Quality Professional Precision Tool Use this indicator to quickly center work pieces or fixtures on your mill. The measuring range is 0.25". The shank is 0.375" diameter and 0.75" long. The indicator is 5.5" long. Can measure internal diameters from 0.12" to 4.25" and external diameters from zero to 4.25". This set includes: Indicator body, restraining rod (prevents rotation), spring-loaded center point feeler, three straight feelers (for IDs), three curved feelers (for ODs). Features: For quick centering on milling and boring set-ups Includes 6 Feelers: 4", 8", 12" Straight 4", 8", 12" Curved 1/8" Feeler Ball Measuring Range: Internal: 5/32" - 4-1/4" External: 0 - 4-1/4" Locating Accuracy is 0.0005" of center Furnished in a fitted wood case Operates at any angle from horizontal to vertical without any change, and from internal to external by simply changing the feeler