Measuring internal threads requires a combination of tools and techniques because you can’t simply wrap a tape measure around threads hidden inside a hole. You’ll need to identify three things: the thread’s diameter, its pitch (how closely spaced the threads are), and whether the threads fall within acceptable tolerances. The approach you use depends on whether you need a quick identification or a precise quality inspection.
The Three Dimensions That Define a Thread
Every internal thread has three critical diameters. The major diameter is the largest measurement, taken from crest to crest of the threads. The minor diameter is the smallest, measured across the deepest part of the thread grooves. The pitch diameter sits between those two and represents the point where the thread tooth width equals the space between teeth. This is the most important dimension for fit because it determines whether a bolt or screw will engage properly.
You also need the pitch, which is the distance from one thread crest to the next. In metric systems, pitch is expressed in millimeters (such as 1.5 mm). In imperial systems, it’s expressed as threads per inch, or TPI (such as 20 TPI). Getting the pitch wrong by even a small amount means the mating fastener won’t thread in at all.
Measuring Diameter With Calipers
A dial or digital caliper is the most accessible starting tool. To measure an internal thread’s minor diameter, place the inside-measuring jaws (the smaller set at the top of the caliper) into the threaded hole and slide them apart until they contact the roots of the threads. This gives you the minor diameter. Make sure the hole and jaws are clean; dirt or metal chips will throw off your reading and can damage the caliper’s rack and pinion mechanism.
Getting an accurate major diameter reading on internal threads is trickier with calipers alone because the jaws sit in the thread grooves rather than on the crests. For a rough identification, the minor diameter reading combined with the pitch is usually enough to look up the thread size in a reference chart. If you need the pitch diameter specifically, you can calculate it from the minor diameter using this formula: pitch diameter equals minor diameter plus 0.4330 multiplied by the thread pitch. For example, if your minor diameter measures 8.376 mm and the pitch is 1.25 mm, the pitch diameter is 8.376 + (0.4330 × 1.25) = 8.917 mm.
Identifying Pitch With a Thread Gauge
A thread pitch gauge is a fan-shaped set of thin metal leaves, each stamped with a specific pitch or TPI value. Each leaf has teeth cut to match that pitch exactly. To use one on internal threads, select a leaf, press its teeth into the threads inside the hole, and check the fit. A white background behind the gauge helps you spot gaps. If you see light between the gauge teeth and the threads, the pitch doesn’t match. Try the next leaf until you find one where every tooth drops perfectly into every groove with no visible gap.
Thread gauges come in metric and imperial sets, so grab the right one based on what you suspect the thread standard is. If you’re unsure, try both. Metric pitches and imperial TPI values rarely overlap in a way that would fool you once you check the fit carefully.
Using Go/No-Go Thread Plug Gauges
For quality control and machining work, thread plug gauges are the standard method for verifying internal threads. These are precision-ground steel plugs that come in pairs: a “Go” end and a “No-Go” end, usually color-coded or marked.
The Go plug checks the minimum acceptable thread size. It should screw smoothly into the threaded hole by hand and pass through the entire threaded length without resistance. If the Go plug won’t enter, the threads are too tight and the part needs rework or rejection.
The No-Go plug checks the maximum allowable size. It’s designed to engage only a few threads before stopping. If the No-Go plug screws fully into the hole, the threads have been cut too large, which means the part could fail under load or assemble loosely. A properly made internal thread accepts the Go plug fully and rejects the No-Go plug after no more than two or three turns.
Measuring Tapered Pipe Threads
Tapered threads like NPT (National Pipe Taper) use a different approach because the diameter changes along the length of the thread. Instead of a Go/No-Go pair, you use a single L1 plug gauge. This gauge has a step or notch ground into it at a specific distance from the small end.
Thread the L1 plug into the internal pipe thread by hand, without forcing it. When seated, the step should sit flush with the end of the part, within a tolerance of plus or minus one full turn. If the gauge goes in more than one turn past flush, the threads are too large. If it stops more than one turn short of flush, the threads are too small. Each nominal pipe size has a specified L1 engagement length listed in reference tables.
Checking Thread Depth in Blind Holes
A blind hole (one that doesn’t pass all the way through the part) adds another measurement challenge: you need to confirm the threads are deep enough for the fastener to fully engage. The most practical method is a modified Go plug gauge with a depth notch or step added at a specific distance representing the required functional thread depth.
Thread depth is measured from the centerline of the first full thread at the bottom of the hole. When you thread the modified gauge in, the minimum depth notch should reach the face of the part or sit just inside the thread entrance. If a maximum depth is also specified, a second notch should remain above the part’s surface. This doesn’t give you an exact depth measurement in millimeters, but it confirms functionally that a bolt or screw will engage to the designed depth.
Features like drill points, chamfers, and radii at the bottom of blind holes can interfere with the gauge. In those cases, the gauge face is sometimes ground back to within half a pitch of the first full thread to avoid false readings from incomplete threads at the bottom of the hole.
Choosing the Right Approach
If you’re trying to identify a mystery thread so you can buy a matching bolt, a caliper and a thread pitch gauge will get you there in a few minutes. Measure the minor diameter, identify the pitch, and cross-reference both values against a thread chart for your standard (metric ISO, UNC, UNF, or others).
If you’re machining parts or inspecting threads for assembly reliability, Go/No-Go plug gauges are the correct tool. They verify that the thread falls within its tolerance class without requiring you to interpret individual measurements. For metric threads, the most common tolerance class for general-purpose internal threads is 6H. For unified inch threads, Class 2B covers most standard applications.
For the highest precision, some shops create silicone replicas of internal threads by casting flexible material into the hole, letting it cure, and then measuring the resulting external impression with standard tools like micrometers or optical comparators. This is specialized work, but it allows detailed inspection of thread form in situations where direct measurement is impossible.