spark plugs

Home, Auto Repair Library, Auto Parts, Accessories, Tools, Manuals & Books, Car BLOG, Links, Index

original equipment spark plugs

Are Original Equipment Spark Plugs Best?

by Larry Carley copyright

When you change your spark plugs, should you use the same brand and part number as the original, or a different brand?

Most professional technicians say they usually replace same with same, unless there is a reason to install a different brand of spark plug. If you use the same spark plug that the engine came equipped with from the factory, you should not have any problems with spark plug Fouling, misfiring , detonation or preignition.

When a vehicle manufacturer chooses a certain spark plug for an engine, the plug has to undergo extensive validation testing to make sure it does not run too cold and foul, or run too hot and cause detonation or preignition, or suffer misfiring due to electrode position or wear. Most original equipment spark plugs today are platinum or iridium and have a recommended replacement interval of 100,000 miles. So the validation testing is designed to simulate that kind of mileage before the plug receives its stamp or approval.


Years ago, most vehicle manufacturers used only one brand of spark plugs in their engines:

AC Delco in GM engines
Motorcraft or Autolite in Ford engines
Champion in Chrysler engines
Bosch in European makes
NGK or DENSO in Asian makes

In most cases the vehicle manufacturer either owned the spark plug supplier, or had a long historical relationship with that supplier. But globilization and competition has changed that once-cozy relationship. Today, you may find some GM or Ford engines factory-equipped with Bosch spark plugs, Mazda engines equipped with Motorcraft spark plugs, Chrysler engines equipped with NGK or DENSO spark plugs, and so on. Most late model Saturn engines use NGK spark plugs. Kia and Hyundai may come factory-equipped with either NGK or Champion spark plugs. Some vehicle manufacturers still rely on a single spark plug supplier, but most use a mix of suppliers, even for the same engine.

For example, a late-model Chrysler V6 may come factory equipped with a Champion spark plug, a NGK spark plug or a DENSO spark plug. All are "factory approved" for the application, so which plug is used on any given day may depend where the engine is assembled, which supplier shipped the vehicle manufacturer a container of spark plugs that day, and so on.


Here is something else many people do not know. Most spark plug supplies sell a full line of spark plugs that fit most makes and models, not just the applications where they are the original equipment supplier.

Many spark plug suppliers buy spark plugs from each other for applications they do not manufacture themselves. Why? So they can offer a broader product line with more complete coverage.

Consequently, most spark plug suppliers catalogs list many vehicle applications for which they were not the original equipment supplier. They also have cross-reference indexes in the back that help you figure out which of their spark plugs can be used to replace another brand of spark plugs.

spark plug firing
Peak ignition performance requires a reliable spark with no misfires.


Here is another fact some people have a hard time digesting: No one brand of spark plug is superior to any other brand. This does not mean any brand of spark plug will work in any engine. The heat range of a replacement spark plug (ANY brand) must be a close match with the original spark plug (ANY brand) for good ignition performance. The electrode configuration can be different as can the materials used to make the electrodes, but the electrode gap(s) must not be too wide or too narrow for the air/fuel mixture or firing voltage capabilities of the ignition system. The electrodes on the replacement spark plugs should also be capable of lasting at least as long as the original spark plugs.

The best advice we can offer is to use the same brand and type of replacement spark plug as the original spark plugs that came in your engine, unless you want to upgrade standard plugs to long life platinum or iridium spark plugs.

If the spark plugs have already been replaced at least once in your engine, and you are not sure they are the same brand as the original, or you do not know what brand the original plugs were, ask an auto parts store employee to look up the plugs for you in their data base. Most list the original equipment spark plug along with any other brands the store carries that may also fit your engine.

If you want a certain brand of spark plug , but the spark plug manufacturer does not list one of their spark plugs for your engine, there is a reason why. Either they do not make a plug that fits your engine, or the plugs they have do not have the correct heat range or electrodes for your engine.

DO NOT try to match plugs by visual appearance. Two different brands of spark plugs that seem to be identical on the outside (same diameter and thread pitch, same type of electrodes, same length) may have differences inside that create a significant difference in heat range.


