3 point internal micrometers of sturdy construction are made highly durable with hardened alloy tipped contact points. Used for measuring deep holes with an extension rod which is available on models over 6mm measuring range. Accuracy: Instrument Error: 2-20mm 2 mu m, 20-100mm 3 mu m, 100-300mm 5 mu m. Each Holtest is supplied with hex-wrench.