How Tight Spark Plugs? What Happens When They Are Too Tight Or Too Loose?

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There is no specific answer on how tight the spark plugs should be, but it should not be too loose to turn with your fingers nor too tight to make it difficult to budge with a wrench. Generally, the tightening angle of the spark plugs should be 1/16th of a turn. This ensures that the spark plugs are not too tight or loose as it could cause issues to vehicles’ engines.

Spark plugs are an essential component in a car; without them, your vehicle would not run. The fuel-air mixture in an internal combustion engine is ignited through the spark from the plugs. All spark plugs are not the same, and thus replacing them requires you to understand the size you need for your car. Here, you get to know how tight spark plugs should be and all needed for proper installation. Read on!

Spark Plugs

Installing The Spark Plugs

Installation of the spark plugs should be done well to avoid serious damage to the car engine. You need to pay keen attention to the torque as it affects the spark plug’s ability to dissipate heat to the cylinder head. Heat dissipation can be slowed down if the spark plug is not seated in the cylinder head due to under-torqueing. This causes the firing end to overheat, posing a pre-ignition risk that can seriously damage the engine.

On the other hand, the plug’s metal shell can stretch if over-torqued. This can cause the threads to crack or break off; thus, the spark plug insulator can fracture or cause the inner gas seals to break. The stretching of the metal shell also reduces heat dissipation resulting in pre-ignition that causes severe damage to the engine.

You should ensure that the spark plugs’ holes are clean before installation to avoid twisting against dirt. This is because the torque wrench might indicate that the plugs have been tightened well while, in a real sense, it is under-torqued that causes serious issues to the engine. The installation of spark plugs should be done when the engine is cool to avoid damaging the threads.

Check The Table Below For Ideal Torque Specs For Cast-Iron And Aluminum Cylinder Heads Of Spark Plugs:

Spark Plug TypeThread DiameterAluminum Cylinder Head (lb.-ft.)Cast Iron Cylinder Head (lb.-ft.)
Flat seat type (with gasket)8 ø mm5.8~7.2
Flat seat type (with gasket)10 ø mm 0 ø mm 10 ø mm 107.2~8.77.2~10.8
Flat seat type (with gasket)12 ø mm 010.8~14.510.8~18.0
Flat seat type (with gasket)14 ø mm 018.0~21.618.0~25.3
Flat seat type (with gasket)18 ø mm 025.3~32.525.3~32.5
Conical seat type (without gasket)14 ø mm 07.2~14.510.8~18.0
Conical seat type (without gasket)18 ø mm 014.5~21.614.5~21.6

Steps Of Installing Spark Plugs

Step 1: Get The Required Equipment

Before you proceed to install the plugs, you should ensure you have the following:

1. New spark plugs in case the old ones are faulty or old

2. Gloves

3. Clean rags

4. Right-size spark plug wrench or torque wrench (refer to the table above)

5. Cylinder-head cleaning kit or thread chaser

6. Penetrating oil

7. Feeler gauge

Step 2: Wait For The Engine To Cool Down

Put on gloves to check whether the engine is cool. If not, give it time to cool down to avoid damaging the thread during installation.

Step 3: Disconnect The Battery   

It is safer to work when the battery is disconnected to avoid getting shocked. To do this, detach the negative terminal and keep the connector aside.

Step 4: Remove The Loose Dirt And Debris

Take the clean rag and wipe on and around the plugs to remove all the loose debris from the gasket seal.  This ensures that you don’t twist the plugs against debris that can result in loose spark plugs.

Step 5: Remove The Spark Plugs

Now, remove the spark plugs boot and cables attached to it. Label the wires if not labeled to avoid confusion or work on them one by one.

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Unscrew it using the wrench of your choice while applying penetrating oil in case of any difficulty. Do it slowly until loose to avoid breaking the plugs. The oil helps to soften the crud that holds the plugs to the bolt hole.

