I have to agree w/‘Sea-Rat’ as I have never seen the horns used for leverage. On that reg. a possible damage risk??? ( ..I may be WAY out in left field. Experienced DHR techs, please feel free to ‘chime in’ w/advice or insight. ) ..again you may want to consult Fred Roberts ‘Basic SCUBA’ 2nd Edition or VDH archives.The horns should not be used for leverage. Use, or build, a tool such as is shown in the March 1970 U.S. Navy Diving Manual for that purpose. It is not hard to make, and you can use the drawing below to do so.
But I have to ask, why are your removing the "big nut"? It doesn't have to be removed unless it is leaking through that area.
SeaRat
Scott G. Bonser