4MN-WD Metric Male Captive Seal Hollow Hex Plug – 4MN-WD Hollow Hex Plug

The 4MN-WD hollow hex plug is a single-port blanking plug with a metric male thread, an internal hex drive and a captive elastomer seal held in the body behind the thread. It seals on that captive seal against a flat port face rather than on an O-ring in a recessed port. The 4MN-WD hollow hex plug is driven from inside the body with an Allen key, which matters on manifold blocks and cartridge housings where a wrench cannot reach around the plug.

Core Advantages

• Captive elastomer seal held in the body, so the seal stays with the plug when it is removed.
• Internal hex drive across S1, for ports where no external wrench clearance is available.
• Twelve part numbers from M10X1 through M48X2, covering large cartridge and housing ports.
• Metric parallel thread to the stud end practice described in DIN 3852-2.
• Machined from bar stock on CNC lathes, with thread gauges and dimensional checks applied batch by batch.
• Body options of carbon steel with trivalent chromium zinc plating, or 304 / 316 stainless steel.

1. What a 4MN-WD Hollow Hex Plug Does

A 4MN-WD fitting is a hydraulic plug that terminates a single metric port and is driven from inside its own body. The thread is a metric parallel male thread, the drive is an internal hex at the tail, and the sealing element is a captive elastomer seal seated at the rear of the thread. The body closes behind the thread, so there is no second connection.

The sealing end must match its mating port exactly, because that match is the precondition for eliminating leakage at the joint. This plug seals on an elastomer seal pressed against a flat face, so it belongs on a port that presents a plain machined face. It is not the correct part for a port with a 60° cone seat, and it is not the correct part for a port recessed to take an O-ring, because in both cases the face that the captive seal needs is not there.

The sealing principle is worth stating precisely, because this series is often confused with O-ring plugs. The captive seal is an elastomer element held captive in the plug body, and it compresses axially against the flat port face when the plug is tightened. There is no O-ring in a machined recess on the port, and no metal-to-metal cone contact. DIN 3852-2 covers metric stud ends that seal with or without an elastomer seal, which is the family this stud end belongs to, and the same flat-face sealing idea underlies the bonded seal washer described in DIN 7603. The thread is parallel and does not seal; it only clamps the seal against the face.

2. 4MN-WD Specifications

2.1 Connection 1 – Metric male stud end with captive seal

• Thread standard and family: metric parallel thread to the metric stud end practice described in DIN 3852-2.
• Thread size range: M10X1 through M48X2, as listed in the dimension table.
• Gender: male.
• Sealing method: captive elastomer seal held in the body and compressed against a flat port face.
• Mating port standard: a metric port with a flat machined face that accepts an elastomer seal per DIN 3852-2.

This stud end is the E end in the dimension table. Its thread is the size code in the part number and its threaded length is dimension A.

2.2 Connection 2 – Closed end, no second port

The 4MN-WD has no second connection. The body terminates behind the hex, and the only opening on that end is the internal hex drive, which is a recess for the Allen key rather than a port. There is no bore through the plug, no thread and no seal face on the closed end.

This is what the 4MN-WD hollow hex plug is: a plug whose drive is sunk into the body instead of standing proud of it. The internal hex does not weaken the pressure boundary, because it is a shallow recess in a solid head, and the wall between the recess and the pressure path remains part of the one-piece machined body.

Because the closed end carries no thread and no seal, you cannot stack a second fitting onto this plug, and you cannot use it as a coupling. If a port has to feed two lines, or if a line has to continue through the port, the correct part is a metric adapter with a stud end at each side rather than this hydraulic plug.

2.3 Construction and Materials

The blanking duty and the drive feature both sit in the body. The body is machined from bar stock on CNC lathes in one piece, so the pressure path contains no brazed or welded joint, and the internal hex is cut into solid metal at the tail. Each thread is produced to the metric parallel form, so the flanks do not wedge and do not deform the port, and the captive seal groove is turned at the rear of the thread in the same operation that keeps it concentric with the thread axis.

