4H Metric Male L-Series ISO 6149-3 Blanking Plug – 4H Plug

The 4H plug is a metric male blanking plug that closes a metric O-ring port with an elastomer O-ring seated in the machined recess of the port face. It is the plugging form of the ISO 6149-3 L series stud end, so the parallel metric thread only clamps and the O-ring does the sealing. This page gives the printed dimension table, the sealing principle, the part number logic and the installation rules for the 4H plug, and it describes the closed end of the part honestly.

Principais vantagens

• Seals with an O-ring in the port recess to ISO 6149, so the thread is a clamp and never a sealing element.
• Metric L series ISO 6149-3 stud end, matching an ISO 6149-1 or an ISO 9974-1 port per the printed note.
• Eight sizes from M12X1.5 to M42X2 with one external hex across the range.
• No second connection and no through bore, so a plugged port has one joint instead of two.
• Carbon steel with trivalent chromium zinc plating, or 304 / 316 stainless steel for corrosive service.
• Seal material options of NBR, FKM and EPDM to match the fluid and the temperature window.

1. What a 4H Fitting Does

A 4H plug is a one-ended hydraulic blanking plug: a short steel body with a metric male stud end on one side and a closed end on the other, used to seal a metric O-ring port that is not in use. It is installed into a metric port cut to ISO 6149-1, where the port face carries a machined recess that holds the O-ring, and it is run in until the stud end shoulder comes down onto that face. The sealing surface is the elastomer O-ring compressed in the recess.

The core value of this part is dimensional match. The sealing end has to match its mating port exactly, because that match is the precondition for eliminating leakage at the joint. A stud end made to ISO 6149-3 has a defined thread length, groove position and shoulder, and a port made to ISO 6149-1 has a matching recess; when the two are matched, the O-ring is compressed by a controlled amount and the shoulder stops against the port face. If the thread pitch or the recess depth is wrong, the O-ring is either over-compressed and cut, or under-compressed and weeping.

The sealing principle of this series is a straight metric thread with an elastomeric seal. This ISO 6149 adapter family clamps with the thread and seals with the O-ring: the thread holds the plug in position and resists pressure load, while the ring compressed in the port recess does the sealing work. Because the thread is parallel and not tapered, the plug can be removed and refitted without changing the joint geometry.

2. 4H Specifications

2.1 Connection 1 – Metric Male ISO 6149-3 L Series Stud End

Connection 1 is the only connection on the part. It is a metric male stud end with a parallel metric thread made to ISO 6149-3, the light duty metric stud end standard used across this ISO 6149 adapter range. The printed range covers M12X1.5, M14X1.5, M16X1.5, M18X1.5, M22X1.5, M27X2, M33X2 and M42X2, eight sizes. The thread is male and external, and the stud end carries an O-ring groove behind the lead thread.

Sealing is by an elastomer O-ring on the stud end, compressed in the machined recess of the mating port face. The mating port is a metric port with an O-ring recess as described by ISO 6149-1, and the printed note under the chart records that the stud end also fits an ISO 9974-1 port, or an ISO 6149-1 port when the retaining ring is not used. The O-ring column of the table lists the ring for this end as O-RING E.

2.2 Connection 2 – Closed End

The 4H plug has no second connection. Behind the hex the body is a solid closed end with no bore, no thread and no second sealing face. There is nothing to connect to on that side and no flow path through the part, because the purpose of the item is to stop a line or a manifold branch rather than to join two lines.

That closed end has two practical effects. It removes one sealing joint from the assembly, so a plugged port has fewer places to leak than a port closed with an adapter plus a cap. It also fixes the working length: because there is no second thread, the overall length L in the table is simply the stud end plus the hex body, so the plug fits close to a manifold wall or inside a tight enclosure.

2.3 Construction and Materials

As an O-ring adapter, the body is machined from bar stock on CNC lathes, so the thread, the O-ring groove and the hex are produced in one setup and stay concentric. The external hex is dimension S1 in the table and runs from 17 mm on 4H-12 to 50 mm on 4H-42, and it sits between the stud end and the closed end so the plug is gripped from the side.

A plug of this type carries no drive feature inside the body. The 4H is an external hex part rather than a hollow hex part: the hex is the drive, and the closed end is a plain machined face with no internal socket. That is the practical difference between the 4H and the hollow hex version in the same programme, and it decides which one can be used where access is limited. The printed Chinese name on the catalogue page does not separate the two forms, while the English name does, so the drawing is the reliable reference when the two are compared.

