Custom Wiring Harness

Take you back to the history of terminal crimping

Apr 17, 2024

Electrical conductors have been used to transmit signals throughout aircraft since the dawn of flight and early propulsion systems. As air travel evolves, so do the requirements for the wiring harnesses and connectors used in these systems.


With the rapid progress in automobiles, aerospace, medical equipment, industrial equipment and other fields, the requirements for electrical systems are becoming higher and higher. The increasing number of electrical components and functions integrated into modern devices has resulted in a significant increase in the number and type of wires and cables in wiring harnesses. This complex wiring requires connectors, terminals, wires, fiber optics and other technology that requires precision tools.



In the 1940s all terminals were soldered

1953 AMP introduces crimp barrel terminals

1957 Cannon brothers experiment with machined terminals with crimp barrels

1960 Buchanan introduces 4-notch crimp tool with ratchet (Ref. MS3191)

1961 Boeing adopts Minuteman program’s ML-C-26500 standard

1963 MS3191-1 is released as the first crimp tool standard

In 1965 Daniels Manufacturing introduced the MS3191-4.

1969 MIL-T-22520 published and dated, superseding all previous specifications

1974 Changed to MlL-C-22520, added slash table

1996 Changed to MIL-DTL-22520

2016 to present Completed SAE AS22520 changes




Crimping: Then and Now

The first multi-pin connectors were terminated by soldering conductors to non-removable terminals. However, high temperature applications and the need for simple and reliable field service led to the introduction of connectors with removable terminals. They are crimped to the conductors rather than soldered.

The first standard crimping tools for crimping these new terminals were introduced in the early sixties. MS3191-1 is a military drawing that defines the tool and its accessories.

The MS3191-1 features a four-indentation crimp pattern and a positive stop positioner that controls indenter travel (crimp depth).

The MS3191-1 design is a compromise between operational simplicity and crimp performance because the crimp depth of any given terminal cannot be adjusted to accommodate the different diameters of the conductors being crimped. However, it fits the crimp connectors of that era. An improved tool design with independently adjustable crimp depth was soon introduced as the MS3191-4.

The MS3191-4 features internal adjustments that are completely independent of the positioner, allowing selection of seven independent crimp depths, allowing for optimal crimping of conductors ranging from AWG 12 to 26, regardless of the contact’s barrel size . MS3191-4 also introduces the use of a dual-ended indenter to produce eight indentation crimp patterns that consistently achieve superior tensile break values and mechanical properties.

The MS3191-4 introduces the concept of a turret head containing three positioners that can be used without the need to separate any one of the positioners from the base crimp tool.


In 1969, military specifications for two crimping tools were developed to replace existing military drawings. They are MIL-T-22520C (Navy) and MIL-T-83724 (U.S. Air Force), which define standard size crimp tools similar to MS3191-4, but with an expanded eight-step crimp depth range. These specifications also define a micro crimp tool for crimping conductors as small as AWG 32. The two documents were merged into MIL-C-22520D in 1971. All previous military standards for crimp tools, including MS3191, were subsequently cancelled. This list includes specifications for notch crimp tools, terminal lug crimp tools, air tools, coaxial cable crimp tools, and other specialty crimp tools.

MIL-DTL-22520 establishes a single specification that defines the performance requirements for all crimp tools used on military-standard electrical connectors. This eliminates the waste and confusion caused by the overlapping application of many different "standard" crimping tools due to numerous unrelated military drawings.

Crimp concept


As an electrical connection method, crimping is widely used in the electronics and electrical industries. It tightly combines the metal barrel with the conductor through pressure, ensuring that the terminal or contact end can be firmly connected to the electrical conductor.

To achieve a satisfactory crimp, the selection and combination of conductors, crimp barrels and tools are critical. Conductors need to have good electrical conductivity and mechanical strength to withstand the electrical current and mechanical stress during operation. The crimping barrel needs to have sufficient elasticity and plasticity so that it can be tightly combined with the conductor under pressure to form a strong connection.

When correctly matched tools are used, the joints created have good electrical and mechanical properties. The tool will meet these requirements consistently and reliably, ensuring repeatability with quality cycle control tools.

The resistance of a properly designed and controlled crimp connector should be equal to or less than that of wire of equal cross-section. Specifications specify requirements for millivolt voltage drop at specified currents.

The mechanical strength of a crimp joint and its pullout force (tensile strength) varies with the applied deformation (i.e., the crimp die of the tool determines the crimp configuration and deformation). Therefore, by properly shaping the deformation, high pullout forces can be obtained. The die in the tool determines the complete crimp configuration, which is often an element of the contact and/or connector design.

Some design considerations include:

a) contact type, its size, shape, material and function,

b) the type and size of wires to be accommodated,

c) The type of tool that must be configured.

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