Spi 29-421-5 Class Zz Plus Plug Gage 0.421 in Dia

Product Description

The SPI 29-421-5 is a Class ZZ Plus plug gage used to verify the inside diameter of holes to a nominal size of 0.421 inches. It is manufactured from steel and holds a tolerance of 0.0002 inches, making it suitable for go/no-go inspection work in production and quality-control environments. Machinists, toolmakers, and QC inspectors use plug gages like this one to confirm bore dimensions on machined parts, castings, and assemblies. Class ZZ is the standard tolerance class for general-purpose inspection, and the Plus designation indicates the gage is sized at the upper end of the tolerance band.

This plug gage is used in machine shops, manufacturing facilities, and maintenance environments where hole diameter verification is part of a standard inspection process. It is appropriate for checking drilled, reamed, or bored holes in metal, plastic, or composite workpieces. The 0.421-inch nominal diameter makes it a direct fit for inspection of holes in that size range without requiring a gage block stack or indicating bore gage setup.

Key Features

• Nominal diameter of 0.421 inches for direct hole inspection at that size
• Class ZZ Plus tolerance class rated at 0.0002 inches for general-purpose go/no-go work
• Steel construction for durability and dimensional stability under shop conditions
• Compact and lightweight at 0.2 pounds for easy single-hand use at the inspection station
• Single-piece packaging suitable for individual replacement or toolbox stocking

Compatibility

Designed as an OEM-style replacement or direct addition to any Class ZZ plug gage set requiring a 0.421-inch nominal diameter member.

Installation Notes

Plug gages are used by machinists and quality-control inspectors and require no installation tools. Handle with clean, dry hands or inspection gloves to prevent corrosion from skin oils, and store in a protective case when not in use. Before use, verify the gage surface is free of burrs, chips, or debris that could affect the measurement reading. Periodic recalibration against certified masters is recommended to maintain traceability.