A 12 mm cast aluminium enclosure, an M20 entry and a standard gland with an 8 mm thread. The gland goes in, the sealing washer sits flat, and the locknut on the inside catches about two turns before it runs out of thread. It feels tight with a spanner, so it passes the visual check. Six months later the entry weeps.

That gap between “it threaded on” and “it is properly clamped” is where most thick-panel sealing failures start. A long thread cable gland exists to close it, but only if the thread length is matched to the actual panel stack rather than guessed from the enclosure’s nominal wall thickness.

This guide gives you the arithmetic, the two different calculations that apply to through-holes and tapped entries, the engagement figures that standards actually require, and the four ways an over-long thread causes its own problems.

Quick Answer: When You Need a Long Thread Cable Gland

You need one when the thread cannot reach through the wall and still leave the locknut fully engaged. In practice that happens in three situations:

The important thing to notice is the third row. Thick wall does not automatically mean longer thread. It means you have to know which of the two joint types you are installing into, because they are calculated differently.

Published catalogue data makes the scale of the problem concrete. In one common European range, standard entry threads run 6 mm on M12, 7 mm on M16, 8 mm on M20 and M25, 9 mm on M32 to M50, and 14 mm on M63. The extended versions of the same range run 12 mm on M12 to M25, 15 mm on M32 to M50, and 18 mm on M63. So on an M20 entry, “standard” gives you 8 mm of thread to pass through a wall, a washer and a locknut. On a 10 mm panel there is nothing left.

The Four Numbers That Decide Thread Length

Selecting a long thread cable gland is not a single measurement against a single wall thickness. It is a stack-up, and every layer in the stack consumes thread.

1. Finished panel thickness. Measure the actual wall at the specific entry point with a caliper, not the drawing’s nominal figure. Powder coating, hot-dip galvanising and paint each add 0.1–0.5 mm per face, and castings vary across a single face. Take three readings around the hole and use the largest.
2. Sealing washer or gasket thickness, uncompressed. A flat sealing washer is typically 1–2 mm; a profiled gasket or O-ring seat can be more. This layer sits between the gland shoulder and the panel face and cannot be compressed to zero.
3. Locknut height. Standard locknuts for M20 run in the 4–6 mm range depending on material and whether it is a serrated or plain type. You need the nut’s full height on the thread, not 70% of it.
4. Minimum engagement beyond the nut. At least two full threads should protrude past the locknut so the nut is fully engaged rather than hanging on a partial thread. Two threads at a 1.5 mm pitch is 3 mm.

Add them and you have the minimum thread length for a through-hole installation:

Required thread length = panel thickness + washer thickness + locknut height + (2 × thread pitch)

A worked case: 10 mm panel, 1.5 mm washer, 5 mm locknut, M20 × 1.5 thread. Required length = 10 + 1.5 + 5 + 3 = 19.5 mm. An 8 mm standard thread is 11.5 mm short. A 12 mm extended thread is still 7.5 mm short. You need a gland with roughly 20 mm of thread, or a change of approach.

That is the point at which a long thread cable gland stops being optional. This is also the calculation most selection guides skip. They compare thread length to wall thickness alone and conclude that a 12 mm extended thread covers a 12 mm wall — ignoring that the locknut and washer have to fit in the same thread length.

Through-Hole vs Tapped Entry: Two Different Calculations

These are genuinely different joints, and mixing up the arithmetic is the second most common reason a long thread cable gland is ordered in the wrong length — after ignoring the locknut.

For a tapped entry the thread length you need is roughly the boss depth you intend to use plus a millimetre or two of margin. More thread than the boss is deep is not a safety factor — it is a leak waiting to happen, because the gland stops turning on the bottom of the hole while the shoulder is still 3 mm off the face.

How Much Thread Engagement Is Actually Required?

This is where published guidance diverges, so it is worth being precise about what each standard does and does not say.

• Flameproof (Ex d) entries, IEC 60079-1 Clause 5.3 and Tables 3–4: a minimum of five fully engaged threads, or 8 mm of axial engagement, whichever is greater. This applies to parallel metric and to tapered NPT entries.
• The M20 trap: at a 1.5 mm pitch, five full threads is 7.5 mm — below the 8 mm alternative. An M20 × 1.5 entry therefore cannot satisfy the rule on thread count alone; it needs 8 mm of engagement. M25 and larger pass on the five-thread count because the same pitch yields more than 8 mm.
• North American practice: NEC 501.4 requires threaded joints in classified locations to be made up with at least five threads fully engaged, with the joint wrench-tight.
• IP-only installations, IEC 60529: the standard defines the test and the rating. It does not specify a thread count. What governs is the gland manufacturer’s installation instruction, the compression of the sealing washer and the applied torque.

