4B-WD BSP Male Captive Seal Plug Adapter — 4B-WD Adapter for Capping Spare Ports

1. What a 4B-WD Adapter Does

The 4B-WD adapter is a single-ended BSPP male plug with a captive bonded seal, used to close a hydraulic port that is not in use. The captive seal is a bonded seal washer held on the plug so that it stays in place when the plug is handled, carried or fitted. The stud end carries a G (BSPP) parallel thread to ISO 228-1 and screws into a BSPP port on a manifold, valve block, pump, cylinder or filter head.

A plug has one interface only, so that interface decides whether the port stays dry. The thread form, the thread size and the sealing face must match the port exactly, because the joint between the plug and the port is the only barrier between the fluid system and the atmosphere.

The captive design removes the most common failure mode of a loose washer: the washer that drops off the plug while the technician works, falls into the port or onto the workshop floor, and is refitted dirty, damaged or not at all. With this captive seal design the washer is retained on the plug for the whole life of the part.

2. 4B-WD Specifications

2.1 Connection 1 — BSPP male stud end (into the port)

• Thread standard: ISO 228-1 parallel pipe thread, G series, the thread family used by ISO 1179 ports.
• Gender: male.
• Size range: G1/8″x28 up to G1.1/2″x11, eight sizes in the published table.
• Sealing: bonded seal washer (Dowty-type) pressed against the flat face of the port.
• Mating port: a flat-faced BSPP port with a spot face large enough for the washer to seat square.

2.2 Connection 2 — closing head (no line connection)

• Function: the head closes the stud end, retains the washer and provides the drive for tightening.
• Drive: external hex, dimension S1 in the table.
• Seal retention: the washer is captured on the plug, so it cannot be lost or dropped during fitting.
• Mating part: none. The head connects to no hose, tube or fitting.

2.3 Construction and Materials

The body is machined from bar stock on CNC lathes. Standard material is carbon steel with trivalent chromium zinc plating (Cr3+) for general hydraulic service; zinc-nickel plating, 304 stainless steel, 316 stainless steel and brass are available for corrosive, offshore and marine duties. The captive bonded seal is supplied in NBR, FKM or EPDM according to the fluid and temperature of the application, and carries its own captive seal code in the table so that the correct washer can be reordered separately.

2.4 Pressure and Temperature

Pressure is not fixed by the part number alone. It is governed by the thread size, the wall thickness of the plug, the sealing method used at the port and the body material, so working pressure for these sizes is confirmed per order by HENGHUA engineering, based on the sealing method, thread size and material selected. Seal material temperature limits apply as material facts: NBR approximately -30°C to +100°C, FKM approximately -20°C to +200°C, EPDM approximately -40°C to +120°C.

2.5 Part Number and Dimension Table

Every value below is taken from the published series chart. All dimensions are in millimetres. E is the thread designation of the stud end, the captive column gives the code of the bonded seal washer retained on the E end, A is the thread length, L is the overall length of the plug, and S1 is the hex across flats of the head.

Note on thread notation: a designation such as G1.1/4″x11 means a 1 1/4 inch BSPP thread with 11 threads per inch. The number after the x is the thread pitch in threads per inch.

3. How to Read a 4B-WD Adapter Part Number

A 4B-WD part number has a fixed structure: series prefix, size code, and the captive seal suffix.

• 4B identifies a BSPP male plug for a 60° cone seat or bonded seal port.
• WD is the captive seal suffix. It marks the version in which the bonded seal washer is retained on the plug, so the same part number covers the plug and its captive seal.
• The two digits before WD are the size code, expressed as the nominal bore in sixteenths of an inch. 02 is 2/16 inch, or 1/8 inch; 04 is 4/16, or 1/4 inch; 08 is 8/16, or 1/2 inch; 12 is 12/16, or 3/4 inch; 24 is 24/16, or 1 1/2 inch.
• The captive seal code in the table tells you which washer is fitted, for example WD-B08 for the 1/2 inch size, so the washer can be ordered as a spare part.