Step 6: Clean Cylinder Heads

Use the cleaning kit or thread chaser tool to remove all debris that might still be present in the bolt hole.

Step 7: Check That The New Plugs Aren’t Damaged

Before putting the new plugs, check one more time that they aren’t any chips or crack on it. You do not want to do all the work to put the damaged plugs that cause issues as soon as the installation is done.

Step 8: Gap The New Plugs

Use the feeler gauge to double-check that the new plugs are gapped well. If not, adjust accordingly.

Step 9: Screw In The New Plugs Using Your Fingers

Position the new unit well and screw it in using your fingers. If you face resistance, then reposition it at a good angle until it screws in smoothly.

Step 10: Tighten Carefully

Loose or over tighten spark plugs can damage the engine. Therefore, you should tighten carefully using the torque recommended by the manufacturer. Tighten it approximately ½ to 2/3 turn more until it is secure. The plugs should not be too loose to turn with your fingers nor too tight to make it difficult to budge with a wrench.

Step 11: Re-Connect The Boot And The Wires

Once the spark plugs are secure, re-connect the boot and the wires as they were before. Your spark plugs should be working well now.

What Happens If You Over Tighten Spark Plugs?

Too Tight Spark Plugs Can Result In The Following:

Impossible Or Hard To Remove The Unit

Unscrewing over-tightened spark plugs is a difficult task. It may appear as if it has melded with a cylinder head. In some instances, it might be impossible to remove the plugs unless done by a professional that could involve more than sing the wrench. Potential corrosion, debris, and other deposits can behave like glue, making removal hard.

Foreign Objects Can Get Into The Combustion Chamber

If the spark plugs are overtightened, its insulator and electrode can get their way into the combustion chamber. This can cause damage to parts that these objects knock.

Damage Of The Cylinder Head

The bolt hole where the spark plugs are installed needs to stay intact. If deformed or stripped, the installation of new spark plugs cannot be done. Bolt holes can easily get damaged when removing overtightened spark plugs. If this happens, there is a need to seek repair from expert mechanics that can be costly.

What happens If Spark Plugs Are Loose?

The spark plugs can get loose and affect the performance of the cylinder. If the issue is not dealt with sooner, the loose spark plugs can cause the following:

Pre-Ignition Of Engine

Overheating can result due to loose spark plugs. As a result, there will be abnormal combustion that causes pre-ignition.

Poor Engine Performance

The engine will not perform to its optimum if the spark plugs are loose. This can be noticed if the vehicle is slow or difficult to accelerate. Also, you might experience trouble in starting the engine plus misfires.

Harmful Detonation

When pre-ignition happens as a result of overheating, there is a possibility of harmful detonation. That is the spontaneous engine combustion that can be noticeable if there is knocking or pinging. Detonation is a serious issue as it can damage the engine or cause total failure.

Dead Cylinder

If the spark plugs are too loose, they can pop out when the vehicle is in motion. This causes shock to you and your vehicle, which is not a good experience.

Can You Change Spark Plugs Without A Torque Wrench?

The torque wrench is an important tool to tighten the spark plugs. However, you can still change the spark plugs without the torque wrench. First, hand screw until tight then uses a plug wrench to tighten it well. For tapered set plug, do 1/12 to 1/18 turns while gasket plus ½ to 2/3 turns is ideal. Note that if the plugs are loose, then it can pop out, damaging the cylinder head,

Final Word

Spark plugs should be installed properly to avoid overtightening or loosening, as that could cause serious damage to the engine. However, the ideal tightness depends on the plug type being used and the model of your car. Of importance is to follow the installation guidelines keenly to avoid damaging the bolt holes or the spark plugs. 

If you are replacing the spark plugs, then be sure to buy the right size for your vehicle. Always refer to the user manual as the plugs are not universal. Once you know the right size, you should get the ideal torque wrench size to help in the installation process. Now, follow the installation steps, and regardless of your vehicle model, you will end up with properly fitted spark plugs.

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