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:
- How Tight Should Spark Plugs Be?
- How To Tighten Spark Plugs (Step-by-Step Guide)
- 5 FAQS About How Tight Should Spark Plugs Be
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 Type | Plug Thread Size (Millimeter) | Recommended Turns For Aluminum Heads | Recommended Turns For Cast Iron Heads |
|---|---|---|---|
| Gasket Spark Plugs | 10MM | 8-12 LB. FT. | 8-12 LB. FT. |
| 12MM | 10-18 LB. FT. | 10-18 LB. FT. | |
| 14MM | 18-22 LB. FT. | 26-30 LB. FT. | |
| 18MM | 28-34 LB. FT. | 32-38 LB. FT. | |
| Tapered Seat Spark Plugs | 14MM | 7-15 LB. FT. | 7-15 LB. FT. |
| 18MM | 15-20 LB. FT. | 15-20 LB. FT. |
B. Without A Torque Wrench:
| Spark Plug Seat Type | Plug Thread Size (Millimeter) | Recommended Turns For Aluminum Heads | Recommended Turns For Cast Iron Heads |
|---|---|---|---|
| Gasket Spark Plugs | 10MM | 1/4 Turn | 1/4 Turn |
| 12MM | 1/4 Turn | 1/4 Turn | |
| 14MM | 3/8 – 1/2 Turn | 1/2 – 5/8 Turn | |
| 18MM | 3/8 – 1/2 Turn | 1/2 – 5/8 Turn | |
| Tapered Seat Spark Plugs | 14MM | 1/16 Turn | 1/16 Turn |
| 18MM | 1/16 Turn | 1/16 Turn |
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:
- Step 1: Clean the cylinder head and the spark plug thread to remove debris.
- Step 2: Once the debris is gone, twist the spark plug in by hand till the cylinder head touches the gasket.
- Step 3: Then tighten the spark plug with the correct torque, either with a torque wrench or without.
If you plan on torquing with a torque wrench, remember the torque specification of different spark plug manufacturers may differ. You can look for the torque spec online and then tighten the plugs.
On the other hand, if you don’t have a torque wrench, you can carry out torquing with a spark plug wrench.
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:
- Your car’s engine mounts should be capable of subduing the internal vibrations.
If the spark plugs are loose, these vibrations aren’t absorbed. Moreover, the plugs loosen up more with distance, affecting the cylinder performance.
- A loose spark plug impacts the overall performance and causes engine damage in the long run.
- With a loose spark plug, you’re risking a high-temperature combustion gasses leak. The gasses will leak past the plug and melt the thread.
Moreover, with a loose spark plug, your engine is highly likely to experience one of many issues, like:
- Problems with engine performance
- Improper combustion in the combustion chamber
- Detonation (unpredictable combustion in the engine)
- A dead cylinder
- Piston damage
2. What Happens When Spark Plugs Are Too Tight?
When you screw in a spark plug too tight, it can cause numerous issues:
- It affects the holes of the cylinder head bolts. Any damage to the holes to these head bolts is bad news.
That’s because the bolt hole damage won’t allow you to install a new plug. As a result, you’ll end up paying more for repairs.
- Tightening a plug too much can stretch the metallic shell on it. This causes the plug thread to crack or break. It can also harm the insulator or break the interior gas seals.
- A tight plug can also damage the spark plug thread (it gets stripped). The only way to fix the plug thread issue is to replace the head — which is an expensive repair.
Other issues that you may face due to tight spark plugs include:
- Unpredictable engine misfire
- Reduced RPM
- Problematic engine start
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 AutoNation Mobile Service.
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 AutoNation Mobile Service today, and our ASE-certified technicians will replace your old spark plugs directly in your driveway!