Spark Plug Threads Sizes Table

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Spark Plug Threads Sizes Table

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Spark Plug Threads Sizes and Tapped Hole

The SAE Standard includes the following sizes: 7⁄8- inch nominal diameter with 18 threads per inch: 18-millimeter nominal diameter with a 18- millimeter nominal diameter with 1.5-millimeter pitch; 14-millimeter nominal diameter with a 1.25-millimeter pitch; 10-millimeter nominal diameter with a 1.0 millimeter pitch; 3⁄8-inch nominal diameter with 24 threads per inch; and 1⁄4-inch nominal diameter with 32 threads per inch. During manufacture, in order to keep the wear on the threading tools within permissible limits, the threads in the spark plug GO (ring) gage should be truncated to the maximum minor diameter of the spark plug; and in the tapped hole GO (plug) gage to the minimum major diameter of the tapped hole.

SAE Standard Threads for Spark Plugs

Size^a Nom. × Pitch	Major Diameter		Pitch Diameter		Minor Diameter
Size^a Nom. × Pitch	Max.	Min.	Max.	Min.	Max.	Min.
Spark Plug Threads, mm (inches)
M18 × 1.5	17.933 (0.07060)	17.803 (0.7009)	16.959 (0.6677)	16.853 (0.6635)	16.053 (0.6320)	-
M14 × 1.25	13.868 (0.5460)	13.741 (0.5410)	13.104 (0.5159)	12.997 (0.5117)	12.339 (0.4858)	-
M12 × 1.25	11.862 (0.4670)	11.735 (0.4620)	11.100 (0.4370)	10.998 (0.4330)	10.211 (0.4020)	-
M10 × 1.0	9.974 (0.3927)	9.794 (0.3856)	9.324 (0.3671)	9.212 (0.3627)	8.747 (0.3444)	-

Spark Plug Tapped Thread Holes

Size^a Nom. × Pitch	Major Diameter		Pitch Diameter		Minor Diameter
Size^a Nom. × Pitch	Max.	Min.	Max.	Min.	Max.	Min.
Tapped Hole Threads, mm (inches)
M18 × 1.5	…	18.039 (0.7102)	17.153 (0.6753)	17.026 (0.6703)	16.426 (0.6467)	16.266 (0.6404)
M14 × 1.25	… …	14.034 (0.5525)	13.297 (0.5235)	13.188 (0.5192)	12.692 (0.4997)	12.499 (0.4921)
M12 × 1.25	… …	12.000 (0.4724)	11.242 (0.4426)	11.188 (0.4405)	10.559 (0.4157)	10.366 (0.4081)
M10 × 1.0	… …	10.000 (0.3937)	9.500 (0.3740)	9.350 (0.3681)	9.153 (0,3604)	8.917 (0.3511)

^a M14 and M18 are preferred for new applications.

Thread Designations

Thread major and minor definitions

In order to keep the wear on the threading tools within permissible limits, the threads in the sparkplug GO (ring) gage shall be truncated to the maximum minor diameter of the spark plug, and in the tapped hole GO (plug) gage to the minimum major diameter of the tapped hole. The plain plug gage for checking the minor diameter of the tapped hole shall be the minimum specified. The thread form is that of the ISO metric

Reference: Society of Automotive Engineering International (SAE)