Bet, i don't get this posted in time.....
The bolt part that goes through the arm is tapered. Put a board on top to keep from damaging anything, remove the grease fitting if you have already screwed it in, if it has one, and tap the joint quite firmly down into the arm.
The taper will bind, and hold long enough for the nut to start tightening, which will draw it farther into the taper, and lock it solid.
J.