Spark Plug Installation

Torque is one of the most critical aspects of spark plug installation. Torque directly affects the ability of the spark plug firing end to dissipate heat to the cylinder head. A spark plug that is under-torqued will not be fully seated in the cylinder head and heat dissipation will be slowed. As a result, the firing end of the spark plug can overheat and pre-ignition can occur. Serious engine damage can follow.

Over torque can cause the metal shell of the plug to stretch. The threads can crack and/or break off.    Stretching of the spark plug metal shell can break the inner gas seals or cause a fracture in the spark plug insulator. The most dangerous result is pre-ignition when the heat dissipation is reduced due to metal shell stretching by over torquing. In short, over torquing can cause severe engine damage.

The spark plug holes must always be clean prior to installation; otherwise, you may be twisting against dirt or debris and the spark plug will be under-torqued, even though your torque wrench indicates the proper torque. Of course, you should only remove and install spark plugs in a cool engine, because metal expands when hot and thread damage can happen. Proper torque specs for both aluminum and cast-iron cylinder heads are listed below.

Spark Plug Type Thread Diameter Cast Iron Cylinder Head (lb-ft.) Aluminum Cylinder Head (lb-ft.)
Flat seat type (with gasket) 18 ø mm 25.3~32.5 25.3~32.5
Flat seat type (with gasket) 14 ø mm 18.0~25.3 18.0~21.6
Flat seat type (with gasket) 12 ø mm 10.8~18.0 10.8~14.5
Flat seat type (with gasket) 10 ø mm 7.2~10.8 7.2~8.7
Flat seat type (with gasket) 8 ø mm 5.8~7.2
Conical seat type (without gasket) 18 ø mm 14.5~21.6 14.5~21.6
Conical seat type (without gasket) 14 ø mm 10.8~18.0 7.2~14.5

Installing Spark Plugs – Lawn & Garden Equipment

  1. Confirm that the thread reach of the spark plug is the right one for your engine.
  2. Remove the dirt at the gasket seal of the cylinder head.
  3. Tighten the spark plug finger-tight until the gasket reaches the cylinder head, then tighten about ½ – ⅔ turn more with a spark plug wrench. (Taper seat: About 1/16 turn more.)