6G vs 6H Thread Tolerance: Differences, Gauges and Ordering Guide

6G vs 6H is a common question because the two symbols often appear on mating metric threaded parts. They are related through the ISO metric thread tolerance system, but they do not identify the same type of workpiece or the same gauge.

Need gauges for this? GageTools makes 6H and 6g metric thread plug and ring gauges with MOQ 1 piece: plug gauges from USD 20, ring gauges from USD 30, and standard stock sizes ship within 72 hours. Send your thread size for a quote.

6G vs 6H at a Glance

What the Letter and Number Mean

In a metric thread tolerance callout, the letter identifies the fundamental deviation and the number identifies the tolerance grade. Uppercase letters are used for internal threads; lowercase letters are used for external threads. That is why 6H is normally seen after an internal-thread designation, while 6g is normally seen after an external-thread designation.

The common pairing M10 × 1.5-6H/6g describes a functional fit between an internal 6H thread and an external 6g thread. The complete drawing, applicable standard, coating condition and inspection plan still govern the correct acceptance limits.

Which Thread Gauge Should You Order?

• For an internal 6H thread: order a GO/NO-GO thread plug gauge matching the complete size and pitch, for example M10 × 1.5-6H.
• For an external 6g thread: order a GO/NO-GO thread ring gauge matching the complete size and pitch, for example M10 × 1.5-6g.
• For coated parts: state whether inspection is before or after plating or coating. Any allowance must be defined by the drawing or approved specification.
• For special requirements: include the governing standard, gauge class, quantity, certificate requirement and shipping country in the RFQ.

GO and NO-GO Acceptance for Straight Metric Threads

For a standard straight metric thread, the GO member checks the functional thread condition across the usable threaded length under the applicable inspection procedure. The NO-GO member checks the opposite limit and should not engage beyond the permitted amount. Follow the governing standard and buyer inspection plan for the correct acceptance method.

Ordering Checklist for 6H or 6g Thread Gauges

• Complete thread callout: size, pitch and tolerance class
• Internal or external workpiece thread
• GO only, NO-GO only or complete set
• Working gauge or master gauge requirement
• Inspection condition before or after surface treatment
• Required documentation, serial marking and quantity

What Does 6H Mean in a Thread? Reading M6-6H and M8x1.25-6H

In an ISO metric designation, 6H is the tolerance class of an internal thread. The number 6 is the tolerance grade, which sets how wide the tolerance band is. The uppercase letter H is the tolerance position, which sets where that band starts relative to basic size. For position H the fundamental deviation is zero, so the smallest permitted pitch diameter of the nut or tapped hole equals the basic pitch diameter.

M6-6H therefore means: ISO metric thread, 6 mm nominal diameter, coarse pitch (1.0 mm, omitted because it is the standard coarse pitch), internal thread, tolerance class 6H. M8x1.25-6H is the same idea with the pitch written out. Fine pitches must always be stated, for example M8x1-6H.

How to read a full designation

1. M8x1.25 – thread profile, nominal diameter and pitch.
2. 5H6H or 6H – the first class applies to the pitch diameter, the second to the crest diameter (minor diameter for internal threads, major diameter for external threads). When both are the same, it is written once: 6H rather than 6H6H.
3. 6H/6g – a slash shows a fit: internal class first, external class second. M8x1.25-6H/6g is the standard medium-quality nut-and-bolt combination.
4. S, N or L – length of thread engagement (short, normal, long). N is assumed when no letter is given.
5. LH – left-hand thread. Right-hand is assumed when nothing is written.

Uppercase letters (G, H) always describe internal threads; lowercase letters (e, f, g, h) always describe external threads. If a drawing calls out a single class with no fit, the letter case tells you whether you are dealing with a hole or a bolt, and therefore whether you need a plug gauge or a ring gauge.

6H vs 5H vs 7H: Same GO Plug, Different NO-GO

Buyers often ask whether a 6H plug gauge can be used on a part toleranced 5H or 7H. The answer depends on which member you mean. All H classes share the same lower pitch diameter limit, because the fundamental deviation of position H is zero. The GO member checks that lower limit, so for H-position internal metric threads the GO plug is identical for 4H, 5H, 6H and 7H. Only the NO-GO member changes, because it checks the upper pitch diameter limit, and that limit moves with the tolerance grade.

In practice this means a plant that already owns an M8x1.25 6H GO plug only needs an extra NO-GO member to inspect 7H holes. It does not mean a 6H NO-GO can stand in for 5H or 7H: a 6H NO-GO used on 7H parts will reject good holes, and used on 5H parts it will pass holes that are too large. Specify the exact class on the order so the NO-GO is ground to the right limit.

6g vs 6h vs 6H vs 6G: Why the Letter Case Matters

The query “6g vs 6h” usually mixes up two different things. Lowercase 6h is an external thread class with zero allowance: the largest permitted bolt pitch diameter equals basic size. Uppercase 6H is an internal class. A 6h bolt and a 6H nut can mate, but with no clearance at the maximum-material condition, which leaves no room for plating or coating build-up.

