A 9/16 wrench fits it, it's 24 threads per inch and the adjuster hole is 3/8's, and the nut is only 3/16th's thick. It's hardened and black.

Unfortunately ATM my computer wont get pic off my camera....

Take care,
Rick
68 Coronet R/T 440 & 68 Charger 528 Hemi,and 5 Challengers! 6 cyl, 318, 360, 383, 451