Abstract: A roller arm centralizer to center a drill string in a well pipe. The arms of the roller arm centralizer include shoulders to limit movement of the arms in the direction of the centralizer body which prevents the arms from being locked in a straight line adjacent to the centralizer body. The shoulders also prevent the arms from being forced into an undesired configuration; wherein one arm is parallel to the body and the other two arms form a "V", by an abrupt decrease in the bore diameter. The roller arm centralizer remains operative when passing through relatively narrow sections of well pipe or through well pipe having abrupt changes in diameter.