Position g has a negative fundamental deviation. Under ISO 965 the upper deviation of position g is es = −(15 + 11P) µm, where P is the pitch in mm. For M6 (P = 1) that is −26 µm; for M8x1.25 it is about −28 µm. That small gap is the allowance that makes 6H/6g the default fit. Position G mirrors it on the internal side, with EI = +(15 + 11P) µm.

The number 6 is the same grade in all four rows, but the tolerance band widths for internal and external threads are not identical, so the grade alone does not tell you the band size. The letter tells you where the band sits; the case tells you whether it applies to a hole or a bolt.

Thread Plug Gauge g vs H: Which Letter Applies to a Plug?

Searches for “thread plug gauge g h difference” often assume a plug gauge can be 6g or 6H. It cannot be 6g. A plug gauge checks an internal thread, so its class always carries an uppercase letter: 6H, 6G, 7H and so on. Lowercase classes such as 6g and 6h are checked with ring gauges. If a drawing or RFQ asks for an “M10 6g plug gauge”, it is almost always a nut or tapped hole toleranced 6H, or a typing error for 6G, and it should be confirmed before the gauge is made.

6H plug vs 6G plug

• 6H GO plug – pitch diameter at the basic size of the internal thread (EI = 0).
• 6G GO plug – pitch diameter shifted up by the fundamental deviation, EI = +(15 + 11P) µm. For an M6 thread that is 26 µm larger than the 6H GO plug.
• NO-GO plugs – each sits at the upper pitch diameter limit of its class, so 6G NO-GO is also larger than 6H NO-GO.

The practical consequence: a 6H GO plug used on a 6G hole will still enter and accept the part, but it does not prove the allowance is there. A 6G GO plug used on a 6H hole can reject holes that meet the drawing. Order the class the drawing states, and mark both members with the full designation so they are not mixed on the shop floor.

Which Gauges to Order for a 6H/6g Thread Pair

A 6H/6g fit covers two parts and needs two gauge types. For each thread size, a complete inspection set looks like this:

Solid ring gauges are common for ISO metric threads, and a setting plug lets your lab or a calibration provider confirm the rings without relying only on the original certificate. If your parts are plated after threading, state whether gauging happens before or after plating, because that decides whether the gauge should be 6g or a class with more allowance.

What to send with an RFQ

• Full designation, for example M8x1.25-6H/6g, including LH if left-hand.
• Which parts you inspect: nut only, bolt only, or both.
• Quantity per size and whether you need an ISO/IEC 17025 calibration certificate.

Standard metric sizes ship from stock within 72 hours, custom classes take 10–15 working days, and MOQ is 1 piece. For other classes such as 4H, 7H or 6e, see the tolerance class guide, or request a quote and we reply within 24 hours.

Frequently Asked Questions

Is 6H internal or external?

6H is an uppercase tolerance designation normally used for an internal metric thread, such as a threaded hole or nut.

Is 6g internal or external?

6g is a lowercase tolerance designation normally used for an external metric thread, such as a bolt, stud or threaded shaft.

Can I use a 6H plug gauge for a 6g bolt?

No. A 6H plug gauge is for internal threads. An external 6g thread is normally inspected using the applicable ring gauge system.

What does M6-6H mean on a drawing?

M6-6H is an internal ISO metric thread with a 6 mm nominal diameter, standard 1.0 mm coarse pitch and tolerance class 6H. Grade 6 sets a medium tolerance band and position H means zero fundamental deviation, so the minimum pitch diameter equals basic size. It is inspected with a GO/NO-GO thread plug gauge.

Can I use a 6H thread plug gauge for a 7H tapped hole?

Only the GO end. For H-position internal metric threads the GO plug is the same for 4H to 7H because all share the basic lower limit. The 7H NO-GO is larger than the 6H NO-GO, so using a 6H NO-GO on 7H holes will reject parts that are within tolerance. Order a 7H NO-GO member.

Is 6h the same as 6H?

No. Lowercase 6h is an external thread class with zero allowance, checked with a ring gauge. Uppercase 6H is an internal thread class, checked with a plug gauge. Both have zero fundamental deviation, but they apply to opposite parts of the fit and use different tolerance band widths, so the gauges are not interchangeable.

Why is 6g used for bolts instead of 6h?

Position g leaves a small allowance below basic size, es = −(15 + 11P) µm, so a 6g bolt always has clearance in a 6H nut. That clearance helps assembly and leaves room for thin coatings. Position 6h has no allowance, so a bolt at maximum material and a nut at minimum material fit with zero clearance.

Additional Buyer Notes

• How 6H/6g Affects Gauge Selection: If you need to inspect an M12 x 1.75 – 6H tapped hole, you need an M12 x 1.75 – 6H thread plug gauge.

Related Buyer Guides

• Thread Gauge Tolerance Classes Explained: 4H, 6H, 7H, 6g, 6e and When to Use Each
• What Is a 6H Thread Tolerance Class?
• Thread Gauges Before or After Plating: A Buyer’s Guide

Have a drawing or thread callout ready? Request a thread gauge quote and our engineering team will review your specification.

Working with pipe threads? See our BSP thread gauge guide: BSPP vs BSPT, G and R threads and which gauges to use.