Features and Benefits: Heli-Coil Inserts are packaged according to thread size for easy handling and display The inserts are contained in a sturdy, clear, plastic display box and marked with part number and thread size The length of the inserts in inch and metric packages is 1- 1/2 times the diameter of the bolt This provides sufficient strength in most applications Spark plug hole inserts are packaged in lengths corresponding to the various spark plug reaches. .