bucking bar
A heavy bar held against the bucktail of a solid rivet, which has been heated if necessary to soften it, and passed through the items to be held together, while a rivet gun hammers on the rivet's factory head; the impulses from the hammer, reflected by the bucking bar, then upset the bucktail to form a field head which is typically pancake-shaped.
bucking bar: the anvil that shapes hot rivets from behind
A bucking bar is a heavy steel tool held firmly against the tail end of a rivet while a pneumatic or hand-operated rivet gun pounds the factory head on the opposite side. The bar absorbs and reflects the hammer blows back into the rivet shank, upsetting the protruding tail into a formed head, typically mushroom or pancake-shaped. This method has been the backbone of structural assembly in aircraft, boilers, bridges, and shipbuilding for over a century.
The bars themselves are machined from hardened steel, usually weighing between 2 and 8 pounds depending on the rivet size being set. One face is shaped to match the type of field head desired, whether a shallow pan head, a universal head, or a flush head. The opposite end is typically knurled or roughened to prevent slipping from the gloved hand. The bar must be held perpendicular to the rivet axis and pressed firmly backward with steady pressure; poor contact results in incomplete upsetting, loose rivets, or damage to surrounding material.
Heat treatment of the rivet before bucking depends on the alloy and diameter. Aluminum rivets and steel rivets under 0.375 inches are often driven cold. Larger steel rivets, particularly those in boiler work, are heated to between 1200 and 1600 degrees Fahrenheit to soften the material and reduce the force needed for upsetting. The heated rivet must be driven while still glowing; once it cools below roughly 800 degrees, it becomes too hard to form properly and will shear or tear instead.
Shop practice and wear
In production riveting, the bucking bar operator (the "backer") works as a team with the riveter holding the gun, communicating through noise and feel. The backer must develop a sense for maintaining consistent pressure and alignment; even small angles cause uneven upsetting that creates stress concentrations. Over time and repeated impacts, the formed face of the bar can crack, pit, or mushroom at the edges, requiring dressing with a grinder to restore the surface.
The name "bucking" reflects the verb "to buck": to resist or push back against force. The bar literally bucks or resists the hammer blow, converting it into forming pressure on the rivet tail. Modern riveting has largely been displaced by bolting and welding in new construction, but bucking bars remain essential in aircraft maintenance, restoration work, and in regions where riveted joints are still preferred for fatigue resistance and damage tolerance.