Worn or sloppy axles allow wheels to lift off the rails and cause derailments. Truck frame and axle specifications vary widely between various manufacturers. The Coffman Truck Caliper allows for precise measurement of the truck frame so proper length axles/wheelsets can be installed.
An "S" Scale truck that uses needle bearing axles for its wheels has cone shaped holes in the journals where the axle points ride. For needle bearings to work properly, the points of the axle should ride all the way into these cone shaped holes. Otherwise, the axle just flops around loosely in the journal holes causing poor operation and possible derailments.
It is easy to measure the overall length of a wheel set axle with a Vernier, Dial or Digital Caliper but it is very difficult to measure to the bottom of the journal holes to know how long the wheel set axle should be for proper operation.
The "S" Scale Truck Caliper is an adjustable tool that reaches into the journal holes with axle stubs to help get a correct dimension. With the axle stubs in the journal holes, you carefully adjust the Thumb Nut until there is almost no axle slop at the bottom of the holes. Without changing the adjustment, you squeeze the Truck Caliper and remove from the truck frame. The spring returns the Caliper to the setting it had while in the truck. Now with a Vernier, Dial or Digital Caliper you can measure the point to point distance on the Truck Caliper and know how long the wheel set axle should be for that truck.