Body material options are carbon steel, 304 stainless steel, 316 stainless steel and brass. The standard export build is carbon steel with trivalent chromium zinc plating (Cr3+), with zinc-nickel plating available where a harder finish is preferred. Stainless steel is selected for corrosive environments, offshore and marine work and water-based fluids. Plating thickness and salt-spray performance are confirmed per order. Seal options are NBR, FKM and EPDM, and the O-ring can be supplied separately as a spare. Every batch is checked against the drawing with thread gauges and dimensional checks before packing.

2.4 Pressure and Temperature

Working pressure for these sizes is confirmed per order by HENGHUA engineering, based on the thread size, sealing method and material selected. The printed chart for this series carries no pressure column, so no per-size rating is quoted here. On an ISO 6149 adapter the pressure capability is governed by the port wall and the mating housing as much as by the plug itself.

Temperature is a material fact rather than a property of the fitting. NBR suits approximately -30°C to +100°C, FKM approximately -20°C to +200°C, and EPDM approximately -40°C to +120°C. The steel body is not the limiting element in normal hydraulic service; the elastomer is. Select the compound against the fluid first, then confirm the temperature window.

2.5 Part Number and Dimension Table

Dimensions are millimetres unless the column states a thread size. Printed note under the table: “Fit for ISO 9974-1 port or ISO 6149-1 port when useless of retaining ring.”

What each column means

• N.º DA PEÇA. – the ordering code for the size, built from the series prefix 4H plus a two digit size code.
• E – the thread size and pitch of the single stud end, written as the nominal metric diameter followed by the pitch, for example M22X1.5.
• O-RING E – the O-ring for the stud end, printed as O plus inside diameter in millimetres and cord diameter in millimetres, so O19.3X2.4 is a 19.3 mm inside diameter ring with a 2.4 mm cord.
• A – the thread length of the stud end, measured from the sealing face to the end of full thread.
• L – the overall length from the sealing face of the stud end to the outer face of the closed end.
• S1 – the width across the flats of the external hex, which decides the spanner size.

There is no pressure column and no second thread column in the printed chart, because the part has one connection and a closed end.

Thread size and dash chart

3. How to Read a 4H Plug Part Number

A 4H plug part number has two fields: the series prefix and the size code. The prefix 4H identifies the family, a metric male ISO 6149-3 L series O-ring stud end supplied as a blanking plug with a closed end and an external hex. The second field is a two digit code that follows the nominal metric thread size.

Worked example, 4H-22: 4H is the series prefix for a metric L series O-ring blanking plug, and 22 is the size code, shown in the table as the M22X1.5 size with thread length A of 15 mm, overall length L of 27.5 mm, hex S1 of 27 mm and an O19.3X2.4 O-ring. Worked example, 4H-12: the same prefix with size code 12, the smallest size in the printed range, M12X1.5 with A of 11 mm, L of 19.5 mm, S1 of 17 mm and an O09.2X2.4 ring.

There is no suffix in this series. The letter suffix H used elsewhere in the metric programme marks the heavy duty S series stud end, and the -HN suffix marks a hollow hex drive, so a request written 4H-22H or 4H-22HN belongs to a different part and should be confirmed against a drawing rather than matched to this table. The printed range is the L series, which is the light duty form of the ISO 6149 stud end.

4. Installation and Fitment

This plug is a straight-thread O-ring part, so the O-ring is the seal and the thread is only the clamp. Never ask the thread to seal. Do not wrap the thread with PTFE tape, do not brush on a thread sealant or paste, and never fit the plug without its O-ring. A sealant adds nothing to a straight-thread joint and can push into the recess and change how the ring is compressed, and contamination in that same recess is one of the most frequent causes of a weep on an otherwise correct assembly.

Torque practice is a sequence rather than a number. Clean the thread and the recess, fit the O-ring dry or with a light film of system fluid, start the thread by hand to prove it is not cross-threaded, then bring the plug down to the port face with the correct spanner and stop when metal contact is reached. Never use the hex to force a plug that has picked up on the thread, and never keep tightening after the shoulder lands. An over-tightened hydraulic plug distorts the O-ring and can flare the port; an under-tightened one leaves the ring short of its designed compression.

