Please check the attach image it's a kind of seesaw. But as from image the black bodies have same density. And the horizontal rectangle is attached with the triangle using "Revolute" joint. But still not working any suggestion. in the current situation it's need to be balanced.
Due to tiny imbalances in the layout caused by the limitations of floating point precision etc, it's highly unlikely that this will ever balance in the middle reliably (just like real life). One thing you could try is to give the beam some angular damping, which would make it less easy to swivel around, so it would slow down quicker and sleep earlier. That might be enough to get it to come to rest without falling to one side or the other.
I don't think it is the problem of floating point precision. At least it can't appear that fast. As far as I know Box2D resolves contacts (including resting contacts) one by one. It is much faster than simultaneous contact resolve, but less precise at the same time since resolving one contact create impact on the others.
I would try adding a motor with a small maximum torque to your revolute joint and controlling it's speed to balance the system.
