It is a 3/4 allen. I tried rigging up but then my 1/2 impact wouldn't break it loose even my 3/4 impact would not break it loose so i called a friend that runs a transmission shop and thankfully he had a Snap On 1/2 drive with a nice 3/4 shank on. Broke it loose instantly, glad to temove the screen and clean it. Wasn't real bad but had some green paint chips and some other junk. Thirteen gallons later no leaks, was able to take my transmission shop buddy 20 gallons of used oil when i took the tool back, good day.