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How Tight Should Spark Plugs Be? (+5 FAQs)

December 21, 2021

Spark plugs deliver the spark that ignites the fuel-air mixture in the combustion chamber. 
As they play such an important role, your spark plug installation needs to be carefully handled to ensure that it has the proper torque (tightness).

But how tight should spark plugs be?

Many spark plugs are tightened between 8-40nm (nanometer) and should be installed per the torque setting recommended by spark plug manufacturers. 

Moreover, it should also be done with a torque wrench (a particular socket wrench type) to avoid any over or under-tightening

But that isn’t all you need to know.
In this article, we’ll cover in detail how tight spark plugs should be, how to tighten them, and answer some FAQs to help you better understand this.

This Article Contains:

Let’s begin!

How Tight Should Spark Plugs Be?

In the past, cars had iron cylinder heads. 
With them, you could install plugs by turning them in by hand with new copper washers. 

After the plugs were seated, they could turn them with a plug wrench until they felt resistance. And then twist the plugs an additional ¼ to ½ turn to compress the copper washer.

But now, many cars have aluminum cylinder heads. 

Over-tightening a spark plug in an aluminum cylinder head can entirely damage the spark plug thread, requiring spark plug replacement.

So the best solution is to use a torque wrench and follow the recommended torque mentioned by your manufacturer.

Irrespective of whether your car has an iron cylinder head or an aluminum cylinder head, you should be careful when torquing spark plugs.

Here are quick tables to give you a general idea of spark plug tightness based on plug thread size, wrench use, cylinder head material, and plug type. 

Check if your car supports a gasket or tapered seat spark plug type to understand the tables better.

A. Using A Torque Wrench:

Spark Plug Seat TypePlug Thread Size
(Millimeter)
Recommended Turns For Aluminum HeadsRecommended Turns For Cast Iron Heads
Gasket Spark Plugs10MM8-12 LB. FT.8-12 LB. FT.
12MM10-18 LB. FT.10-18 LB. FT.
14MM18-22 LB. FT.26-30 LB. FT.
18MM28-34 LB. FT.32-38 LB. FT.
Tapered Seat Spark Plugs14MM7-15 LB. FT.7-15 LB. FT.
18MM15-20 LB. FT.15-20 LB. FT.
Recommended turns when using a torque wrench

B. Without A Torque Wrench:

Spark Plug Seat TypePlug Thread Size
(Millimeter)
Recommended Turns For Aluminum HeadsRecommended Turns For Cast Iron Heads
Gasket Spark Plugs10MM1/4 Turn1/4 Turn
12MM1/4 Turn1/4 Turn
14MM3/8 – 1/2 Turn1/2 – 5/8 Turn
18MM3/8 – 1/2 Turn1/2 – 5/8 Turn
Tapered Seat Spark Plugs14MM1/16 Turn1/16 Turn
18MM1/16 Turn1/16 Turn
Recommended turns when not using a torque wrench

Note: If you’re dealing with an old plug, you may have to estimate the torque setting a bit when torquing by hand. It may not be the exact number of turns mentioned in the table above. The exact turns will work for a new spark plug.

How To Tighten Spark Plugs (Step-by-Step Guide)

Now, if you’re wondering how to tighten spark plugs, here’s a quick step-by-step guide:

And that’s how you tighten spark plugs!
All you have left is reconnecting the spark plug wire or ignition coil to the plug. Do this entire process for each spark plug, and you’re all set.

With that said, let’s move on to some related FAQs.

5 FAQS About How Tight Should Spark Plugs Be

Here are the answers to some common spark plug and torque related questions:

1. What Happens When Spark Plugs Are Too Loose?

Here are some of the things you may notice as a result of a loose spark plug:

Moreover, with a loose spark plug, your engine is highly likely to experience one of many issues, like:

2. What Happens When Spark Plugs Are Too Tight?

When you screw in a spark plug too tight, it can cause numerous issues:

Other issues that you may face due to tight spark plugs include:

Moreover, it won’t be easy to remove a tight spark plug without causing damage because the compression in the gasket is designed for single use. 

In such a case, it’s best to call a mechanic. 
They’ll clean the debris in the spark plug area and remove the tight old plug. Then place a new spark plug using the correct torque (recommended torque by your manufacturer).

3. How To Know If My Spark Plugs Are Too Tight Or Loose?

The best way to check spark plug tightness is by inspecting the spark plugs themselves. 

Try to twist the spark plug. 

If you can easily turn it around, you have a loose spark plug.
If you can’t turn it even with a wrench, the plug is too tight.

4. What Is Spark Plug Torque?

Spark plug torque is the measure of force that can cause a spark plug to rotate about its axis. 

It’s one of the most critical aspects of spark plug installation as it directly influences the capacity of the spark plug firing end to disperse heat to the cylinder head. 

The proper torque for spark plug installation depends on the thread diameter of the plug.

5. Tapered Seat Vs. Gasket Spark Plugs: What’s The Difference?

There are two spark plug seat types: flat and tapered.
Flat seat plugs use a crushable gasket (hence the name gasket spark plugs) to form a seal between the plug and combustion chamber.

On the other hand, tapered seat plugs use the spark plug’s outer shell to create a seal between the plug and the combustion chamber.

Note: Spark plug manufacturers recommend using a new gasket each time an old plug is re-installed.

Final Thoughts

The answer to how tight spark plugs should be is entirely based on the recommended torque by spark plug manufacturers. 

You should always follow their torque spec, or you might experience misfire or ignition issues or something worse — like severe engine damage.

Moreover, spark plug installation isn’t simple, even if you have the correct torque value. 
That’s why it’s best to call a professional mechanic for the job, like RepairSmith.

We’re a mobile automotive repair and maintenance company that offers service 7 days a week via simple online booking. We also offer competitive, upfront pricing and the highest quality replacement parts.

Contact RepairSmith today, and our ASE-certified technicians will replace your old spark plugs directly in your driveway!