Choosing a metric thread is not only a question of nominal diameter and pitch. The thread class fit determines how an internal thread and an external thread assemble, how much clearance is available for coating or contamination, and how reliably the joint can be inspected. For industrial fasteners, flanges, fittings and machined components, a clear thread designation prevents costly mismatches on the shop floor.
This practical guide explains metric thread tolerances, ISO 965-style class notation, common 6H/6g pairings and the questions buyers should answer before releasing a drawing or purchase order. It is a technical overview; the current project standard, engineering drawing and approved inspection plan always govern the final requirement.
Metric thread classification: what the designation controls
A metric thread is commonly identified by its nominal diameter and pitch, such as M16 × 2. That tells the manufacturer the basic thread form and pitch, but it does not completely define the permitted dimensional variation. The metric thread class adds a tolerance grade and a fundamental deviation so that mating parts have a defined functional relationship.
In ISO metric practice, the number normally indicates the tolerance grade, while the letter indicates the tolerance position. Uppercase letters identify internal threads; lowercase letters identify external threads. The class applies to thread diameters, especially the pitch diameter that has the greatest influence on assembly. It should not be treated as a generic machining tolerance pasted onto every dimension.
| Callout element | Example | What it communicates |
|---|---|---|
| Nominal designation | M16 | Basic major diameter of 16 mm in the metric thread system. |
| Pitch | × 2 | Distance between adjacent thread forms; it must match the mating component. |
| Internal class | 6H | Thread-hole tolerance grade and position, written with an uppercase letter. |
| External class | 6g | Bolt or stud tolerance grade and position, written with a lowercase letter. |
| Complete metric thread callout | M16 × 2 – 6H/6g | One defined internal/external pairing; use the drawing to state which part receives which class. |
How metric thread classes create a fit
Thread standards establish a basic profile and then allow controlled limits around it. A smaller grade number generally represents a tighter tolerance band than a larger grade number, but grade alone is not the whole story. The letter shifts the tolerance zone in relation to the basic profile. Together, the number and letter create the thread class of fit.
A widely specified production pairing is internal 6H with external 6g. It provides an assembly-friendly relationship for many uncoated commercial fasteners and machined parts, subject to the governing component standard. The H position is commonly used as an internal reference position, while an external g position provides an allowance that supports practical interchangeability. The exact limits depend on nominal size and pitch and must be taken from the applicable standard table—not guessed from another diameter.
| Class feature | Internal thread example | External thread example | Practical effect |
|---|---|---|---|
| Letter case | H, G and other uppercase positions | h, g, e and other lowercase positions | Confirms whether the callout belongs to the tapped hole or mating screw, bolt or stud. |
| Tolerance grade | 6H | 6g | The number defines the width of the permitted zone for applicable thread diameters. |
| Fundamental deviation | H is a common internal reference position | g commonly includes practical assembly allowance | Controls where the tolerance zone sits in relation to the basic profile. |
| 6h thread tolerance | Not an internal callout—the lowercase h is external | 6h places an external zone at the basic position | May be selected only when the specification supports the lower allowance; coating and assembly conditions must be reviewed. |
6H, 6g and 6h: do not confuse the letters
Capitalization is functional, not cosmetic. 6H identifies an internal-thread class. 6g and 6h identify external-thread classes. A drawing that says “M12-6H” for a threaded hole is communicating something different from a bolt drawing marked “M12-6h.”
The term 6h thread tolerance is often searched when an external thread needs minimal deviation from the basic profile. It should not automatically replace 6g. Finish condition, thread rolling or cutting method, plating thickness, service temperature and mating-part tolerance all influence whether the specified fit remains functional. For coated fasteners, the drawing should state the coating sequence and the required post-coating gauging method rather than relying on an assumed class conversion.
What is the tolerance for thread holes?
What is the tolerance for thread holes? The useful answer is that a tapped hole is accepted as a thread, not simply as a drilled diameter with a single plus/minus value. The drill size is a manufacturing input; the required internal thread class, pitch, engagement length, material and inspection method define the functional result.
