The difference between "g" and "h" in bolt go and stop gauges

In high-end fields such as aerospace and automotive manufacturing, the precision of bolt go and stop gauges directly determines the assembly quality of fasteners. The International Organization for Standardization (ISO) classifies thread go and stop gauges into two grades: G (General) and H (High), with the core differences lying in manufacturing tolerances and applicable scenarios.

I. Precision Classification under the Standard System

Tolerance zone positioning difference

Grade G: Complies with ISO 1502 standard, with the tolerance zone located below the basic median line (6H/6g grade), allowing a deviation of ±26μm, suitable for conventional fastening scenarios

Class H: Complies with ASME B1.2 standard, with the tolerance zone closely adhering to the centerline (4H/4h class), allowing only ±12μm deviation, used for high-load connections

Comparison of Detection Principles

Parameters: G-class go/stop gauge, H-class go/stop gauge. The pass rate of the through end is ≥95%. The thread engagement is 100% without resistance. The stop end is judged to be qualified if the screw-in is ≤2.5 turns. Any screw-in is strictly prohibited

Ii. Differentiation of Typical Application Scenarios

G-class economic choice

Connecting bolts for building steel structures

Non-critical parts of agricultural machinery

A production line scenario with an average daily inspection volume of over 500 items

The strict requirements of Class H

Aircraft engine rotor bolts (Case: After a certain aviation engine factory adopted H-class specifications, the scrapping rate decreased by 37%)

Fixing bolts for battery packs of new energy vehicles

Sealing flange connection for nuclear power facilities

Iii. Three-dimensional Model of Procurement Decision-making

Cost dimension

The unit price of H-grade specifications is 40-60% higher than that of G-grade, but it can reduce the subsequent quality claim costs (a certain transmission enterprise saves 820,000 yuan in quality inspection costs annually).

"Efficiency dimension"

The detection speed of G-class gauge is 30% faster than that of H-class gauge, making it suitable for large-scale initial inspection. The H-class needs to be used in conjunction with a pneumatic measuring instrument

Risk control

It is suggested to establish a hierarchical testing system:

Iv. Key Points for Maintenance and Care

Wear compensation: H-class gauges require laser calibration every 2,000 tests, while G-class gauges allow correction after 5,000 tests

Storage conditions: Under a constant temperature of 20±2℃, the accuracy retention period of H-class gauges is 30% shorter than that of G-class gauges

Industry trend: Digital go and stop gauges (such as Zeiss SmartCheck) are gradually replacing traditional mechanical gauges