to add to Rays oint above
I would recomend for anyone actualy buying new shackls to get 3 piece bow shackles .
not the 2 piece type ( shackle and a threaded pin that screws into the shackle ) 2 piece are a pain . they tighens them selfs up ( even when backed of 1/2 a turn or wil become loose ) .
Get the 3 piece one's . ( seperate Shackle , Pin and Nut .) one of these with a safety clip ( or split pin if your real keen ) is heeps better . they will not tighten or loosen up . and Seem to be the only accepted standard most places I work these days . ( for above said reasons )
I've got mostly these in my recovery gear these days and havn't had any trouble loosing the nut for the shackle .
you can do what I've done and used 2.5mm s/s braided wire ( Fishing trace ) to attach the Pin / Shackle / and safety clip together .