This special 24 mm flare nut type wrench will slide back into the recessed area of the cylinder head where the fuel line retainer screws into the head. Without this special depth and angles flare jut wrench, you cannot hold the 24 mm retainer in place while you loosen the 19 mm coupler to the high-pressure fuel lines. This tool is special for the Cummins common fuel rail set up on the 5.9L motors 2003 and later. This Cummins 5.9L diesel motor can be found in Dodge trucks starting in 2003 and other trucks using the Cummins 5.9L motor with a common fuel rail.