DR Power RapidFire 5.5 HP Subaru (July 2013 - Present) Engine Manual - Manual (Page 13)

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Don’t touch the muffler when the engine is running running and just gasoline engine stopping stage to avoid burn.
(2) Clear away dirt from around the spark plug base.
(3) Only remove the spark plug with a spark plug wrench.
(4) Visually check the spark plug. Clean with a steel brush. If the insulator is damaged, replace the spark plug.
(5) Measure the spark plug clearance with a feeler gage. The clearance should be 0.7~0.8mm. If adjustment is necessary,
bend the side electrode carefully.
(6) Check if the spark plug gasket is in good conditions. Screw in the spark plug as far as you can by hand to prevent thread
damage.
(7) Tighten the spark plug with a spark plug wrench and compress the gasket.
-- If a new spark plug is used, turn it 1/2 turn after compressing the gasket.
-- If reinstalling the used spark plug, turn it 1/8-1/4 turns after compressing the gasket.
The spark plug must be tightened securely or it may become very hot and damage the engine.