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Summary
  • To fix a stripped spark plug hole, consider preparing the necessary tools first, using a Helicoil insert, and coating the tap with grease.
  • Spark plugs ignite the fuel inside the combustion chamber by creating a high-temperature spark that exceeds the fuel’s flashpoint.
  • Overtightening the spark plug to the cylinder can strip the spark plug hole and damage its threads. Failing to replace the spark plug and leaving it connected to the cylinder for too long can also damage the hole.

Rethreading a spark plug hole isn’t going to be a cakewalk, but there are ways you can make it easier. Here are some tips to consider when fixing stripped or damaged spark plug threads.

Thread Sizes for Spark Plugs

First of all, if the threads are only damaged but not stripped, you simply need to use the right tap with some grease applied to repair the existing threads. But make sure you get it going in straight. Here is a list of thread sizes for spark plugs:

  • M18 x 1.5: Typically used on some engines up to but not usually after 1986.
  • M14 x 1.25: This size is used on most vehicles.
  • M12 x 1.25: Used in 3 valve 5.4L Fords, newer Mercedes, Nissan, and Toyota Scions.
  • M10 x 1.0: Used in some motorcycles

Make sure you know the right size before you even begin to repair the threads with a tap. Also, before you begin, gettyour toolbox, gloves, some grease, and the proper kit ready.

Keep in Mind: A Regular Helicoil Insert is Not for Spark Plugs

You’ll find different types of repair inserts for your spark plug holes, like a Helicoil. But remember, spark plug inserts and bolt inserts aren’t the same. Not by a long shot.

helicoil for bolts illustration
A Helicoil for bolts (see illustration) is a coiled metal insert that requires the hole to be tapped out slightly larger with a special tap that comes with the Helicoil so that after the Helicoil is inserted, the hole will take the same size fastener that was there before. | Image Source: Richard McCuistian
spark plug Helicoil kits illustration
But spark plug Helicoil kits (see illustration) have inserts rather than regular Helicoils. Some spark plug thread inserts are aluminum (not good) and others are steel (better). | Image Source: Richard McCuistian

Coat the Tap With Grease

One trick you can try is coating the tap with grease. Metal chips from the repair insert can enter and damage the cylinder. By coating the insert with grease, the metal chips will stick to the tap as you pull it out.

, How To Repair a Stripped Spark Plug Hole

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: Some professionals will slowly turn the engine until the intake valve is open on that cylinder and have air blowing into the throttle body so the filings are blown up and out instead of falling into the cylinder. The tap enlarges the hole making threads for the insert.

Blowing the Chips Out With Air

Even if you used grease to trap the filings, before you install the insert, spin the engine over and let the action of the piston blow any chips out that may have fallen in. The engine may start, but you can shut it off quickly and you’ve done what you needed to. Then you can screw the insert onto a spark plug with a bit of grease between the spark plug threads and the insert and thread it into the head where you tapped the threads to receive it.

Make sure to use an insert of the right length. Once the insert is in place, some kits have a tool to stake the insert so it won’t come out the next time somebody removes a plug. To do this, you have to remove the spark plug from the insert while it’s still in the head and use the stake tool (it looks like a thick punch specially made to expand the knurled neck of the insert) to make the insert permanent.

Make sure to use an insert of the right length. Once the insert is in place, some kits have a tool to stake the insert so it won’t come out the next time somebody removes a plug.

– Richard McCuistian, ASE Certified Master Automobile Technician

Keeping Your Engine Safe

Under no circumstances is it ever safe for metal filings or other debris to enter the combustion chamber. When foreign substances enter the spark plug hole, you can expect the worst. Various engine parts are likely to malfunction and won’t be easy or cheap to repair.

If you aren’t sure how to remove and fix a stripped spark plug without any metal filings falling into the cylinder, consider asking a mechanic for help.

How Long Do Spark Plugs Last?

Spark plugs can last for around 60,000 -120,000 miles before needing a replacement, but this can still vary depending on several factors. How often you drive, the type of spark plug you use, and your vehicle’s year, make, and model can all affect the spark plug’s lifespan.

What Causes Stripped Spark Plug Holes?

If your spark plug hole is stripped, then it might have something to do with your last trip to the mechanic. Overtightening the spark plug to the cylinder can strip the spark plug hole and damage its threads.

, How To Repair a Stripped Spark Plug Hole

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: Unlike lug nuts, spark plugs aren’t typically overtightened. The standard torque for a spark plug is about 15 lbs. ft.

Additionally, cross-threading the spark plug and then trying to crank it in is a rookie error that ruins the threads. Sometimes this happens when the spark plugs are in a well and somebody just throws the spark plug in there, then shoves a socket in there, starting to thread the spark plug in a haphazard way.

Can You Drive With a Stripped Spark Plug Hole?

Just because your vehicle can still run with a stripped spark plug hole doesn’t mean you should drive it. However, until the spark plug blows out, you probably won’t know you have a problem, anyway. A stripped spark plug hole is like a ticking time bomb. It’s only a matter of time before it causes your spark plug to blow, leading to engine failure and other costly damage. For your own safety, it’s best to avoid driving your car until you repair your stripped spark plug hole.

How Much Do Spark Plugs Cost?

The final price of spark plugs will depend on several factors, such as your vehicle’s year, make, and model. On average, however, you can expect spark plugs to cost anywhere from $10 to $640. Depending on your vehicle, they can cost more than a spark plug rethread kit, which typically go for around $30.

About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

File Under : DIY , Ignition System Tagged With :
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