Two practical consequences follow. First, on an IP-only job you cannot cite a standard to justify two threads of locknut grip — you are relying on the manufacturer’s instruction, so read it. Second, thread sealant is not a substitute for engagement. PTFE tape or liquid sealant applied to make up for a short thread does not create a certified flame path, and on a through-hole joint it does not compress the washer either.

Worked Examples Across the Common Wall Thicknesses

The examples below use the through-hole formula. Substitute your own washer and locknut dimensions; the structure of the calculation does not change.

1.5 mm painted sheet steel

Standard gland, locknut on the inside. Required length = 1.5 + 1.5 + 5 + 3 = 11 mm. An 8 mm standard thread is short, but the locknut sits inside a thin wall where the panel itself is the only obstacle, and a standard gland plus locknut is the normal, correct answer here. Watch the paint: the sealing washer must sit on bare, flat metal, so mask the entry face before coating or fit a serrated washer to bite through.

6 mm GRP or reinforced plastic

Required length = 6 + 1.5 + 5 + 3 = 15.5 mm. This is the crossover point. A standard 8 mm thread leaves the locknut hanging; a 12 mm extended thread gives you 4.5 mm less than the calculation wants but works if you use a low-profile locknut. The clean answer is a 15–16 mm thread, or a standard gland with a mounting plate that brings the entry face forward.

12 mm cast aluminium enclosure

Required length = 12 + 1.5 + 5 + 3 = 21.5 mm. This is squarely long-thread territory. Note that cast walls are not uniform — measure at the entry, because a boss may be 12 mm while the surrounding wall is 8 mm, or the reverse.

50 mm insulated sandwich panel

Required length = 50 + 1.5 + 5 + 3 = 59.5 mm. No standard catalogue gland reaches this, and a custom thread of 60 mm on an M20 body is a poor engineering answer: the gland body would sit 60 mm off the wall, creating a long lever arm on a foam-cored panel that has almost no local stiffness. The right solution is a gland plate or a short through-bulkhead nipple that bridges the panel and gives the gland a rigid, flat face to seal against. Treat the sandwich panel as an enclosure problem, not a gland problem.

Wall Thickness to Gland Specification

Use this cable gland panel thickness table as a starting point for through-hole installations, then confirm against the actual drawing. The thread length column assumes a standard 5 mm locknut, a 1.5 mm washer and two protruding threads.

Thread pitch and hole size matter as much as length. Under IEC 60423 the metric conduit thread uses a 1.5 mm pitch from M12 through M75, stepping to 2 mm at M90 and M110, and the clearance hole is nominally the nominal size plus 0.2–0.4 mm. Published installation guidance allows a clearance hole 0.1–0.7 mm larger than the thread’s major diameter. If you are switching thread systems — for example a PG entry being replaced by metric — check the conversion rather than assuming the diameters are close enough; a metric, PG and NPT thread comparison will save you a mis-ordered batch.

The last column is the point worth carrying into a specification review. At a 1.5 mm pitch, five threads is always 7.5 mm, so on every metric size up to M75 the eight-millimetre clause is the controlling one. A drawing that says “minimum five threads” on an M20 Ex d entry does not meet IEC 60079-1 unless 8 mm is also achieved.

When the Thread Is Too Long

Ordering a long thread cable gland with more thread than the joint needs is not a free safety margin. Four specific problems appear when the thread is longer than the joint is deep.

The gland bottoms out in a blind tapped boss

On a tapped entry with a shallow boss, a long thread reaches the bottom of the hole before the shoulder reaches the face. The installer feels resistance, stops, and the sealing washer is never compressed. The joint looks assembled and holds pressure on a bench test with no cable fitted, then leaks in service. If the boss depth is 10 mm, specify roughly 10 mm of usable thread — not 20 mm.

Excess thread protrudes inside the enclosure

On a through-hole, the extra length ends up inside the box, exactly where the wiring, terminal blocks and DIN rail are. On a small junction box that can block the lid or reduce the wiring space below what the cable needs. It is also a cut hazard during maintenance.

Exposed thread corrodes and gets damaged

Unused thread is a crevice that holds water and dirt, and the crests are the first thing to be damaged when a gland is knocked or a spanner slips. Damaged thread makes the gland difficult to remove years later, which matters on equipment that will be re-terminated.

The gland body sits further off the wall

Every millimetre of thread between the shoulder and the panel face adds to the cantilever. On a thin or foam-cored panel, that load transfers into a face with very little local stiffness, and the entry starts to work loose under vibration. Long thread is the right answer for a thick, rigid wall — not for a thin wall that happens to have a deep recess.

Long Thread Gland, Reducer, or Custom Thread?