The internal hex drive is the feature that separates this plug from an external hex blanking plug. An Allen key enters the recess from the tail, so no wrench has to pass around the outside of the body. That makes this port plug suitable for a manifold block where adjacent ports, fittings or machined bosses sit close to the plug, and for a recessed housing where an open-end wrench would foul the bore. S1 in the dimension table is the width across the internal hex flats, and it is the Allen key size for the row.

The captive seal is an elastomer element held in the body behind the thread. Because it is captive, it does not drop off when the plug is unscrewed and it cannot be forgotten on reassembly, which is a practical advantage on a plug that may be removed many times in service. Seal material options are NBR, FKM and EPDM.

Body material options are carbon steel, 304 stainless steel, 316 stainless steel and brass. Carbon steel bodies are supplied with trivalent chromium zinc plating (Cr3+) for general hydraulic service, and zinc-nickel plating is available for more demanding conditions. Stainless steel bodies are supplied unplated, and plating specification is agreed per order.

2.4 Pressure and Temperature

Working pressure for the 4MN-WD sizes is confirmed per order by HENGHUA engineering, based on the thread size, seal material and body material selected. The printed dimension chart for this series carries no pressure column, so no per-size rating is published on this page. What the plug holds depends on the seal compression actually achieved against the port face and on the port material.

Temperature capability follows the seal material rather than the body. NBR suits approximately -30°C to +100°C, FKM approximately -20°C to +200°C and EPDM approximately -40°C to +120°C. These are material facts for the captive seal. For corrosive, offshore and wash-down duty, 304 or 316 stainless steel is the appropriate body choice, and the seal material is then matched to the fluid.

2.5 Part Number and Dimension Table

The table below is reproduced in full from the printed dimension chart for this hollow hex plug series. All dimensions are in millimetres.

What each column means

• PART NO. – the ordering code for the row. The digits after the series prefix are the nominal size of the threaded stud end.
• E – the thread of the stud end, given as nominal diameter by pitch, for example M10X1.
• CAPTIVE – the code of the captive seal held in the body for that row, which is the seal to quote when ordering spares.
• A – the threaded length of the stud end in millimetres, measured from the nose.
• L – the overall length in millimetres, including the threaded end and the head.
• S1 – the width across the internal hex flats in millimetres, which is the Allen key size for the plug.

Thread size and dash chart

3. How to Read a 4MN-WD Part Number

A 4MN-WD part number has three elements, read from left to right.

• Series prefix 4MN – a metric male plug with a captive seal and an internal hex drive. The N marks the hollow hex form, so the drive is inside the body rather than across an external hex.
• Size code – the nominal size of the stud end, which is the E end in the table. 10 means M10X1, 12 means M12X1.5 and 33 means M33X2.
• Suffix WD – the captive seal version. Both characters stay in the part number, so a 4MN-16WD is the M16X1.5 plug with an internal hex drive and a captive seal.

Worked example: 4MN-33WD. The prefix 4MN denotes a metric male hollow hex plug, the size code 33 sets the thread at M33X2, and the WD suffix confirms the captive seal version. Reading across the row, the captive seal code is WD-B16, the threaded length A is 18 mm, the overall length L is 24.5 mm and the internal hex across flats S1 is 17 mm.

4. Installation and Fitment

The 4MN-WD hollow hex plug seals on the captive seal against a flat port face, never at the thread, so installation practice concentrates on that one face. Check the face before assembly: it must be flat, clean and free of scoring, because an elastomer seal compressed against a damaged face will weep no matter how tight the plug is. Blow out the port, remove swarf and old sealant residue, and confirm that the captive seal is present in its groove and undamaged.

Start the plug by hand with the Allen key, keeping the plug axis aligned with the port axis, and turn it in until the seal touches the face. Tighten until the seal is evenly compressed, without over-driving the internal hex or distorting the head. A thread sealant must never be applied to compensate for a damaged or missing captive seal, because a liquid sealant cannot reproduce the controlled compression of an elastomer element, and sealant residue on the face will prevent the seal from seating. HENGHUA confirms the recommended tightening torque for your thread size, body material and port material per order.

Reuse policy follows the seal. The captive seal stays with the plug and does not fall off when the plug is removed, but it is still a consumable: an elastomer element that has taken a compression set or has been cut by a damaged face is replaced before the plug goes back into service. The plug body itself can be reused after the seal is renewed, provided the thread and head are undamaged.

