Thread Gauge Tolerance Classes Explained: 4H, 6H, 7H, 6g, 6e and When to Use Each

• Aerospace and defence fasteners
• Medical device threaded components
• Precision instrument assemblies
• High-load, vibration-critical joints
• When drawing explicitly specifies 4H

Tighter tolerance band than 6H — the machined thread must be closer to the nominal pitch diameter. Requires more precise tapping and higher-quality tooling. Parts that pass 4H also pass 6H. The 4H and 6H GO gauges are the same size — for H-position classes the minimum pitch diameter is the basic size — but the 4H NO-GO gauge is smaller than the 6H NO-GO gauge.

• Automotive engine and powertrain components
• Where 6H is too loose but 4H is unnecessarily tight
• Fine pitch threads requiring closer fit
• When drawing explicitly specifies 5H

Less common than 4H or 6H in standard engineering. Gauge limits fall between those of 4H and 6H. Typically specified by automotive customers following IATF 16949 control plans. Confirm from the drawing — do not assume 5H when only a tolerance grade number is given.

• All general engineering tapped holes
• Standard nuts and threaded inserts
• Machine frames, brackets, housings
• Any application where tolerance class is not specified
• The overwhelming majority of metric internal threads

The default and most widely used internal thread tolerance class in ISO metric engineering. Mates correctly with 6g external threads. Provides a practical balance between ease of tapping, assembly clearance, and joint strength. If your drawing shows no tolerance class for an internal metric thread, 6H is the correct assumption.

• Tapped holes in parts that will be zinc plated or hot-dip galvanised
• Parts with paint or powder coat inside the thread
• Where easy hand assembly is required
• Low-precision or non-critical structural joints

Larger tolerance band than 6H — the thread can be machined looser and still pass inspection. Used when the thread will receive a coating that adds material thickness to the flanks, effectively tightening the thread after plating. A 7H thread that receives zinc plating typically ends up at approximately 6H after coating.

• All standard metric bolts, screws, and studs
• Threaded shafts in general engineering
• Any application where tolerance class is not specified
• Mates correctly with 6H internal threads

The default external thread tolerance class. The g position provides a small negative offset from the basic size — giving assembly clearance and accommodating minor surface contamination. If your drawing shows no tolerance class for an external metric thread, 6g is the correct assumption.

• Bolts or studs that will receive zinc, nickel, or chrome plating
• Hot-dip galvanised external threads
• When the bolt will be used with a 7H nut after plating
• When drawing explicitly specifies 6e

The e position provides a larger negative offset than g — the unplated thread is machined smaller so that after plating adds material to the flanks, the final dimensions fall within the 6g range. Used in conjunction with 7H internal threads in plated assemblies.

• Aerospace fasteners and structural bolts
• Precision instrument and optical components
• High-load joints requiring close thread fit
• When drawing explicitly specifies 4g

Tighter tolerance band than 6g — the machined thread must be closer to the nominal pitch diameter. Mates with 4H or 5H internal threads in precision assemblies. Requires more precise thread rolling or cutting and more stringent quality control.

• Close precision fits where 6g clearance is too large
• Setting gauges and master gauges
• When a zero-deviation external thread is required
• When drawing explicitly specifies 6h

The h position has zero fundamental deviation — the maximum material limit of the external thread touches the basic size. Less common in production fasteners. More frequently used in gauge manufacturing (setting plugs) and precision instrument threads where a near-zero clearance fit is required.