Upgrading from standard spark plugs to platinum or iridium spark plugs usually makes sense in many instances, but downgrading from platinum or iridium to a standard plug does not unless you have a reason for changing plugs frequently (such as an engine that burns oil).


Replacement spark plugs must have the same physical dimensions as the original to fit the cylinder head. The thread diameter, pitch and seat configuration must be identical. The "reach" of the electrode (how far it protrudes into the combustion chamber) must also be similar to avoid mechanical interference with the pistons or valves.

The replacement spark plug must also have a heat range that is similar to the original spark plugs. If the wrong replacement spark plug (ANY brand) is installed in an engine, the spark plugs may experience rapid fouling and misfiring (if the replacement plug is colder than the original), or preignition and/or detonation (if the replacement plug is hotter than the original), or possible misfiring due to electrode erosion or metal transfer.

For example, the best type of park plugs for "waste spark" distributorless ignition systems are ones with double platinum (platinum on BOTH the center and ground electrode), or iridium on the center electrode and a hard alloy on the outer ground electrode, or multiple ground electrodes.

Ford Motorcraft PZT 2FE Platinum spark plug

Watch Out for Ford Motorcraft Two-piece Spark Plugs That Break!

The original equipment Motorcraft brand spark plugs that were factory installed in many late model (2004 to 2008) Ford trucks with 5.4L V8 and 6.8L V10 engines, 2005 to 2007 Mustang GT 4.6 & 5.4L V8 engines, and 2008 Mustang GT models built prior to 11/30/07) can break when you attempt to remove them! The spark plugs have a crimped lower electrode shell that becomes coated with carbon, causing it to stick in the cylinder head. When you attempt to unscrew the plug, the lower shell breaks off and stays in the head. Removing the broken shell requires a special Ford, Snap-On or Lisle extractor tool. Worse yet, if any shell or electrode fragments fall into the cylinder and can't be fished out, you may have to remove the cylinder head to get the debris out. Many experts recommend replacing the original equipment Motorcraft spark plugs before the get too many miles on them (over 35,000). Replacing the plugs at low mileage will reduce the risk of them sticking and breaking. Waiting until the original equipment spark plugs have 100,000 miles on them is asking for trouble!

Ford Technical Service Bulletin 08-7-6 covers the recommended removal procedure for these spark plugs, as well as the repair procedure if one or more plugs break ( Click Here to View Ford TSB 08-7-6). Essentially, it says to remove the spark plugs when the engine is COLD (room temperature). Loosen the plugs about 1/8 to 1/4 turn, stop and spray some WD-40 or penetrating oil into the spark plug well. Allow the oil to soak into the threads so it can loosen the carbon around the electrode shell. Wait at least 15 minutes, or longer (overnight is recommended if the plugs have over 80,000 miles on them). Then slowly loosen the plugs, applying no more than 35 ft. lbs of torque to your wrench. If a plug sticks, retighten it half a turn, apply more penetrating oil, wait, then try again.

Do NOT reinstall the same Motorcraft spark plugs (PZT 2FE Platinum). Replace them with a much improved one-piece spark plug from Champion (7989), or a similar spark plug from NGK, Denso or Bosch. Apply nickel anti-seize to the outer surface of the lower electrode shell (the smooth part) before installing the plugs.

For more information about the Ford spark plug breakage problem Click Here.

spark plugs More Spark Plug & Ignition Articles:

Spark Plugs

Don't Neglect the Spark Plugs

Why Spark Plugs Still Need To Be Replaced

Spark Plug Fouling

Analyzing Ignition Misfires

Spark Plugs & Ignition Performance

Spark Plug Technology

Bosch Platinum +4 Spark Plugs

Spark Plug Wires

Distributorless Ignition Systems

Coil-Over-Plug Ignition Systems

Ignition Coil Diagnosis & Testing

Engine Won't Start, No Spark

Diagnosing An Engine that Won't Crank or Start

To More Technical Info Click Here to See More Carley Automotive Technical Articles

Be sure to visit our other websites:
original equipment spark plugs

Auto Repair Yourself


Scan Tool Help