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The Insight uses a new, iridium spark plug that is unique to this vehicle.
There are four different part numbers for the spark plugs, and each has the electrode aligned differently with the plug threads. This allows the orientation of the ground electrode in relation to the intake valve to be optimized for maximum economy and power, and optimum combustion efficiency.
Due to variations inherent to the manufacturing process, each engine's head will differ. After each head is manufactured, it is measured and the optimal spark plug for each cylinder in that engine is determined. This information is then stamped on the cylinder head.
Each spark plug has an identification mark (A, B, C or D) located on the top of the solid post terminal that is to be matched with the corresponding identification mark for each cylinder, as stamped on the cylinder head of the engine in question. The cylinder head marking location is shown below.
To ensure proper electrode position, only replace spark plugs with letter codes that match the stamping on the cylinder head. If the spark plugs are to be reused, they must be returned to the same cylinders.
To ensure proper electrode position, it is also doubly important that the plug is torqued to specifications.