IN THE KNOW SPARK PLUG THREADS AND REPAIR
On rare occasions, it may be necessary to repair the spark plug hole in a cylinder head because of either cross- threading of the spark plug or incorrect installation torque. Today’s smaller spark plug thread sizes are especially susceptible to breakage when over-tightened. The aluminum cylinder heads, used on most engines, are also vulnerable to damage from cross threading. NGK Spark Plugs strongly recommends following the torque recommendations and installation procedures provided in either the NGK Spark Plugs catalog or the application’s manufacturer’s repair guide. Currently, there are four common industry standard spark plug thread sizes; 18mm, 14mm, 12mm and 10mm thread. Identifying thread size on NGK spark plugs can usually be achieved by looking at the first alphanumeric character in the part number. Using KR5E as an example, will notate the thread size as being 14mm. As seen below, there are several letters used to identify thread size. While there are a few exceptions, this rule this will hold true for the majority of NGK spark plugs.
Thread Size Measurement
The industry currently uses these different thread pitches. Thread pitch is defined by the amount of threads per millimeter.
top view of spark plug
A, W = 18mm 18mm X 1.50 B, F, G, T, U, X, Y = 14mm 14mm X 1.25 C, M = 10mm 12mm X 1.25 D, J, K = 12mm 10mm X 1.0
thread size
Thread Reach Measurement
In the event of damage to the cylinder head spark plug hole, thread repair may be possible. In some cases, a thread-chaser tool can repair the threads of the cylinder head. In more severe situations, a thread repair insert may be required. One would need to know the thread reach of the spark plug to purchase the proper thread repair kit. Thread reach is defined as the measurement from the seat of the spark plug to the end of the threads (near the firing electrode). This measurement can be obtained by calling NGK technical support at 1-877-473-6767, or by using the NGK spark plug specification chart in the NGK Spark Plugs catalog.
thread reach
For further information, call NGK Technical Support at 1-877-473-6767 ext. #2 , or visit us at ngksparkplugs.com.
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