In ISO metric threads, 6H is the standard tolerance for internal threads (nuts/tapped holes), while 6g is the standard tolerance for external threads (bolts/screws), with the number '6' indicating a medium tolerance grade and the letter specifying the fit position (H for internal, g for external), defining acceptable variations for a secure, general-purpose fit. These designations ensure proper fit, preventing threads from being too tight or too loose, especially important for materials like aluminum or steel.
A 6G gauge is typically used for external threads such as screws or bolts. The tighter tolerance helps the male component seat securely into its counterpart. A 6H gauge is designed for internal threads like tapped holes or nuts, where a smoother fit facilitates assembly, especially in production settings.
6H Tolerance (Internal Threads): The 6H tolerance is used for nuts or the internal threads of a bolt and nut assembly. It ensures that the internal threads fit precisely with the external threads. The 'H' designation indicates that the tolerance is on the higher side of the nominal diameter, ensuring a tighter fit.
6g is the short-form expression for the external thread fit tolerance class 6g6g, which is quite common. In our article about Unified Inch threads, we talk about the 2A and 3A thread fit. In metric, 6g is roughly comparable for 2A in the Inch system.
The “G” in a G1/4″ thread refers to British Standard Pipe (BSP) threads, specifically the cylindrical type. This designation indicates that both the internal and external threads are parallel rather than tapered, which is typical of other thread types like R or NPT (tapered) threads.
"G" designates Ground Thread. "H" designates the pitch diameter is on high side of basic.
The 5g indicates the tolerance class for the pitch diameter and 6g is the tolerance class for the major diameter. A fit between threaded parts is indicated by the nut thread tolerance designation followed by the bolt thread tolerance designation separated by a slash.
6 H is therefore the designation for the tolerance of the taps, with which an average fit between screw and nut is produced. The number stands for the degree of tolerance and the letter for the position of the tolerance field.
M12 – 6H. 12mm internal thread, grade 6, position H (tapped hole/nut) M12 – 6g. 12mm external thread, grade 6, position g (bolt/screw)
M6x1. 0 - 6H is the thread and tolerance to be tapped into the drilled hole - again, through all. With blind holes it's often the case that a minimum thread depth is specified that's less than the hole depth to avoid multiple tapping operations using a bottoming tap.
What is the 3-Thread Rule? The 3-thread rule ensures that a bolt or stud extends at least three full threads beyond the nut to guarantee full engagement. This prevents improper fastening, which could lead to loosening or joint failure.
The number in a screw size generally refers to its diameter, with higher numbers indicating a larger diameter. Therefore, a #8 screw has a larger diameter than a #6 screw.
H and L limits are used for inch size ground thread taps. H (high) represents a pitch diameter that is larger than the basic pitch diameter, and L (low) represents a pitch diameter that is smaller. The number after the H or L, indicates the amount it is different than the basic pitch diameter.
What is thread tolerance 6H and 6g? Thread tolerance 6H is the recommended tolerance for metric internal threads. Thread tolerance 6g is the recommended tolerance for metric external threads.
Tolerance letter designation
Internal threads only use G and H, and external threads use a-h. As seen in Figure 2, H describes tolerance with zero deviation from the basic profile, and G describes tolerance with deviation above the basic profile.
The "M" stands for metric, "8" is the nominal diameter in millimeters, "1.25" is the pitch in millimeters (distance between threads), and "6H" refers to the tolerance class.
H means (high) above basic pitch diameter and L (low) below basic pitch diameter. Tap limits have been established to provide a choice in the selection of the tap size best suited to produce the class of thread desired.
Between classes 6H and 6G, as well as between classes 6G and 7G, tap manufacturers produce taps with tolerance 6HX and 6GX. These taps are used for tapping abrasive materials, such as cast iron or Al-Si alloys, in order to increase their tool life.
6G will be significantly more energy-efficient, turning off components and scaling down capacity when the demand is lower. Energy efficiency will be a major design criterion in 6G along with the other metrics such as capacity, peak data rate, latency, and reliability.
Size for size, a fine thread is stronger than a coarse thread. This is both in tension (because of the larger stress area) and shear (because of their larger minor diameter). Because of the smaller pitch, they allow finer adjustments in applications that need such a feature.
6g Screws Features:
Wide range of screws for timber, steel, plasterboard, roofing, decking, and general applications. Available in bright zinc, gold zinc, galvanised, ceramic coated, and stainless steel 304/316 finishes. Multiple head types and point styles for secure, material-specific fastening.