This is a 24 mm flare nut wrench for use on Cummins 5.9 liter, and 6.7 liter diesel engine with common fuel rail setup. This will work on the 5.9L 2003 and later, and the 6.7L 2007 and later. This will also work on the Dodge 6.7L diesels with a common rail set up 2003 year. This wrench is a special 24mm flare nut type wrench that will slide back into the recessed area of the cylinder head where the fuel line retainer screws into the head. This wrench holds the 24mm retainer in place while you loosen the 19mm coupler to the high-pressure fuel lines.