When installing a new spark plug, always follow the instructions listed in the manufacturer's literature. Improper torquing can lead to engine damage. The information provided below has been estimated for standard engines:
In most engines, replacement plugs can be installed without using a torque wrench but extra care should be taken to ensure the new spark plug seats and seals properly. If you are inspecting and replacing an existing plug, be sure to use a wire brush to clean any debris from the plug's threads.
A tapered-seat spark plug should be tightened by 1/16th of a turn past finger tight. See the illustration above.
NOTE: If you have high performance alloy or aluminum heads, always use a torque wrench to eliminate any possibility of damaging the engine's internal threads.