How to Read the Tolerance Band Code

A metric thread tolerance band code has three parts: a number, an uppercase or lowercase letter, and sometimes another number. The number is the tolerance grade (from 4 for fine to 8 for coarse). The letter is the fundamental deviation: lowercase letters (g, h, k, js) apply to external threads (bolts, screws), and uppercase letters (G, H, K, JS) apply to internal threads (nuts, tapped holes). The letter g or h means an external thread with a negative or zero fundamental deviation, sitting below the basic profile. The letter H means an internal thread with a zero fundamental deviation, sitting on the basic profile. This metric thread tolerance-band system is defined by GB/T 197 (general tolerances for screw threads).

The most common default band is 6g for external threads and 6H for internal threads. This is the general-purpose medium-precision fit used for the vast majority of bolts and nuts. The grade 6 corresponds to a reasonable manufacturing tolerance that standard threading dies and taps can achieve without excessive cost.

Where 6g / 6H / 6h Are Used

The practical difference between 6g and 6h is small but real. A 6g bolt has a slightly smaller pitch diameter to leave room for plating: after zinc plating, the thread grows by a few micrometers, and the 6g starting size ensures the plated bolt still fits a 6H nut. A 6h bolt is used unplated, where precision fit matters more than plating clearance. This is why plated bolts are almost always specified 6g rather than 6h.

Choosing Common Thread Fits

The most common fit classes are 6g/6H (general purpose), 5g/6H (fine precision), and 6g/5H (tight). For standard structural fasteners, 6g/6H is the default and matches commercial bolts and nuts. For precision adjustment applications such as micrometer screws or instrument leads, a finer grade (4h or 5h) may be specified. For soft materials that need extra clearance, a 6G internal thread may be used.

When threads are coated or plated, the fit must account for coating thickness. A zinc plate of 8-12 µm adds roughly 8-12 µm to the thread profile on each flank, so the external thread must be made oversize before plating (or under-cut, i.e., 6g instead of 6h) to ensure the plated thread still assembles. This is a common source of assembly trouble: a 6h bolt that is zinc-plated without adjusting the pre-plate size will not screw into a 6H nut.

GO/NO-GO Gauge Inspection

Thread inspection uses two gauges. The GO gauge checks the pitch diameter and must screw in freely by hand. The NO-GO gauge checks that the pitch diameter is not too far out of tolerance and must NOT screw in (or screw in no more than two turns). For external threads, you use a GO ring gauge and a NO-GO ring gauge. For internal threads, you use a GO plug gauge and a NO-GO plug gauge.

A fastener that passes GO but fails NO-GO has a pitch diameter outside the tolerance band. A fastener that fails GO is oversize on an external thread (or undersize on an internal thread) and will not assemble. This simple go/no-go check catches the vast majority of thread manufacturing errors and is the standard incoming inspection for fastener lots. The plug and ring gauges themselves are manufactured and calibrated in accordance with GB/T 3934 (gauges for general metric screw threads).

What Happens When You Pick the Wrong Band

If you specify 6h on a bolt that will be zinc-plated, the plated thread may be too large to assemble into a standard 6H nut, resulting in hard assembly or cross-threading. If you specify 6G on a standard nut that should be 6H, the thread is slightly oversized, the fit is loose, and the nut may rattle or strip more easily. If you mix a 6g bolt with a 5H nut (too tight), assembly is difficult and the threads gall easily. If you mix a 6g bolt with a 7H nut (too loose), the joint is sloppy and fatigue life drops.

Designing for manufacturability. When specifying thread tolerance, remember that 6g/6H is the standard that commercial fasteners are made to. If you specify 6h or a finer grade, you are moving into specialty territory with higher cost and longer lead time. Reserve the finer grades for actual precision fits. When threads will be plated, specify 6g for external threads so the plating thickness does not push the thread over the limit. When in doubt about which band a commercial bolt uses, check the manufacturer's datasheet; most stock bolts are 6g/6H by default.

Thread fit in assembly practice. When a bolt does not thread into a nut smoothly, the cause is often a tolerance issue rather than a damaged thread. If a 6g bolt will not enter a 6H nut, check whether the bolt was plated without adjusting the pre-plate size - the plating may have added thickness that pushes the thread over the upper limit. If a bolt threads in loosely and the joint feels sloppy, the tolerance band may be too wide for the application, and a closer fit should be specified. When in doubt, measure the pitch diameter with a thread micrometer or compare the bolt against a known-good nut. Most thread fit problems disappear once you stop blaming the operator and start measuring the actual tolerance.

Thread gauge and inspection practice. Production incoming inspection of fasteners should include GO/NO-GO thread gauges for every lot. The GO gauge must screw in freely; the NO-GO gauge must not screw in more than two turns. For precision applications, also measure the pitch diameter with a thread micrometer. Keep gauges calibrated: a worn gauge gives false accept or false reject. When a thread fit problem appears on the assembly line, check the gauges first before blaming the fastener supplier. Most thread complaints trace back to a worn gauge, a dirty thread, or a plating thickness issue rather than a manufacturing defect.

Designing for manufacturability. When specifying thread tolerance, remember that 6g/6H is the standard that commercial fasteners are made to. If you specify 6h or a finer grade, you are moving into specialty territory with higher cost and longer lead time. Reserve the finer grades for actual precision fits. When threads will be plated, specify 6g for external threads so the plating thickness does not push the thread over the limit. When in doubt about which band a commercial bolt uses, check the manufacturer's datasheet; most stock bolts are 6g/6H by default.

Thread fit in assembly practice. When a bolt does not thread into a nut smoothly, the cause is often a tolerance issue rather than a damaged thread. If a 6g bolt will not enter a 6H nut, check whether the bolt was plated without adjusting the pre-plate size - the plating may have added thickness that pushes the thread over the upper limit. If a bolt threads in loosely and the joint feels sloppy, the tolerance band may be too wide for the application, and a closer fit should be specified. When in doubt, measure the pitch diameter with a thread micrometer or compare the bolt against a known-good nut. Most thread fit problems disappear once you stop blaming the operator and start measuring the actual tolerance.

Thread gauge and inspection practice. Production incoming inspection of fasteners should include GO/NO-GO thread gauges for every lot. The GO gauge must screw in freely; the NO-GO gauge must not screw in more than two turns. For precision applications, also measure the pitch diameter with a thread micrometer. Keep gauges calibrated: a worn gauge gives false accept or false reject. When a thread fit problem appears on the assembly line, check the gauges first before blaming the fastener supplier. Most thread complaints trace back to a worn gauge, a dirty thread, or a plating thickness issue rather than a manufacturing defect.