in a butt fusion machine for fusing two sections of large diameter plastic pipe wherein said sections are clamped in axially aligned clamps one of which is fixed against linear movement and one is relatively linearly movable , a generally l - shaped frame is provided on which said clamps are mounted , part of the frame defining a reference plane . the clamps are mounted to open at an acute angle to the reference plane . the movable clamp is moved by a hydraulic system , which includes an accumulator . a carriage mounted on the upright portion of the frame for movement parallel to the axis of the clamps carries hingedly mounted on it a facer and a heater . both the facer and the heater are mounted on an axis radially outboard of and substantially parallel to the axis of the clamps , and are so arranged as to be selectively swingable between a position intermediate the clamps and a position totally outboard of them . means are provided for maintaining the face and heater selectively and independently in either of said positions .