The AllSource Back Tap thread repair tool lets you repair damaged threads in spark plug holes without having to pull the cylinder head or use a coil insert. 18mm size is useful for thread repairs on oxygen sensors, too. 18mm size.Collapsible design.Back Tap is inserted through spark plug hole into chamber, expands, then threads back up the hole.Repairs the damaged threads and brings up debris and metal filings with it.Leaves no metal filings in engine.Especially useful for bore thread repairs on oxygen sensors.