DIY Spark Plug Change Guide
For most 4-cylinder engines, this is one of the easiest repairs you can do. Allow 45 minutes your first time.
Tools You Need
Spark plug socket
$8
Has a rubber insert that grips the plug. Standard sockets will not work. 5/8" for most cars, 13/16" for older ones.
Socket wrench + extension
You probably have one
A 3/8" drive ratchet. You may need a 3-6" extension to reach the plugs.
Torque wrench
$25
The most important tool. Prevents stripping threads on aluminum heads. Click-type is easiest to use.
Step by Step
Let the engine cool completely
Work on a cold engine. Hot spark plug threads in a hot aluminum head can seize - meaning you pull the threads right out with the plug. Give it at least 2 hours after driving. Morning of the next day is ideal.
Locate the spark plugs
On a 4-cylinder, they are in a row along the top of the engine. You will see either rubber coil boots sitting directly on the head (coil-on-plug) or spark plug wires running into the head. Count them - there should be one per cylinder.
Remove the coil or wire
Coil-on-plug: grip the rubber boot, not the coil body, and pull firmly straight up. It is sealed and will resist a bit. For wire-equipped engines: grip the boot at the plug end and pull - never yank the wire itself. Inspect the boot for cracks before reinstalling later.
Remove the old plug
Insert the spark plug socket and turn counter-clockwise. If the plug is tight, do not force it. Apply a small amount of penetrating oil around the base and wait 10 minutes. Work one plug at a time - never leave multiple holes open at once in case something falls in.
Check or confirm the gap
Most modern iridium and platinum plugs come pre-gapped from the factory for common applications. Check the gap against the spec on the sticker under your hood. Use a feeler gauge to check - not a coin-style gapping tool, which can damage fine iridium electrodes.
Install the new plug
Thread in by hand for the first several turns. If it does not go easily by hand, it is cross-threading - stop, remove it, and start again. Once hand-tight, use the torque wrench. Typical spec: 15-20 ft-lbs. Seat the coil boot firmly until you feel it click or seat fully.
The Anti-Seize Debate
Anti-seize is a compound you apply to the threads to prevent the plug from seizing in the head. Some mechanics put it on every plug. NGK, the largest spark plug manufacturer, says do not use it on their nickel-plated plugs.
The reason: NGK calculates their torque specifications for a dry thread. Adding anti-seize reduces friction, meaning the same torque value applies more clamping force. You effectively over-tighten and risk stripping threads.
Practical guidance
- +If you do not use anti-seize: torque to spec (15-20 ft-lbs for most cars)
- !If you want to use anti-seize: reduce torque by approximately 20%
- iOn aluminum heads where you are worried about future removal: finger-tight plus 1/4 turn is a common alternative
Mistakes that cost money
Over-tightening on an aluminum head can strip the threads. That repair costs $500+. A $25 torque wrench is cheap insurance. Never use an impact driver on spark plugs.