A cable gland nut can appear to fit yet bind after a few turns or fail to clamp the gland securely against an enclosure. The correct match depends on the gland’s mounting thread: its standard, nominal size, pitch, and whether it is parallel or tapered. Diameter alone is not enough.

Before selecting a round nut for cable gland installation, identify the enclosure-entry thread rather than the thread beneath the cable compression cap. Then check that enough usable thread remains through the enclosure wall and any washers or seals. A nut that screws onto a loose gland can still be unsuitable for the installed assembly.

Identify the thread that actually passes through the enclosure

A cable gland may have more than one threaded section. The mounting thread is the external thread on the entry end that passes through the enclosure wall or screws into a threaded entry. The compression cap at the cable end performs a different job: it compresses the gland’s sealing components around the cable. Its thread is not a reliable reference for choosing a mounting locknut.

Check the gland’s part number, packaging, or dimensional drawing for an entry-thread designation such as M20 × 1.5, PG13.5, or 1/2–14 NPT. A cable clamping range such as 6–12 mm describes the cable diameter the gland accepts, not the mounting-thread size.

Also establish whether a locknut is required. In a plain clearance hole, the gland shoulder and an internal nut generally provide the clamping arrangement. A threaded enclosure entry may retain the gland directly. Adding a nut does not correct a mismatched threaded entry, and some installations require a specific assembly arrangement.

Read the full designation, not just the size number

Thread families can be close enough in diameter to start together without being compatible. Their pitch, thread profile, or taper can differ. That initial engagement is not proof of a match.

The dimensions above are identification references, not acceptance limits. Manufacturing tolerances, coatings, wear, and—in tapered threads—the measurement position affect caliper readings.

Metric and PG threads also differ in profile: ISO metric threads use a 60-degree included angle, while PG threads use an 80-degree profile. BSP threads use a 55-degree profile, while NPT uses 60 degrees. Matching diameter and an apparently similar spacing therefore does not establish compatibility.

For metric glands, never assume that a nut marked only “M20” has the required pitch. General-purpose M20 nuts commonly use a 2.5 mm coarse pitch, whereas M20 × 1.5 is used on many cable-gland entries. Both share the same nominal diameter, but they cannot be substituted.

Measure an unmarked gland without treating measurements as proof

When documentation is unavailable, clean the mounting thread and inspect it under good lighting. Remove loose dirt without cutting or reshaping the thread. Look for flattened crests, burrs, damaged starting threads, or coating buildup before attempting identification.

Use a caliper to measure across the external thread crests, keeping the jaws square to the gland axis. Take readings at several positions. On a parallel thread, the diameter should remain broadly consistent along the threaded section. A systematic diameter change can indicate taper, although a short entry thread may be difficult to assess accurately with calipers.

Measure pitch with a thread-pitch gauge. The correct leaf should sit against several consecutive threads without visible gaps or rocking. A metric gauge reads spacing in millimetres; an imperial gauge reads threads per inch. Their relationship is:

Pitch in millimetres = 25.4 ÷ threads per inch.

For example, 18 threads per inch corresponds to approximately 1.411 mm pitch, not 1.5 mm. That small difference explains why a PG thread and a nearby metric size may begin to engage but then bind.

If no gauge is available, measure across several thread intervals and divide by the number of intervals. Measure from one crest to another and count the spaces, not the crests: eleven crests span ten intervals. This is only a screening method, especially on short or fine threads. If the result falls between plausible standards, obtain the manufacturer’s drawing or have the thread checked with appropriate gauges rather than choosing the nearest-looking nut.

Check engagement with the actual enclosure stack

A correct thread match does not guarantee sufficient holding strength. The nut must engage enough complete threads after the gland passes through the enclosure wall, sealing washer, and any specified accessory.

Compare the gland’s usable mounting-thread length with the complete installed stack. Allow for the enclosure thickness, external sealing components, internal washers or bonding accessories, and the engagement required by the nut. The tapered starting portion of a thread and the incomplete threads near a shoulder do not provide the same engagement as fully formed threads.

