The Threads of Fasteners (4)

1.4 Thread Fit Grades

Thread fit refers to the degree of looseness or tightness between mating threads. The fit grade is a specified combination of deviations and tolerances applied to external and internal threads.

For unified imperial threads, external threads have three grades: 1A, 2A, and 3A, while internal threads have three grades: 1B, 2B, and 3B. All are clearance fits. The higher the grade number, the tighter the fit. For imperial threads, deviations are specified only for grades 1A and 2A; the deviation for grade 3A is zero, and the deviations for grades 1A and 2A are equal.

A higher grade number indicates a smaller tolerance.

Metric threads: External threads have three grades: 4h, 6h, and 6g; internal threads have three grades: 5H, 6H, and 7H.

In metric threads, the fundamental deviation for H and h is zero. The fundamental deviation for G is positive, while for e, f, and g, it is negative.

1. H is a commonly used tolerance zone position for internal threads, typically not used for surface plating or with very thin phosphating layers. The G position with fundamental deviation is used for special applications, such as thicker plating layers, and is generally seldom used.

2. g is often used for thin plating layers of 6–9 μm. For example, if a product drawing specifies a 6h bolt, the pre-plating thread should use the 6g tolerance zone.

3. The recommended thread fit combinations are H/g, H/h, or G/h. For precision fastener threads such as galvanized bolts and nuts, the standard recommends using a 6H/6g fit.