The 150mm inside spring calipers are a hand tool used to measure and transfer internal dimensions such as the inside diameter of holes, recesses and bores. The tool comprises two concave legs joined by a spring-loaded hinge that opens to a set span; its primary use is to take quick comparative measurements and to transfer those measurements to vernier or digital calipers, micrometers or scribing tools for final inspection. It is intended for workshop and fabrication use where reliable repeatable internal measurement is required.

These calipers are manufactured from precision hardened and tempered carbon steel with a typical hardness range of approximately HRC 45–55 to resist wear at the contact points. The working surfaces are ground and the body is finished in a corrosion-resistant black oxide to reduce surface rust; some variants are nickel-plated for additional protection in humid environments. The hinge contains an internal tempered coil spring to provide consistent opening tension, and a knurled fine-adjuster (thumb nut) allows the user to set and lock the span for repeatable measurements. As a hand tool the caliper has no thread or drive type in the sense of powered tools; the adjuster uses a fine screw thread for locking.

Manufacturing follows recognised workshop tooling and quality practices and is produced in factories operating under ISO 9001 quality controls; the design and dimensions conform to common DIN/BS conventions for dividers and calipers to ensure interchangeability with other workshop measuring equipment. The instrument is simple to use but precise enough for applications where an accurate transfer of internal measurement is required prior to final gauging.

Typical applications include metalworking and engineering workshops, toolroom setting, automotive cylinder and bore checks, plumbing and pipe fitting, pattern making and general maintenance and inspection tasks. The key benefits are the durable hardened leg tips for long life, the protective finish for corrosion resistance, and the consistent spring action plus fine adjuster for accurate, repeatable transfers compatible with vernier and digital measuring instruments.

Duchy Fasteners stocks this style of inside spring caliper in 75mm, 100mm, 150mm and 200mm lengths; they are available singly or in trade packs to suit workshop and trade requirements. If you require alternative finishes or protective cases we can offer matching options from our hand tools range.