For example, a drawing may require M10 × 1.5–6H. The machinist selects an appropriate tapping process and pre-drill strategy for the material and thread-forming method, then verifies the completed hole with the specified gauges. A GO gauge must enter as required, while a NO-GO gauge must not accept the thread beyond the permitted condition. This approach evaluates pitch diameter and form far more effectively than checking only the drilled hole.
| Thread-hole question | Correct control | Why it matters |
|---|---|---|
| Which drill diameter should be used? | Follow the approved process data for size, pitch, material and cutting or forming method. | A drill chart alone does not establish the finished thread class. |
| How is the thread tolerance verified? | Use the specified GO/NO-GO plug gauges and any drawing-specific checks. | Functional gauging checks the finished internal thread rather than one preliminary feature. |
| Does coating affect a threaded hole? | State coating sequence, thickness allowance and final acceptance condition. | Deposits can change fit and reduce usable engagement. |
| Can a pitch change be substituted? | No; retain the exact metric designation and mating component requirements. | Similar nominal diameters with different pitches will not assemble correctly. |
Selecting a thread fit class for real service
Good metric thread classification begins with the function of the joint. General-purpose bolts in clean, uncoated conditions can often use the standard class called out by the applicable fastener specification. Precision assemblies may need controlled runout, engagement or preload behavior, but a tighter class is not automatically better: insufficient assembly allowance can make a joint difficult to build or sensitive to minor damage.
- General mechanical assembly: use the internal and external pairing stated by the relevant fastener or component standard, often a 6H/6g-style arrangement where appropriate.
- Coated or galvanized components: define the finish process first, then establish thread allowances and final gaging requirements with the coating supplier and engineer.
- Corrosive process equipment: consider material compatibility, lubricants, galling risk and required engagement together with the thread fit class.
- Repeated maintenance joints: account for field contamination, handling damage and the ability to inspect replacement parts.
- High-consequence service: include the governing standard edition, material certification, heat treatment and acceptance plan in the purchase documentation.
Inspection points that protect thread tolerance
Thread tolerances can be lost after machining. Burrs at the entry, damaged lead threads, poor surface finish, rolled-thread distortion, incomplete cleaning and uncontrolled plating may all affect assembly. An effective inspection plan combines functional gauges with appropriate visual and dimensional controls.
For a purchased bolt or stud, verify the callout, pitch, material marking, coating condition and gauge acceptance. For a threaded hole, verify the internal designation, depth and usable engagement in addition to plug-gauge results. If a drawing includes a special thread tolerance, clarify whether it controls the full thread length, a specific gauging plane, or a localized fit requirement before production begins.
Procurement checklist for a metric thread standard
| Item to state on the order or drawing | Example of clear information |
|---|---|
| Thread standard metric designation | M16 × 2, including right- or left-hand requirement where relevant. |
| Thread location | Internal tapped hole, external bolt, stud or threaded fitting end. |
| Metric thread classes | 6H internal and/or 6g external, with the applicable standard identified. |
| Material and process | Specified alloy, heat treatment, thread cutting or rolling method and required certificates. |
| Coating condition | Finish type, sequence, thickness expectation and final thread acceptance after coating. |
| Inspection | GO/NO-GO gauges, sampling level, traceability and any customer witness requirements. |
Frequently asked questions
Are metric thread tolerances the same for every diameter?
No. The class format is consistent, but the numerical limits vary by diameter and pitch. Use the applicable ISO 965 tables or the project-controlled standard for the exact size.
What is a thread fit class?
A thread fit class combines the tolerance grade and tolerance position used for internal or external threads. The selected mating classes determine functional assembly clearance and inspection limits.
Can I specify only “metric thread” on a purchase order?
That is usually incomplete. State nominal size, pitch, internal or external location, the required class, applicable standard, material, finish and inspection expectations. A complete metric thread callout reduces interpretation risk.
Final takeaway
Metric thread tolerances are a system for making threaded parts interchangeable without ignoring real manufacturing and service conditions. Define the nominal thread, pitch, internal or external class, coating sequence and functional inspection method as one package. When the application involves pressure equipment, industrial flanges or custom fasteners, Songhai can review the thread specification and component interface before production.