What must be consistent for replacement spark plugs?

For replacement spark plugs, it is essential to have a consistent plug thread size. The thread size ensures that the spark plug fits properly into the engine's cylinder head. A mismatched thread size can lead to improper installation, which can cause several issues, including loss of compression, poor engine performance, and potential damage to the engine.

Having the correct thread size is critical for maintaining the integrity of the engine's combustion chamber. Different engines have specific thread sizes designed to match the load and performance requirements. Therefore, when replacing spark plugs, compatibility with the existing thread size is paramount for safe and effective operation of the vehicle.

Other options, while they may contribute to the overall compatibility of the spark plug, do not have the same direct impact on the installation and functionality as thread size does. The inner casing material and outer coating finish can vary significantly among different spark plugs without affecting the fitment. While using the same brand can sometimes ensure compatibility, it is not strictly necessary as long as the plug meets the specified requirements, including the correct thread size.