Assembly steps

1. Confirm the port presents a flat machined face, not a 60° cone seat and not a recessed O-ring groove.
2. Check that the captive seal is present in the groove and undamaged, and replace it if it has taken a set.
3. Clean the port, the thread and the flat face, and confirm they are free of swarf and old sealant.
4. Select the Allen key that matches S1 and start the plug by hand until the seal touches the face.
5. Tighten until the seal is evenly compressed, without over-driving the internal hex.
6. Pressurise the circuit and check the port for weeping before returning the machine to service.

Common fitment errors and their symptoms

• Fitting the plug to a recessed O-ring port or a 60° cone port: the captive seal never seats on a face and the joint leaks immediately.
• Damaged or scored port face: a steady weep that increases with pressure and temperature.
• Over-tightening with a long Allen key: the internal hex is deformed and the plug has to be replaced.
• Leaving old sealant on the face: the seal sits on a ridge of cured sealant and leaks at low pressure.

5. Applications and Industries

Application tags for this series are hydraulic systems, water jetting, oil and gas, industrial equipment, construction machinery, agriculture, marine and material handling.

The internal hex drive is the reason most buyers choose this hydraulic plug. On a hydraulic manifold, ports are often spaced tightly or sit below the level of neighbouring bosses, and an external hex plug would need wrench clearance that the block does not offer. An Allen key works inside the plug footprint, so this port plug can be installed and removed without disturbing any adjacent fitting.

In industrial equipment and machine tools, cartridge valve housings, cylinder end caps and test points are closed with this blanking plug because the seal stays with the plug during maintenance. A technician who removes the plug to take a pressure reading does not have to hunt for a dropped seal or a loose bonded seal washer, which keeps contamination out of the system.

Large metric threads are common on the housing ports of hydraulic pumps, motors and accumulators, and the 4MN-WD hollow hex plug runs up to M48X2 for exactly that reason. In construction machinery, agriculture and material handling, the same plug seals unused ports on valve sections and reservoir blocks in a service environment where the surface around the port may be oily and awkward to reach. In water jetting, oil and gas and marine service, the stainless steel versions are chosen where wash-down, spray or salt exposure would attack a plated carbon steel part.

6. Replacement and Cross-Reference Guide

Cross-reference for this family is organised by standard, thread and sealing method, never by brand. To replace this port plug, identify the port rather than the part that was removed.

Measure these three things

1. Thread – measure the outside diameter over the thread and the pitch, and confirm the thread is parallel rather than tapered. M26X1.5 and M27X2 sit next to each other in this range and will not interchange.
2. Seal type at the port – this plug needs a flat machined face. If the port has a 60° cone seat, or a machined recess for an O-ring, a different family is required.
3. Drive size – measure across the internal hex flats and compare it with S1 in the table before ordering, so the correct Allen key is on hand.

Where the port has a 60° cone seat or takes a bonded seal washer on a metric thread, the correct part is a metric plug from the 4M series, which seals on the cone or on a DIN 7603 washer and is driven by an external hex. Where the port carries an O-ring in a machined recess and the thread is metric, the correct family is the metric ISO 6149 plug, covered in this range by the 4H series, and the L series heavy duty O-ring equivalents sit alongside it. Where the port carries a G thread to ISO 228-1 with an elastomer seal in a recess to ISO 1179-1, the correct part is a BSPP plug such as the 4G series.

Two structural differences are worth remembering. This hollow hex plug is one of two closed-end parts in the metric plug group: the 4M plug has an external hex and a cone or bonded seal stud end, while the 4MN-WD has an internal hex and a captive seal. Where the port must connect onward instead of being closed, neither plug is suitable and a metric adapter with a thread at each end is required. The 4MN-WD series has no light and heavy duty split, because it does not seal on a recessed O-ring; the choice here is the thread size and the seal material.

7. Ordering, Packaging and Delivery

To order a 4MN-WD hollow hex plug, give five pieces of information: the full part number including the WD suffix, the thread size of the stud end, the captive seal code from the CAPTIVE column, the seal material, and the body material and plating.

