Rick's Free Auto Repair Advice

Cause of spark plug failure

What causes spark plug failure?

Improper torque is the #1 cause of spark plug failure

I bet you’ve never used a torque wrench to install spark plugs. Even in the days before aluminum cylinder heads do it yourselfers and professional technicians never used a torque wrench when installing spark plugs. But just because they installed and fired up doesn’t mean it wasn’t the cause of their eventual failure. Here’s why.

Excessive torque damages spark plug seal

Spark plugs consist of a hollow threaded shell and a porcelain insert. When you tighten a spark plug too much, you distort the shell and break

spark plug failure from improper torque

Broken spark plug due to excessive torque

the shell-to-porcelain gas seal or cause a hairline fracture that results in misfires. In fact, spark plug manufacturer NGK says that improper torque is responsible for 90% of spark plug warranty claims. Bad plug? Yeah, you probably caused it yourself.

If you don’t clean around the spark plug wells before removing the old spark plugs, the dirt and grit can fall into the combustion chamber or lodge on the seat or threads. That dirt can cause improper tightening even if you use a torque wrench.

Don’t use anti-seize on new spark plugs

Yes, I realize you’ve been taught to use it. But that advice is OBSOLETE. Read this post about the change in spark plug installation techniques. Most new spark plug threads are

too much anti-seize on spark plug threads

Whoa dude, that’s WAY TOO much anti-seize

treated with a metallic nickel anti-seize coating AT THE FACTORY. The coating prevent thread galling during installation and prevents seizing in the cylinder head. Adding anti-seize only throw off your torque calculations, causing you to over torque.

Tightening a spark plug by hand is crazy

If you undertighten a spark plug, it won’t seat properly, causing combustion gasses to leak out. It also wont’ make full contact with the cylinder head, which means it can’t properly dissipate heat. That results in pre-ignition and detonation and possible engine damage. Under tightening can also cause the spark plug can eventually blow right out of the cylinder head, taking the cylinder head threads with it. In addition to breaking the shell to porcelain seal, overtightening can distort the cylinder head threads or even rip them out.

What’s the proper torque specification?

The proper torque depends on the type of spark plug seat, whether tapered or washer, the cylinder head material and the spark plug thread diameter. Refer to the shop manual or spark plug manufacturer’s website for the proper torque.

©, 2017 Rick Muscoplat

Posted on by Rick Muscoplat


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