When a long thread cable gland is not available in the length you need, three routes get you out of the problem, and they are not equivalent.

The reducer route is the one that gets used too often because it is available off the shelf. It solves a diameter mismatch. It does not solve a length problem, and stacking an adaptor into a shallow entry can make the length problem worse while adding a joint that nobody inspects.

For reference, the two extended-thread series most often specified for this job differ mainly in body material and certification route. The PA66 nylon extended thread gland gives IP68 at 10 bar, −40 °C to +100 °C with 120 °C instant, and is available in metric, PG, G and NPT threads — the usual choice where the enclosure is non-metallic and weight or insulation matters. The nickel-plated brass extended thread gland runs M8 to M100, IP68, −40 °C to +100 °C with 120 °C short-time, with PA claw and NBR seals, and is the choice for cast metal enclosures, marine and offshore work and machinery. Both are offered with custom thread lengths, which is the practical answer when a standard extended thread still falls short of the stack-up.

Installation and Inspection Checklist

Thread length is only half of a reliable entry. These eight checks catch the rest.

1. Hole perpendicular to the face. An angled hole means the washer compresses unevenly and the joint leaks on one side.
2. Face flat, rigid, clean and dry. Paint, grit, oil or a curved surface under the washer defeats the seal regardless of thread length.
3. Clearance hole within tolerance. Nominally the thread size plus 0.2–0.4 mm; 0.1–0.7 mm oversize is generally accepted, beyond that the washer may not cover the gap.
4. Full locknut height engaged, with two threads protruding. Count them; do not judge by feel.
5. Cable outside diameter centred in the clamping range. A cable at the extreme of the range compresses the seal unevenly — check the sealing range measurement method before ordering.
6. Torque applied to the correct component. Body and locknut have different limits; over-torquing the body crushes the seal rather than improving it. Follow the gland’s torque specification and sealing points.
7. No sealant used to compensate for short engagement. On flameproof joints this is a non-conformance, not a fix.
8. Entry protected after installation. Cap unused entries with a blanking plug of the same rating, not tape.

Common Mistakes

Conclusion

The thread length question has a clean answer once you stop comparing it to wall thickness alone. Measure the finished wall at the entry, add the uncompressed washer, add the full locknut height, add two threads of protrusion, and you have the number to put on the purchase order. Then check which joint you have: a through-hole needs that total, a tapped boss needs the engagement depth plus clearance so the shoulder still seats.

If your cable gland panel thickness falls between roughly 7 and 25 mm and the entry is a standard metric size, an extended thread gland is the direct answer, and a standard gland remains the correct baseline for thin sheet where the locknut sits inside. If the stack-up runs past 25 mm, the enclosure, not the gland, is what needs changing.

Send us the finished wall thickness, the thread specification and the cable outside diameter, and we will confirm the thread length against the drawing before anything is ordered — contact our engineering team with those three numbers and a photo of the entry from both sides.

FAQ

How do I know if I need a long thread cable gland?

Measure the finished wall at the entry. If the wall plus the sealing washer plus the locknut height plus two threads of protrusion is longer than the gland’s thread, you need an extended thread. On an M20 entry with a typical 8 mm standard thread, that point arrives at roughly a 4 mm wall once you allow for the nut and washer.

What is the minimum thread engagement for a cable gland?

For flameproof Ex d entries, IEC 60079-1 requires five fully engaged threads or 8 mm of axial engagement, whichever is greater. For IP-rated but non-hazardous installations, IEC 60529 does not specify a thread count, so the gland manufacturer’s installation instruction, the washer compression and the applied torque govern.

Can I use a reducer to fit a standard gland to a thick panel?

A reducer changes thread size or thread system. It does not extend thread length, and stacking one into a shallow entry can make seating worse. It also adds a second threaded joint and seal interface inside the IP boundary. Order the extended or custom thread instead.

Is a longer thread always the safer choice?

No. In a blind tapped boss, a thread longer than the usable depth stops the gland before the shoulder seats, so the washer never compresses. In a through-hole, excess thread protrudes inside the enclosure where it obstructs wiring and collects moisture. Size to the stack-up.

What thread length do I need for a 12 mm enclosure wall?

With a 1.5 mm washer, a 5 mm locknut and two protruding threads, the calculation is 12 + 1.5 + 5 + 3, so about 21.5 mm of thread. That is beyond most standard extended ranges on small metric sizes, which is why 12 mm walls often need a custom thread length or a larger body size with a longer standard entry.

Does thread length affect the IP rating?

Indirectly. The IP rating comes from the compressed sealing washer at the shoulder and the cable seal, not from the thread itself. But if the thread is too short for the locknut to clamp the joint, or too long to let the shoulder seat, the washer never reaches its specified compression and the rated protection is not achieved.