GB/T 6791-2025Road vehicles - Spark-plugs and their cylinder head housings - Basic characteristics and dimensions (English PDF)

3.1 Installed height; l1

The distance from the contact point between the spark plug and the cylinder head to the top of

the high-voltage terminal, including the gasket thickness after installation to the specified

torque.

NOTE. For conical-type spark plugs, the contact point refers to the reference point of the sealing surface.

3.2 Spark plug thread size

Nominal dimensions of connection threads of the spark plug and cylinder head.

NOTE. Except for M14×1.25 threads, all threads conform to the metric thread requirements specified in

GB/T 193.

3.3 Hexagon

The characteristics of the contact area between the spark plug housing and the mounting sleeve

when the spark plug is installed in the cylinder head.

3.4 Bi-hexagon

When a spark plug is installed on the cylinder head, the spark plug housing and the mounting

sleeve have the following characteristics at the contact point. there are 12 mounting contact

surfaces, and the mounting sleeve shall also have 12 mounting contact surfaces.

3.5 Conical seating

The spark plug housing seals against the cylinder head using a conical seal.

NOTE. There is no gasket for the sealing surface between this type of spark plug and the cylinder head.

3.6 Flat seating

The spark plug housing seals against the cylinder head in a planar manner, with the sealing

surface perpendicular to the spark plug axis.

NOTE. There is a gasket for the sealing surfaces between this type of spark plug and the cylinder head.

3.7 High voltage terminal

The connection between the spark plug and the high-voltage ignition source.

NOTE. It is abbreviated as "terminal". The connection between the high-voltage ignition source and the

spark plug terminal may be threaded, use a spring clip to connect to a fixed terminal, or use a spring-

loaded mechanical contact.

3.8 Installation tightening torque

The rotational torque that is applied to the spark plug housing to ensure a seal between the spark

plug and cylinder head.

NOTE. The appropriate installation tightening torque value varies depending on the friction conditions

between the spark plug and cylinder head. These conditions include the cylinder head material, the spark

plug plating, thread lubrication, and combustion deposits. Avoid excessive torque during installation, as

it can damage the spark plug's integrity and potentially damage the engine.

3.9 Insulator diameter; d1

The nominal diameter of the insulator between the top of the spark plug housing and the high-

voltage terminal; this is the area where the spark plug contacts the high-voltage wire or the

sheath of the ignition coil.

NOTE. The insulator diameter is a critical dimension for preventing high-voltage leakage (flashover) on

the spark plug insulator surface.

3.10 Spark plug reach; l10

After the spark plug is correctly installed, the distance is measured from the sealing surface

(flat seating) or reference plane (conical seating) of the spark plug to the point where it is flush

with the surface of the cylinder head combustion chamber.

NOTE. The spark plug reach needs to match the cylinder head mounting holes to ensure the spark plug

is correctly installed in the combustion chamber.

3.11 Spark plug projection; l11 – l10

The distance the spark plug extends beyond its reach and into the combustion chamber.

NOTE. The design needs to avoid interference between the spark plug and the engine piston at top dead

center.

4.1 General

The used type of high-voltage terminal shall be determined by the spark plug manufacturer and

the engine manufacturer.

4.2 Dimensions of integral terminal

The dimensions of integral terminal shall conform to Figures 1 and 2.Compact spark plug

terminal may use the type specified in Appendix A.

NOTE. The nuts used on screw-type terminals have the same external dimensions as integral terminals.

Before assembly with screw-type terminals, their internal thread tolerance is 6H.

a 0.7mm pitch shall conform to GB/T 192 and GB/T 193.

b Effective thread length.

c Cylindrical portion.

d According to the manufacturing process, tolerance grades 7e6e are acceptable for the finished

product.

Figure 5 -- Screw-type terminal

5.1 Spark plug reach

The spark plug reach is shown in Figures 6 and 8, and shall conform to Tables 6 or 9.

The following are the definitions of the basic types of spark plug reach.

--- Short. S;

--- Medium. M;

--- Long. L;

--- Extra Long. EL;

--- Special Long. XL;

--- Ultra Long. EXL.

5.2 Gaskets

The function of the gaskets is to ensure the airtightness between the engine cylinder head and

the spark plug under all operating conditions. The airtightness test method refers to GB/T 7825.

For new flat-seat spark plugs with clean, smooth, and dry threads, when applied the torque

values specified in Table 3 according to the methods specified in Clause 7, the gasket thickness

shall conform to the provisions of Table 3.In special cases, non-tethered gaskets may be used.

5.3 Threads, limit dimensions, and tolerances

Spark plug threads and corresponding cylinder head threaded holes shall conform to GB/T 192,

GB/T 193, GB/T 197 and GB/T 2516; and their limit dimensions, pitch diameter, basic profile

and initial clearance shall conform to the provisions of Tables 1 and 2, respectively.

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