Worked example from the table: 4B-08WD. The prefix 4B shows a single-ended BSPP plug; WD shows that the bonded seal is captive on the plug; 08 gives a 1/2 inch nominal bore, so the thread is G1/2″x14 and the captive seal is WD-B08. The thread length A is 14 mm, the overall length L is 25 mm, and the hex across flats S1 is 27 mm.

4. Installation and Fitment

Hand-start the plug for the first turns, then tighten with a wrench on the hex until the washer contacts the port face, and stop there. A bonded seal joint seals at the washer, so extra rotation does not improve sealing and can crush the elastomer or bow the metal backing ring. Do not use a thread sealant on a BSPP bonded seal joint; the seal is at the face, not on the threads.

Check the port spot face for burrs, swarf, paint and old sealant before fitting. The washer must sit square and must not be trapped on the lead thread. Because the seal is captive, there is nothing to drop into the system, but the washer should still be inspected for cuts, hardening, swelling and embedded debris before the plug is reused after service.

Common fitment errors and their symptoms:

• Fitting a captive seal plug into a port with a 60° internal cone instead of a flat face. Symptom: the washer bridges the cone and weeps at the head.
• Fitting the plug into a scored or damaged spot face. Symptom: leakage that continues after retightening.
• Reusing a washer that has taken a permanent set. Symptom: a slow weep after the first thermal cycle.
• Cross-threading into an aluminium casting. Symptom: the plug stops hard before the washer touches the port face.
• Over-tightening a small size. Symptom: a distorted washer or a stretched port boss.

Treat the captive bonded seal as a wearing part. Where the plug is removed regularly, keep spare washers in the matching captive seal code so that the plug can be returned to service with a new seal.

5. Applications and Industries

Typical applications and industries for this plug include hydraulic systems, water jetting equipment, oil and gas service equipment, industrial machinery, construction machinery, agriculture, marine hydraulics and material handling equipment.

Four concrete uses:

1. Capping a spare port. Manifolds and valve blocks are often cast with more ports than a given build needs. This plug closes those ports on a flat spot face so the block can go into service without a blanking flange.
2. Isolating an unused branch. When a circuit branch is removed during a rebuild, the port that fed it remains open. A captive seal plug isolates that port from the rest of the system.
3. Capping a line during commissioning. On a test bench, sub-circuits are pressurised one at a time while the other ports stay closed, which lets a technician test part of a machine without dismantling the rest.
4. Protecting a port during transport. Pumps, cylinders and blocks ship with their ports closed to keep out moisture, dust and insects and to protect the spot face during handling.

The captive seal is of most value where plugs are fitted on site or at height, where a dropped washer stops the job, and where a loose washer could find its way into a reservoir or a valve.

6. Replacement and Cross-Reference Guide

Identify a replacement by standard and sealing method, not by the source of the original part. BSPP threads are defined by ISO 228-1 and BSPP port sealing geometry by ISO 1179, so any captive seal plug that matches the thread, the washer size and the hex clearance performs the same duty. Separate the thread families first: BSPP (parallel, G) to ISO 228-1, BSPT (taper, R) to ISO 7-1, JIS gas thread to JIS B 0203, and the metric and SAE families to their own standards.

Measure these three things before ordering:

1. The thread. Measure the outside diameter of the male port thread and match it to the size code table, then confirm the pitch with a thread gauge. A parallel thread measures the same diameter along its whole length; a taper thread narrows toward the end.
2. The seat and the spot face. Look at the port face. A flat machined spot face takes a bonded seal washer, which is what the captive seal version is built for. A machined 60° internal cone takes a cone seat plug. Measure the spot face diameter to be sure the washer sits fully on it.
3. The space and the drive. Measure the clearance around the port and compare it with the hex S1 and the length L. Where a wrench will not fit, the hollow hex captive seal plug of the 4BN-WD series uses an internal hex key with the same thread and the same captive seal.

