You should tighten spark plugs to a specific tightness using a process involving hand tightening and then a specific additional turn. Here's the detailed guide:
Spark Plug Tightening Procedure
Step 1: Hand Tightening
- First, screw in the spark plug by hand until it stops, or "seats" against the cylinder head. This means the plug is as tight as you can get it with your fingers, but isn't applying any significant torque.
Step 2: Additional Turning
- For used plugs: After the spark plug is finger-tight, turn it an additional ¼ turn.
- For new plugs: After finger-tightening a new plug, turn it an additional 2/3 to 3/4 turn. This additional turn is necessary because it crushes the washer between the plug and the head, ensuring a tight seal.
Important Notes
- The Crush Washer: The additional turn is crucial to properly compress the crush washer. This creates the gas-tight seal that's necessary for the proper function of the engine.
- Proper Torque: While the "turn" method is common, using a torque wrench is the most accurate method for tightening spark plugs. Refer to your vehicle's service manual for the exact torque specification.
- Over-tightening: Overtightening can damage the spark plug, the cylinder head, or both. Always follow the correct tightening procedure or use a torque wrench to ensure accurate tightening.
- Avoid Cross-threading: Make sure the spark plug is properly aligned before you begin screwing it into the cylinder head. Cross-threading can damage the threads in the cylinder head and make it difficult or impossible to install the plug correctly.
| Spark Plug Type | Additional Turn After Finger Tight |
|---|---|
| Used Spark Plug | ¼ turn |
| New Spark Plug | 2/3 - 3/4 turn |
