Jesus nut or Jesus pin are colloquialisms for the main rotor retaining nut [1] that holds the main rotor to the mast of some helicopters, such as the UH-1 Iroquois helicopter. The term may have been coined by American soldiers in Vietnam; the Vietnam War was the first war to feature large numbers of soldiers riding in helicopters. Other sources suppose that the term may be coined as early as by Igor Sikorsky, a pioneer of rotor wing aircraft, who was a deeply religious person[2].
The term may have come from the idea that, if the Jesus pin were to fail in flight, the helicopter would detach from the rotors and the only thing left for the crew to do would be to pray to Jesus. In addition, a person must have faith in the Jesus bolt to do its intended job without failure. Real examples of the Jesus pin failing are few and far between. However the pin must be checked before the flight[3]. Some more recent helicopter systems do not have a Jesus nut.
More recently, it has come to be a generic engineering term referring to any single component of a system whose failure would cause catastrophic failure of the whole system.
In literature the term Jesus Nut was used in Chickenhawk by Robert Mason, a narrative about his experiences in the Vietnam War. A Jesus Nut figures prominently in the climax of Clark McCann’s thriller Black Air.[4]
The Rattler/Firebird Association is a Vietnam War veterans organisation. It awards a chrome plated Jesus nut to the member who traveled the longest distance to their reunions. [5]
Mountain bikers often use the term to describe the screw that holds the rear wheel pivot bar in place on a rear suspension bike. This bar holds the back suspension onto the frame of the bike. If the screw falls out, the pivot bar tends to slide out, leading to catastrophic rear-wheel failure, ending in unavoidable loss of the back wheel. Since the pivot bar will only come out under heavy vibrations, it tends to fail at high speed, therefore leading to serious injury.
Another use for the term is found in rock climbing, in which it refers to the first piece of protection placed on a pitch. This piece must be placed to resist an outward pull as well as a downward pull in order to avoid the possibility of a "zipper", in which the outward pull on the rope from the belayer arresting a falling climber pulls protection pieces from the bottom up. In addition, the Jesus Nut prevents the possibility of a factor two fall onto the belay anchor.[6]
Jesus nut is also used in reference to the nut on the front of the recoil mechanism on some artillery pieces. Should this nut fail, the gun tube will come out of battery upon firing, potentially leading to death for the gun crew.
A derivative term used in radio-controlled helicopters is the "Jesus Bolt", which refers to a bolt whose function is analogous to the Jesus nut in a full size helicopter.