7. Ordering, Packaging and Delivery

To place an order or request a quotation, send the series code, the full part number, the thread size, the captive seal code, the seal material, the plating or material grade, and the quantity. Example: 4B-08WD, G1/2″x14, captive seal WD-B08 in NBR, carbon steel with trivalent chromium zinc plating, 500 pieces. Replacement washers can be ordered on their own under the captive seal code. Sizes or thread combinations outside the published table are quoted as specials from a drawing or from the mating port dimensions.

Plugs are packed in sealed polyethylene bags inside cartons, with the part number, thread size, captive seal code, material and quantity printed on the label. Bulk cartons and labelled shelf packs are available for distributor stock. Sample orders are accepted for validation before a production order. Lead time is confirmed per order, since it depends on the size, the material grade, the seal selection and the order quantity.

8. FAQ

Q1. What is the difference between BSPP and BSPT, and can they be interchanged? BSPP is a parallel pipe thread to ISO 228-1, where the pressure-tight joint is not made on the threads; sealing comes from a bonded seal washer or a 60° cone seat. BSPT is a taper pipe thread to ISO 7-1, where the joint is made on the threads and needs a sealant. The two cannot be interchanged: a parallel plug in a taper port leaks, and a taper plug in a parallel port damages the port thread.

Q2. What is the difference between a 60° cone seat and a bonded seal, and which should I choose? A 60° cone seat seals metal to metal between the cone on the stud end and the cone in the port, as defined for BSPP stud ends in ISO 1179. A bonded seal washer seals on a flat face, with the elastomer bonded to a metal backing ring. Choose the geometry the port is machined for. This series uses the flat face bonded seal arrangement and is the right choice where the port face is flat.

Q3. What is the difference between a captive seal plug and a plain plug? On a plain plug the bonded seal washer is a separate loose part that must be placed on the plug before fitting and can fall off during handling. On a captive seal plug, such as this series, the washer is retained on the plug and stays with it. The captive version reduces the risk of a washer being dropped into a port, a reservoir or a valve, and it speeds up assembly on site.

Q4. How do I identify the correct 4B-WD adapter size? Measure the outside diameter of the male port thread and match it to the size code table; the two-digit code is the nominal bore in sixteenths of an inch, so 08 is 1/2 inch and the thread is G1/2″x14, with captive seal WD-B08. Confirm the pitch with a thread gauge, and confirm that the spot face is wide enough and that the hex S1 clears the surrounding casting.

Q5. What seal material should I select for my fluid and temperature? NBR suits mineral hydraulic oil and general service at approximately -30°C to +100°C. FKM suits higher temperatures and aggressive fluids at approximately -20°C to +200°C. EPDM suits water, water-glycol and steam service at approximately -40°C to +120°C, but is not the first choice for mineral oil. Match the elastomer to the fluid first and to the temperature second.

Q6. How is the working pressure of this adapter determined? Working pressure is governed by the thread size, the wall thickness of the plug, the sealing method at the port and the body material, so it is not one figure for the whole series. Working pressure for these sizes is confirmed per order by HENGHUA engineering, based on the sealing method, thread size and material selected.

Q7. What is the minimum order quantity and can I get samples? Sample orders are accepted so that a plug and its captive seal can be validated against the actual port before a production order. Minimum order quantity depends on the size, the material grade and the seal selection, and is confirmed with the quotation. Send the part numbers and the expected annual volume so that the sample and the production batch are quoted together.

Q8. Can a 4B-WD adapter be reused after it has been in service? The plug body is reusable if the thread is undamaged, the cone or shoulder face is not scored and the hex is not rounded. The captive seal washer is a single-use item in practice, because the elastomer takes a permanent set after the first tightening. Order replacement washers in the matching captive seal code and refit the plug with a new seal.

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