It is very important to properly tighten spark plugs after replacement or cleaning – especially on small engines with aluminum heads. If spark plugs are overtightened, the threads in the head can be easily stripped; if undertightened, the spark plug can blow out under engine compression. The table below gives approximate torque values for different sizes of spark plugs in both cast iron and aluminum heads. It is best to use the torque value recommended by the manufacturer in the owner’s manual, but if you don’t have that figure available, this table will serve as a general guide. Use a torque wrench to obtain the recommended torque.