Its a weird set up. I believe it takes a 3/8" extension (I guess it could be 1/4", but for some reason I remember it being 3/8"), just the square end of the extension, and you put that in and then twist and pull. Its a strange set up indeed, and it will come out with a little finagling. The new radiator I have has a wing nut on it and I like that SOOOO much more.