For example, 4MN-16WD is the M16X1.5 plug with captive seal code WD-M16 and an 8 mm internal hex. Because the captive seal code differs between some sizes, always quote the code rather than describing the seal, and order spare seals at the same time as spare plugs so that a service kit is complete. If you are replacing an existing plug and the marking is worn, send the port drawing and the internal hex size and the row will be confirmed for you.

Packaging follows two routes. Distributor orders go into labelled cartons with part number, thread size and quantity printed on the box, so parts go straight onto a shelf. OEM and direct-to-line orders are packed in bulk and labelled to the customer reference. Samples are available so that a new size or seal material can be validated against the port before a production quantity is released. Lead time is confirmable per order, because it depends on the size, material, plating and quantity. Every batch is checked against the drawing with thread gauges before packing.

8. FAQ

What is the difference between a straight-thread O-ring seal and a tapered pipe thread?

A tapered pipe thread seals by wedging the thread flanks against each other as the joint is tightened, so the thread itself is the seal. A straight-thread joint such as the metric stud end on this plug seals on a separate element, here a captive elastomer seal compressed against a flat face, and the parallel thread only clamps the seal in place.

Can BSP and NPT be interchanged?

No. BSP threads are Whitworth forms with a 55° thread angle, covered for the parallel family by ISO 228-1, while NPT is a tapered thread with a 60° thread angle. They differ in diameter and in threads per inch, and a G thread will not seat correctly in an NPT port. Forcing one into the other damages both threads, and any joint that appears to hold will not do so reliably under pressure.

How do I identify the correct 4MN-WD size?

Measure the thread outside diameter and pitch at the port, confirm that the port presents a flat face, then read the size code from the table in 2.5 and match the internal hex across flats to S1. Because the captive seal code changes with size, also note the CAPTIVE value for the row you select.

What seal material should I select for my fluid and temperature?

Select the captive seal material from NBR, FKM or EPDM. NBR suits mineral hydraulic oil over approximately -30°C to +100°C, FKM extends the range to approximately -20°C to +200°C and handles a wider spread of aggressive fluids, and EPDM covers approximately -40°C to +120°C and is the usual choice for water and many water-based fluids. EPDM is not compatible with mineral oil, so it is selected only when the fluid calls for it.

How is the working pressure of this fitting determined?

Working pressure for the 4MN-WD sizes is confirmed per order by HENGHUA engineering, based on the thread size, seal material and body material selected. The printed dimension chart carries no pressure column, so no per-size rating is published here. In practice the limit is set by the seal compression achieved against the port face and by the port material.

What is the minimum order quantity and can I get samples?

Minimum order quantity is confirmable per order and depends on the size, the body material and whether the part is a standard row or a drawing-based special. Samples are available so that a new size, seal material or hex drive size can be validated against the actual port before a production quantity is committed.

Does the 4MN-WD hollow hex plug seal with an O-ring?

No. The 4MN-WD seals with a captive elastomer seal held in the plug body, compressed against a flat port face. There is no O-ring in a machined recess on the port and no cone contact. If the port is machined to take an O-ring in a recess, the correct part is a metric ISO 6149 plug such as the 4H series rather than this hollow hex plug.

What is the difference between the 4MN-WD and the 4M plug?

Both are closed-end metric plugs with a single port, but the drive and the seal differ. The 4M plug has an external hex across S1 and seals on a 60° cone seat or a DIN 7603 bonded seal washer. The 4MN-WD has an internal hex drive and seals on a captive seal against a flat face, and its size range extends to M48X2. Choose the 4MN-WD hollow hex plug on the port face type and on the wrench clearance available around the port, and choose a metric adapter when the port must pass flow onward.

HENGHUA machines the 4MN-WD hollow hex plug in carbon steel, 304 stainless steel and 316 stainless steel, with NBR, FKM and EPDM captive seals to match your fluid. Send your port drawing, thread size and required quantity, and HENGHUA will confirm the exact row, captive seal code and material for your application.

Related series in the same family: the 4M plug and the 1M adapter. Browse the full Hydraulic Adapters range for the complete ISO 1179 BSP, ISO 6149 metric, ISO 11926-3 SAE O-ring boss and SAE J513 cone families.