Consider a gland with a nominal 10 mm entry length installed through a 6 mm wall with a 2 mm sealing washer. Only about 2 mm remains geometrically before allowing for thread lead-in and other details. A compatible nut might catch, but that does not establish adequate engagement. The acceptable arrangement must come from the gland’s installation instructions or a suitable engineering assessment.

There is no universal rule that a certain number of turns is sufficient for every gland. Thread diameter, pitch, nut thickness, material, and applied loads all affect the result. Metal and plastic assemblies should not be assigned the same engagement or torque requirements by assumption.

When engagement is insufficient, a longer-entry gland or an approved change to the mounting arrangement is the appropriate direction. Removing a required sealing washer to gain thread length can sacrifice the intended enclosure seal.

A round shape does not define the nut’s function

“Round nut” describes external form, not thread compatibility or installation suitability. A round nut may have slots, notches, or knurling for a particular tool. Its bore must still match the gland’s thread, and its face must bear correctly against the enclosure or specified washer.

Check the nut’s outside diameter against the clearance available inside the enclosure. Nearby terminals, ribs, adjacent glands, or a recessed mounting surface can prevent the nut from seating flat or leave insufficient room for the tightening tool. A nut that can be positioned by hand but cannot be tightened correctly is not a workable selection.

General fastener categories require the same caution. For example, ISO8673 Hex Nuts are fine-pitch metric hex nuts, but that description alone does not establish suitability for a cable gland. The exact diameter and pitch, nut dimensions, material, and gland installation requirements still need to agree. Neither a hexagonal nor a round exterior proves that a nut is the correct mounting component.

Material choice also matters. A metal nut should not automatically replace a plastic gland’s specified locknut: excessive tightening can deform or strip the gland thread. In metal assemblies, assess corrosion compatibility and environmental exposure. Where electrical bonding is required, an ordinary nut is not automatically a substitute for a specified earth tag, bonding washer, or other approved component.

Confirm the fit by hand before applying torque

Make the initial compatibility check with the gland accessible and the thread clean. Offer the nut squarely to the mounting thread and turn it by hand. For a correctly matched parallel-thread pair, it should advance smoothly along the usable thread without progressively increasing resistance.

Stop if the nut tilts, catches repeatedly at the same point, sheds material, or tightens after only partial engagement. Do not use a spanner to “help it find the thread.” Tool force can make an incompatible pair appear assembled by deforming the thread flanks, particularly on plastic glands or softer metals.

The location and pattern of resistance help identify the problem:

• Resistance at the start: check alignment, the first thread, burrs, and contamination.
• Resistance that builds with each turn: suspect a pitch mismatch or an incompatible thread form.
• Resistance at one repeated position: inspect for local thread damage or coating buildup.
• Smooth movement but no firm clamping: check for insufficient engagement, bottoming, or interference with the gland shoulder.

This free-running check applies to ordinary parallel-thread locknuts. Tapered threads and deliberately locking nut designs behave differently. For a tapered entry, use the nut or mounting arrangement specified for that gland rather than applying parallel-thread expectations.

After confirming compatibility, assemble the gland with the required seals and accessories. Hold the gland body as instructed so tightening the mounting nut does not rotate the assembly or twist the cable. Use the specified tool and torque; the mounting nut and cable compression cap perform separate functions and may have different tightening requirements.

Verify clamping and sealing separately

A fully tightened nut should clamp the enclosure stack rather than merely reach the end of its travel. Check that its bearing face is seated evenly and that it has not stopped against thread runout, a shoulder, or an internal obstruction while leaving the gland loose.

The mounting thread is also not necessarily the sealing interface. On a parallel-thread gland installed through a clearance hole, sealing may depend on an O-ring or sealing washer at the enclosure face. A tight internal nut cannot compensate for a missing seal, a damaged sealing surface, or an unsuitable mounting hole.

For hazardous-area or other certified installations, follow the gland’s applicable instructions and certification conditions, including entry type, wall thickness, seals, and accessories. A nut that physically fits does not by itself establish compliance.

When ordering a replacement, record the complete thread designation, required nut material and dimensions, and the gland part number where available. “Round nut, approximately 20 mm” leaves the crucial distinctions unresolved. A defensible match is one whose thread is identified, whose engagement is sufficient in the installed assembly, and whose tightening arrangement preserves the gland’s retention and sealing functions.