Common fitment errors and their symptoms:

• Missing O-ring. Leakage at the shoulder as soon as pressure is applied, however hard the plug is tightened.
• Ring of the wrong size or cord. An immediate low-pressure leak, or a ring that is visibly pinched or cut on removal.
• Over-compressed ring. A slow weep that grows with temperature, with a flattened ring inside.
• Cross-threading. High turning effort before the shoulder lands, with metal particles at the thread.
• Wrong thread form. The plug runs in a short distance and jams, or seats with a visible gap at the shoulder.

Assembly steps:

1. Confirm the port carries a metric thread with an O-ring recess and that the thread matches the plug.
2. Inspect the recess for burrs, chips, old seal material and the retaining-ring detail, and blow it clean.
3. Check the O-ring for size and cord, and fit it to the stud end groove without stretching it.
4. Lubricate the thread and the ring with a light film of system fluid compatible with the compound.
5. Run the plug in finger-tight, then tighten with the correct spanner until the shoulder contacts the port face.
6. Pressure test, inspect the joint and record the seating position for later refits.

The steel body can be cleaned, inspected and refitted when the thread and the sealing shoulder are undamaged, but the O-ring is a single-use elastomer item and should be replaced whenever the plug is removed.

5. Applications and Industries

This hydraulic plug suits hydraulic systems, water jetting equipment, oil and gas skids, industrial machinery, construction machinery, agricultural equipment, marine installations and material handling units wherever a metric O-ring port has to be closed or reserved. It is a hydraulic plug first and a thread protector second, so it fits any position where a metric port must stay sealed and clean between builds, and in maintenance stores it is usually listed as a port plug for the sizes found on that machine.

Concrete scenarios:

• Reserved manifold branches. A manifold is machined with more ports than the first build needs so a second circuit can be added later. Each unused metric port is closed with a 4H plug, which keeps the circuit sealed and the thread protected until the upgrade is ordered.
• Test and purge points. A service port added for pressure testing or flushing is open during commissioning and plugged for normal operation. With no second connection, nothing protrudes once the plug is installed.
• Shipping and storage of finished assemblies. A cylinder or valve with a metric O-ring port is plugged to keep the bore clean and stop oil escaping during transport and shelf storage, and the plug is removed at first installation.

6. Replacement and Cross-Reference Guide

A replacement is identified by standard, thread and sealing method, never by brand. The 4H plug is a metric ISO 6149-3 L series O-ring stud end supplied with a closed end, so a correct substitute has to satisfy three conditions at once: the same metric thread and pitch, the same ISO 6149 O-ring sealing method, and the same stud end dimensions of thread length and hex as the printed table.

L series and S series. Within the ISO 6149 family the port is ISO 6149-1 and is common to both duties, while the stud end is the variable. The light duty form is ISO 6149-3 and the heavy duty form is ISO 6149-2, with a longer thread length and a longer body at the same thread size. The printed 4H range is the light duty form, so it is specified against a port cut for the L series stud end; a plug for the heavy duty form comes from the S series family and carries a different part number. Because both duties share the port, the two can be confused in a parts bin, so check the length figures in the O-ring adapter tables, where the difference is the quickest way to tell them apart.

The printed port note explained. The chart carries the note “Fit for ISO 9974-1 port or ISO 6149-1 port when useless of retaining ring.” It tells the buyer that this L series stud end is not restricted to one port standard: it mates with an ISO 9974-1 port as supplied, and it also mates with an ISO 6149-1 port when the retaining ring is not used. The sealing element is unchanged in both cases, because the O-ring on the stud end still seats in the recess of the port face and still performs the sealing, and the retaining ring is a port-side detail that holds the ring in place. For an O-ring adapter or an O-ring plug this is a fitment statement rather than a change of seal, and it is worth confirming the port standard on the enquiry so the retaining-ring detail is matched to the hardware.

Measure these three things before ordering a replacement:

1. Thread. Measure the outside diameter and the pitch, and record M22X1.5 rather than M22, because the pitch separates a metric stud end from a similar-looking thread of another family.
2. Seal type. Read the port, not only the plug. A metric O-ring port has a machined recess; a 60° cone seat is a metal seat with no recess; a flat face for a bonded seal washer is plain.
3. Seat and length. Record thread length A and overall length L, and note whether the body has to clear an internal component. A plug longer than the original can foul, and one that is shorter may not reach full thread engagement.

