positive stop. you can make them adjustable by using some thick 3/8" flat washers and double nut the lock nuts. i've done this and it works just fine. i'm using 2 very thick flat washers on each stud on mine. crane cams make a kit that is basically exactly what i just described but i think their kit uses one thick spacer instead of the washers but it's basically the same thing. i think it may also use poly locks too but not absolutely certain on that.