* Cross-threading: This is the most common culprit. If the threads on the spark plug or the cylinder head are damaged (perhaps from a previously stripped plug or a dropped tool), the plug won't engage properly. You'll likely feel resistance and it won't turn smoothly.
* Damaged Spark Plug: The spark plug itself might be damaged. A bent electrode, or a warped or damaged shell can prevent it from seating correctly.
* Damaged Cylinder Head Threads: The threads in the cylinder head itself could be stripped or damaged. This is a more serious problem requiring repair.
* Foreign Object in the Cylinder Head: Something small might have fallen into the cylinder head and is obstructing the spark plug hole. This is less likely but possible.
* Incorrect Spark Plug: Using the wrong spark plug (wrong heat range, wrong thread size, wrong reach) will prevent it from screwing in correctly. Always double-check you're using the correct specification.
* Worn or Damaged Spark Plug Well: Over time, the spark plug well can become worn or damaged, hindering proper seating.
What to do:
1. Don't force it: Forcing a spark plug will only worsen the damage.
2. Inspect the Spark Plug: Carefully examine the spark plug for any damage.
3. Inspect the Cylinder Head: Use a small mirror or flashlight to visually inspect the spark plug hole in the cylinder head for damage or obstructions.
4. Check the Spark Plug Thread: Make sure the spark plug threads are clean and undamaged.
5. Try a different Spark Plug (of the correct type): This will help determine if the problem is with the plug or the cylinder head.
6. Consult a Repair Manual: Your owner's manual or a repair manual specific to your vehicle will provide detailed specifications for your spark plugs and may offer troubleshooting advice.
7. Professional Help: If you're uncomfortable inspecting the cylinder head or suspect significant damage, take your vehicle to a qualified mechanic. Ignoring a damaged spark plug well can lead to much more serious engine problems. A damaged cylinder head often necessitates expensive repair or replacement.
