Machinery's Handbook, 31st Edition
2112
Spark Plug Threads Spark Plug Threads
British Standard for Spark Plugs BS 45:1972 (withdrawn).— This revised British Standard refers solely to spark plugs used in automobiles and industrial spark ignition internal combustion engines. The basic thread form is that of the ISO metric (see page 2046 ). In assigning tolerances to the threads of the spark plug and the tapped holes, full consideration has been given to the desirability of achieving the closest possible measure of interchangeability between British spark plugs and engines, and those made to the standards of other ISO Member Bodies. Basic Thread Dimensions for Spark Plug and Tapped Hole in Cylinder Head
Major Dia.
Pitch Dia.
Minor Dia.
Nom. Size
Pitch
Thread
Max.
Min.
Max.
Min.
Max. 12.402 12.912 16.092 16.676
Min.
13.937 a
14 14 18 18
1.25 1.25 1.5
Plug Hole Plug Hole
13.725 13.125
12.993 13.188 16.819 17.026
12.181 12.647 15.845 16.376
14.00
13.368
17.933 a
17.697 16.959
1.5
18.00
17.216
a Not specified All dimensions are given in millimeters.
The tolerance grades for finished spark plugs and corresponding tapped holes in the cylinder head are: for 14 mm size, 6e for spark plugs and 6H for tapped holes which gives a minimum clearance of 0.063 mm; and for 18 mm size, 6e for spark plugs and 6H for tapped holes which gives a minimum clearance of 0.067 mm. These minimum clearances are intended to prevent the possibility of seizure, as a result of combustion deposits on the bare threads, when removing the spark plugs and applies to both ferrous and nonferrous materials. These clearances are also intended to enable spark plugs with threads in accordance with this standard to be fitted into existing holes. SAE Spark-Plug Screw Threads.— 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 diam- eter 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 Max. Min. Max. Min. Max. Min. Spark Plug Threads, mm (inches) M18 × 1.5 17.933 17.803 16.959 16.853 16.053 … (0.7060) (0.7009) (0.6677) (0.6635) (0.6320) … M14 × 1.25 13.868 13.741 13.104 12.997 12.339 … (0.5460) (0.5410) (0.5159) (0.5117) (0.4858) … M12 × 1.25 11.862 11.735 11.100 10.998 10.211 … (0.4670) (0.4620) (0.4370) (0.4330) (0.4020) … M10 × 1.0 9.974 9.794 9.324 9.212 8.747 … (0.3927) (0.3856) (0.3671) (0.3627) (0.3444) …
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