Fuel filter wrench size? - Chevy Message Forum Jul 11, 2012The fuel filter fitting takes a 20 millimeter, and the fuel line fitting is 5/8". Crack the line loose with a 5/8" line wrench while holding the filter with the 20 mm. let the pressure bleed off. go ahead and remove the filter. sure the little o-rings stay Fuel Filter Wrench sizes??? - Diesel Forum - TheDieselStop Mar 05, 2005Water separator plug is 6mm.. make sure the hole is clean, the wrench is bottomed and deliver a couple of sharp raps while holding it hard in the direction you want to rotate. Secondary (engine mounted) fuel filter will accept 1/2" square drive. IIRC, one and seven sixteenths will work in a pinch for the filter in the HFCM.