22 and a second part 23 that are spaced along the longitudinal axis 21 of the cylindrical clamping mechanism 20 and separated by the weld head 30 and the reaction support 40 the weld head 30 and the reaction support 40 are rotationally mounted to the base frame 10 such that the weld head 30 can rotate completely around the longitudinal axis 21 of the cylindrical clamping mechanism 20 and the reaction support 40 can be maintained in a position diametrically opposed to the weld head 30 in the preferred embodiment rollers 24 are mounted on the cylindrical clamping mechanism 20 the rollers 24 provide the mechanism by which the weld head 30 and the reaction support 40 are rotated around the circumference of the cylindrical work piece 70 in essence the cylindrical clamping mechanism 20 functions like a stationary inner bearing race the rollers 24 function like a bearing and the weld head 30 and reaction support 40 function like a movable outer bearing race the weld head 30 is also movable in a radial direction this radial movement capability accommodates a variety of work piece sizes and is necessary for the friction stir weld process itself the reaction support 40 also has a roller 41 to facilitate movement of the reaction support 40 around the perimeter of the work piece 70 in the preferred embodiment the means for transmitting load from the weld head to the reaction support 50 is a radial load bar 51 that is able to rotate and stay aligned with the.