A metric port to ISO 6149 and a BSPP port to ISO 1179 both seal on an O-ring in a recess, but the thread forms and the recess dimensions differ, so a plug from one family will not seal in the other even at a similar nominal size.

7. Ordering, Packaging and Delivery

To order, give the series code, the full part number, the thread size, the seal material, the body material and plating, and the quantity: for example, 4H plug, part number 4H-22, M22X1.5, O-ring in NBR, carbon steel with trivalent chromium zinc plating, 500 pieces. State the port standard at the enquiry stage as well, because the printed note allows an ISO 9974-1 or an ISO 6149-1 port and the retaining-ring detail differs between the two. If the plug goes into a corrosive or offshore application, say so at the same time so stainless steel and the right seal compound are quoted in one reply.

Packaging is arranged for two routes. For distributor shelves, these port plugs are packed in labelled cartons marked with the part number, thread size and seal material so a box can be identified without opening it. For direct-to-line OEM delivery, parts are packed in bulk or in kitting trays for line-side handling.

8. FAQ

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

A straight-thread O-ring joint, such as the 4H plug in an ISO 6149-1 port, seals on an elastomer O-ring compressed in a machined recess in the port face, and the parallel metric thread only clamps the parts. A tapered pipe thread seals by interference between the thread flanks plus a filler such as tape or paste, so its tightness depends on how far the thread has been driven. A tapered port cannot accept an O-ring, because it has no recess to hold one, and a straight-thread joint must never be sealed with thread sealant.

Can BSP and NPT be interchanged?

No. BSP and NPT are different thread families with different forms and flank angles, and they are not interchangeable even when the nominal sizes look close. The same applies to mixing a metric ISO 6149 port with an imperial BSPP port: both seal on a recessed O-ring, but the thread form and the recess dimensions differ, so the joint will not seal. The exception is a dedicated cross standard adapter made with one thread of each family; never force one thread into the other.

How do I identify the correct 4H size?

Read the thread marking on the port or measure it. The printed range covers M12X1.5, M14X1.5, M16X1.5, M18X1.5, M22X1.5, M27X2, M33X2 and M42X2, which correspond to the 4H-12 to 4H-42 part numbers. Measure the outside diameter and the pitch, match the pair to the table, then confirm thread length A and hex S1 so the plug seats and stays accessible.

Is the 4H the same as the hollow hex version in the same programme?

No. The 4H is the external hex part: the hex is dimension S1 in the table and it is the drive, which is turned from outside with a spanner or socket. A hollow hex version carries an internal Allen socket in the closed end and is turned from inside with an Allen key, which suits positions where no spanner can reach the outside of the body. The sealing end and the port standard are the same idea in both cases, but the drive and the part number differ.

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

NBR is the general-purpose choice for mineral hydraulic oil and covers approximately -30°C to +100°C. FKM suits higher temperatures and many aggressive fluids and covers approximately -20°C to +200°C. EPDM covers approximately -40°C to +120°C and is the usual choice for water-based fluids, but it is not suitable for mineral oil. Confirm the compound against the actual fluid and temperature before ordering.

How is the working pressure of this fitting determined?

The printed chart for the 4H series has no pressure column, so working pressure is confirmed per order by HENGHUA engineering, based on the thread size, sealing method and material selected. The reason is that on a straight-thread O-ring joint the capability is set by the port and the housing as much as by the plug. The rule is that the rating must be established against the actual port, material and duty cycle; the exception is that a larger thread size does not automatically permit a higher pressure in every housing.

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

Minimum order quantity is confirmed per order, because it depends on the size, the material and the plating selected. Sample pieces can be supplied before a production order so the thread fit and the O-ring seating can be checked against the real port. Send the part number, thread size and seal material with the request so the sample is quoted in the correct configuration.

Does this port plug need a washer or a bonded seal?

No. The 4H seals with an O-ring in the port recess, so adding a bonded seal washer or a flat gasket at the shoulder would hold the shoulder off the face and reduce O-ring compression. Bonded seal washers belong to flat face ports with no recess, which is a different port style. The correct accessory for this blanking plug is a replacement O-ring of the size listed in the table.

To close, HENGHUA invites your 4H plug enquiry: send the part number, thread size, seal material, body material and quantity, and our engineering team will confirm the configuration, the working pressure for your port and housing, and the lead time for your order.

Related series in the same family: the 1H adapter and the 1H-H 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.