make.

As a first-time motorist, one of the important questions you may ask yourself before buying replacement plugs is; are all spark plugs the same size? To answer that, these ignition components aren’t the same size.

When selecting new plugs, it’s imperative that you go for the correct size. Otherwise, you’ll experience “reach” issues between the new plugs and your pistons. For starters, reach simply refers to how far the plugs can “reach” into the gaps between them and the heads of the pistons.

If the reach is too long, depending on your engine’s type, the pistons are going to bang against the plugs’ electrodes. Consequently, the plugs will misfire the air-gas mixture each time you turn the ignition key clockwise, making it difficult for the engine to come to power.

You’ll also lose battery power as you keep on struggling to start the car. Additionally, there’s going to be a carbon deposit build-up on your spark plug threads within the combustion space. This will consequently make the plugs and the ignition system generally less effective.

Why Do Spark Plug Sizes Differ?

Spark plugs differ in size because of the varying vehicle engines i.e petrol and diesel with which they are meant to work. The most common plug sizes are 5/8”, 13/16”, 9/16’’, ¾’’, and 11/16”. Plugs designed to work with petrol engines will not be compatible with diesel engines, and vice versa.

The More Common Spark Plug Sizes for Petrol Cars

The most common spark plugs for petrol cars are ⅝’’. A good number of newer car models such as Subaru, Nissan, and Chevrolet come with a petrol engine. But there are also ¾” and 9/16” plugs.

These are usually found in small engine cars as well as some Ford cars plus newer Asian cars, respectively. In case you didn’t know, petrol engines tend to be less high-performance engines compared to diesel engines. As such, these engines will perform well with these smaller spark plugs.

The More Common Glow Plug Sizes for Diesel Cars

Diesel engines are more high-performance than their petrol counterparts. As such, they need bigger spark plugs to fire up the diesel-air mixture, which is heavier than the petrol-air mixture, and power on. I’m talking about the 13/16” plugs and 11/16” plugs that can be found in some small diesel engine BMW cars

How Do I Know My Spark Plug Size?

To determine your spark plug size. you could check your car’s manual or handbook. Usually, this document indicates all the technical specifications of every component of the engine. In fact, the standard spark plugs that came with your car from the manufacturer will be defined in the manual.

You could also check the part number of your existing sparks then google their size online. Manufacturers always display the part number on every plug. To do so, just unscrew a plug, and note down the number so that you can quickly google the size on your phone.

What Should I Look for When Choosing the Correct Spark Plugs?

There are 2 key things you should pay attention to when selecting the appropriate spark plugs for your car. These are explained as follows:

The Size

As you already know, spark plugs come in different sizes, each suitable for a category of cars. Vendors will display plugs of all those sizes and it will be your job to choose plugs that are of the same size as your existing plugs. You can’t go about just selecting randomly without paying attention to that.

The Material

Apart from size, spark plugs can also be categorized according to the materials out of which they are made. In light of that, we have copper plugs, platinum plugs, double platinum plugs, and iridium plugs.

These materials have their advantages and disadvantages, but you want to choose the material with the best pros and least cons.

Copper plugs Pros & Cons


Better for cars built earlier than 1980.


  • Wear out fast.
  • Require more voltage.

Platinum Spark Plugs Pros & Cons


  • More durable than copper plugs, and can last as many as 100,000 miles.
  • Generate more heat than other plugs, reducing debris buildup.
  • Recommended for new cars with an electronic distributor ignition system.


Not the strongest spark plug on the market.

Double Platinum Spark Plug Pros & Cons


  • Recommended for waste spark distributor ignition systems.
  • More reliable than other spark plugs, and isn’t affected by environmental conditions like dampness or rain.


Not suitable for distributorless ignition systems(DIS).

Iridium Spark Plug Pros & Cons


  • More durable and long-lasting than other spark plugs.
  • Uses less voltage to generate electric current.
  • Recommended by many car manufacturers.
  • Enhances the overall performance of your engine.


Costlier than other types of spark plugs.


Spark plugs are a critical component of your car engine’s ignition system. It’s these plugs that enable your car’s engine to start every time you put the key into the ignition system and turn it clockwise.

Unfortunately, these mechanical items aren’t meant to last a lifetime of your engine. Over time, they’ll wear out and need replacement.

When you go shopping, you’ll realize that spark plugs are not the same in terms of size. They come in different sizes, each of which is designed for a specific category of engines.

Make sure to choose plugs of the same size as the size of the plugs you want to replace. Use the above information to determine the size of your current plugs.

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