How to Specify Threads on Drawings So the Right Gauges Get Ordered

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Why the Callout Decides Which Gauge Is Ordered

Thread gauges are made to one designation and one class. A GO/NO-GO plug for M10x1.5-6H cannot inspect M10x1.25-6H, and a 2B plug is the wrong gauge for a 3B thread. When a drawing says only “M10” or “1/4 UNC”, purchasing has to guess, and a wrong guess means either wrong gauges or parts accepted to the wrong limits.

A complete callout lets the buyer, the machinist, the inspector and the gauge maker all read the same requirement. It also prevents the most common reason for returned gauges: a gauge made exactly to an incomplete order.

Think of the callout as a purchase specification for three things at once: the cutting tool, the inspection method and the gauge. If a tap can be selected from the callout but a gauge cannot, the callout is incomplete. The sections below list what each thread system needs.

Metric Thread Callouts (ISO 965 / ISO 68-1)

ISO metric threads are designated by the letter M, nominal diameter, pitch and tolerance class. The pitch of coarse threads is often omitted, but writing it out removes any doubt.

For internal threads with tolerance position H, the GO plug is the same for 4H, 5H, 6H and 7H, because the fundamental deviation is zero; only the NO-GO plug differs. For position G the GO plug is different. So the class on the drawing matters most for the NO-GO gauge, and a missing class is not a small detail. More in our guide to thread tolerance classes 4H to 6e.

Unified Inch Thread Callouts (ASME B1.1)

Unified threads are designated by nominal size, threads per inch, series and class, with A for external and B for internal threads.

Class 2A/2B is the general-purpose fit; 3A/3B is tighter. The choice changes both GO and NOT GO gauges for external threads, because class 2A has an allowance and 3A does not. See thread class 2B vs 3B gauge selection.

Pipe Thread Callouts: NPT, NPTF, BSPP and BSPT

Pipe threads are the area where incomplete callouts cause the most trouble, because similar-looking designations need completely different gauges.

Never write just “1/2 BSP” or “1/2 pipe”. G (ISO 228) and Rp (ISO 7) are both parallel internal threads but belong to different standards with different purposes; R and Rc are taper. For external G threads, ISO 228 has two tolerance classes, A and B, which should appear in the callout. Our NPT vs NPTF vs BSPP vs BSPT guide explains the gauges for each.

Thread Length, Depth and Blind Holes

The designation says what the thread is; the length or depth says how much of it must be good. Gauging depends on both.

• Blind holes: give the minimum full-thread depth and the drill depth separately, e.g. “M8x1.25-6H, full thread 16 min, drill 20”. The GO plug must reach the full-thread depth.
• Through holes: state that the thread is through, so the inspector knows the GO plug must pass the whole length.
• External threads: give the minimum full-thread length, and whether a runout or undercut is allowed.
• Deep threads: if the full-thread depth exceeds the length of a standard GO plug, the gauge must be made longer; say so in the RFQ.

Our guide to thread gauges for blind holes covers depth marking and clearance.

Left-Hand, Multi-Start and Special Threads

Anything that is not a single-start right-hand thread must be stated explicitly. A gauge maker will assume right-hand single-start if the callout is silent.

• Left-hand: add LH to the designation, for example M12x1.5-LH-6H or 1/2-13 UNC-2B-LH. Left-hand gauges are made to order; see left-hand thread gauges.
• Multi-start: give lead and pitch, and the number of starts.
• Trapezoidal and Acme: give size, pitch or TPI and class, for example Tr20x4-7H or 1/2-10 ACME-2G.
• Buttress (ASME B1.9) and special forms: reference the standard or provide full profile dimensions.

Coating, Plating and the Gauging System

Two notes on the drawing prevent most gauge disputes: when the thread dimensions apply, and how the thread is to be gauged.

ASME B1.3 defines gauging systems 21, 22 and 23 for unified inch threads. System 21 relies on GO and NOT GO gauges; systems 22 and 23 add measurement of further thread elements. If the drawing specifies a system beyond 21, make sure receiving inspection has the equipment to meet it. For plating, see thread gauges before or after plating.

Common Callout Mistakes and Their Consequences

Each of these gaps can be closed by the design engineer in a few minutes. Left open, they cost days of clarification between purchasing and the gauge maker, or result in gauges that do not match the real requirement.

A simple check helps: before a drawing is released, ask whether an inspector who has never seen the part could pick the correct GO and NO-GO gauges from the callout alone. If not, the callout needs more detail.

What to Send When Ordering Gauges From a Drawing

We make standard thread plug gauges and ring gauges from stock, and special gauges from your drawing. To quote correctly, send:

• The part drawing, or the full thread callouts copied from it
• Tolerance class for every thread, and LH where applicable
• Full-thread depth or length, especially for deep blind holes
• Whether gauging is before or after coating
• Gauging standard or system required by your customer
• Quantity, plug or ring, and certificate requirement (ISO/IEC 17025 available on request)

If anything is missing or unclear, we will ask before quoting, and we confirm the gauge design with you before manufacture.

Frequently Asked Questions