Our Products

Probe Application

Plunger Selection

There are three basic factors in selecting a plunger. They are plunger style, plunger diameter, and barrel diameter. We recommend that the user always uses the largest diameter plunger and barrel possible, keeping in mind the space available on the particular application. Throughout the product pages, we have specified the minimum centers for each spring test probe and switch probe.

Keeping within the plunger’s working travel will assure that the test probe will reach its designed cycle performance.

The plunger style to be selected must take into consideration whether the user is going to test leads, terminals, lands, pads, through holes or smooth surfaces. The user must also consider if contamination is an issue. Solder residue and greases used in watertight connectors pose a particular challenge. In some applications self- cleaning plungers should be considered. Self-cleaning plungers will shed any contaminants picked up from the components being tested.

Application Example

If the objective is to measure an electrical impulse between two round terminals with .045 (1.14) diameter holes and a center to center distance of .110 (2.79), several factors must be considered. The first step is to determine the best-suited test probe. Be sure to take into consideration the center to center distance between the test points. Always use the largest spring test probe or switch probe with their respective receptacles that will fit into the application. The catalog specifies the minimum centers for each test probe. In this example, we find that the best-suited test probe for this application will be the LS054R series. The next consideration should be the distance between the surfaces where the spring test probes and the terminals are mounted. In the example shown, we see a minimum plunger length requirement of .335 (8.51).

Once a series has been selected the plunger style will be determined. Let us suppose that these terminals were exposed to several contaminants such as soldering flux, fingerprints or dust. Being concerned about possible contamination the user may want to contact the inside of the terminal where fewer deterrents to the flow of electric current may be found. We suggest using an LS054R-427-N because the corners of the four-sided pyramid will act as “knives” which will penetrate through any contaminants.

Quality Assurance


The creation of a test probe starts here. It must meet or exceed the customer’s expectations. Our designers use their combined product performance experience and engineering knowledge to develop the best-suited test probe for a given application


  •  Barrel: This is a fundamental component in the operation and life of the test probe. Besides containing all the internal parts, it must resist damage caused by impact, installation forces and wear caused by the friction of the moving plunger and spring. The barrel must also be a good electrical conductor. We use gold plating for minimum resistance, as required by the electronics industry, or nickel plating as used by the wire harness industry.
  •  Plunger: Our plungers are made of heat-treated beryllium copper. This material resists bending forces and wear. We also use steel for the economy in the larger sized probes. The plungers are gold or nickel plated as required by the user.
  •  Spring: This element can be considered as the heart of the spring test probe. It must retain the same force characteristics after one million cycles as it had when it was first installed. It must also have a strong resistance to corrosion. Corrosion on the spring will alter its designed force. The stainless steel and music wire that we use in our springs meets this criteria satisfactory, assuring a long spring test probe life.


We are well aware that in a number of cases, spring test probes are subjected to conditions that are not considered normal. To remedy these conditions, Lone Star Industrial has fortified its spring test probes with the following features:

  •  Thicker walled barrels.
  •  Minimum clearance between plungers.
  •  Contact points with sufficient surface area.
  •  The interior of the spring test probe must be kept as clean as possible. Abrasive material will shorten its functional life and in extreme cases cause the plunger to stick. We install a chrome steel seal ball in the bottom of each barrel to keep out contaminants such as dust airborne particles and solder residue.
Lone Star Industrial