Starrett 68129 Mechanical Micrometer 5/16 Inch Range

Product Description

The STARRETT 68129 is a mechanical bore micrometer designed for precise inside-diameter measurements in tight bores and holes. It features a ratchet stop thimble for consistent gauging pressure and solid carbide measuring faces for long-term wear resistance. The instrument measures up to 5/16 inch (0.3125 inch) with a graduation of 0.0003 inch and an accuracy of 0.000150 inch, making it suitable for high-precision inspection work. Three contact points provide stable, repeatable seating in the bore. This tool is used by machinists, tool-and-die technicians, and quality control professionals who require close-tolerance dimensional verification.

The 68129 is appropriate for shop-floor and inspection-room use where small bore gauging is required. A gauge depth of 2.25 inches accommodates moderate-depth bores. The maximum extension rod length of 6.0000 inches allows measurement reach in deeper features when rods are employed. The included setting ring (0.3125 inch diameter) allows the instrument to be zeroed accurately before use. This micrometer is suited to applications in precision machining, engine rebuilding, hydraulic component inspection, and similar trades where bore geometry must be verified to tenths-level tolerances.

Key Features

• Ratchet stop thimble delivers consistent measuring pressure for repeatable readings
• Solid carbide measuring faces resist wear during high-volume inspection cycles
• 0.000150 inch accuracy with 0.0003 inch graduation for tenths-level resolution
• Three contact points ensure stable centering in round bores
• Gauge depth of 2.25 inches suits a broad range of bore configurations
• Ships complete with case, contacts, setting ring, wrenches, and instructions

Compatibility

Designed for use wherever inside-diameter measurement up to 0.3125 inch is required. The included 0.3125 inch setting ring is matched to this instrument. Extension rods accommodate bore depths up to 6.0000 inches when fitted.

Installation Notes

Bore micrometers are set and serviced by machinists or metrology technicians. Zero the instrument using the included setting ring before each measurement session. Ensure the bore and measuring faces are clean and free of chips or coolant before gauging. Do not overtighten contact points into the bore; use the ratchet stop to apply only rated measuring pressure.