How tight you install any spark plug into an engine head is critical. Small engine spark plugs come in different sizes with the most common being 14 mm, 12 mm, and now even 10 mm.
Many newer lawn and garden engines now take the tiny 10 mm spark plugs. The body wall thickness of these spark plugs are deceptively narrow. Once installed these spark plugs have to come back out eventually and over tightening them can cause them to snap off at the threads because a spark plug, like any bolt, requires more torque to remove than to install.
Plug manufacturers have preached for years about using a torque wrench when installing these 10 mm plugs and for good reason. Removing a broken over tightened spark plug can be a big expense. You shouldn’t rely on just the “feel” of tightness – either use a torque wrench or a “seated rotation” guideline to ensure the proper tightness of any small engine spark plug that you install.