Technical information

Spark Plug Anatomy / Construction

Resistor Spark Plugs

Precious Metal Spark Plugs

Spark Plug Installation

LPG Compatibility

Operating Temperature

Heat Range Explanation


Electrode Gap

Electrode Gap Adjustment

Modified / Performance Engines

Spark Plug Analysis

Technical information

Spark Plug Installation

Spark plugs are designed to operate at an optimal operating temperature and correct tightening is critical to the operation of a spark plug. The following two major problems commonly occur if the incorrect torque is applied:

Installation Instructions for new Spark Plug:

1. Confirm that the thread reach of the spark plug is the right one for the engine.
2. Remove any dirt at the gasket seat of the cylinder head.
3. Tighten the spark plug finger tight first until the gasket reaches the cylinder head, then screw about 1/2 - 2/3 turn more with a plug
    wrench. (Conical seat; about 1/16 turn more). Refer to diagrams below for full details.
5. If using a torque wrench, please refer to the table below.
6. Always remove and install spark plugs when the engine is cool.



It is essential to select a spark plug specified by the vehicle manufacturer from the NGK Recommendation Chart.


     Plug socket troubles:


Thread damage

Metal shell damage

Insulator breakage

Details of problem


Spark plug inserted at an angle.

Excessive tightening.

Spark plug wrench used at an angle or it slipped.

Corrective actions

Tightening the plug by hand first instead of using a wrench from the start.

Tighten to the specified tightening torque

Use a hexagonal socket wrench that does not slip easily.

Torque is one of the most critical aspects of spark plug installation. Torque directly affects the spark plugs' ability to transfer heat out of the combustion chamber. A spark plug that is under-torqued will not be fully seated on the cylinder head, hence heat transfer will be slowed. This will tend to elevate combustion chamber temperatures to unsafe levels, and pre-ignition and detonation will usually follow. Serious engine damage is not far behind.

An over-torqued spark plug can suffer from severe stress to the Metal Shell which in turn can distort the spark plug's inner gas seals or even cause a hairline fracture to the spark plug's insulator, in either case, heat transfer can again be slowed and the above mentioned conditions can occur.

The spark plug holes must always be cleaned prior to installation, otherwise you may be torquing against dirt or debris and the spark plug may actually end up under-torqued, even though your torque wrench says otherwise. Of course, you should only install spark plugs in a cool engine, because metal expands when its hot and installation may prove difficult. Proper torque specs for both aluminium and cast iron cylinder heads are listed below.