4O Plug – SAE O-Ring Boss Plug L-Series ISO 11926-3

The 4O plug is a single-ended SAE O-ring boss blanking plug made to the light duty L series stud end of ISO 11926-3, which is the same stud end published as SAE J1926-3. It carries an O-ring in a machined groove behind an inch straight thread and is turned down with the body hex that is printed as S1. The part closes a SAE straight thread O-ring boss port to ISO 11926-1, the port that is also published as SAE J1926-1, and it holds that port closed with the O-ring as the only pressure barrier on the joint.

Core Advantages

• Correct SAE O-ring boss L series stud end for ports machined to ISO 11926-1 / SAE J1926-1.
• O-ring in the stud end groove seals in the port recess, and the straight inch thread clamps only.
• Thirteen printed sizes from 5/16″X24 to 2-1/2″ SAE straight thread in one range.
• Body hex S1 is printed per row, so the wrench size is known before the part arrives.
• Carbon steel with trivalent chromium zinc plating, or 304 / 316 stainless steel for corrosive service.
• NBR, FKM and EPDM seal options matched to the fluid and the operating temperature.

1. What a 4O Plug Does

A 4O plug is a blanking plug for a SAE straight thread O-ring boss port. The male stud end is machined to ISO 11926-3, the light duty L series SAE O-ring boss stud end, and it enters a port machined to ISO 11926-1. The body then terminates the connection: this port plug has no through bore, no second thread and no outlet, so the part closes the port instead of joining it to a line.

The core value here is the match between the stud end and the port. A SAE O-ring boss joint seals because the O-ring is compressed into the chamfered recess on the port face while the straight thread pulls the stud end down against it. The thread angle and pitch only have to carry the load, and the O-ring does the sealing. That is why a correct 4O plug seats with a firm, repeatable feel and holds, while a hydraulic plug with the wrong groove position cannot be made to seal by tightening it further.

This sealing principle is shared across the whole straight thread O-ring family. The SAE adapter family under ISO 11926, the metric family under ISO 6149 and the BSP family under ISO 1179 all seal on an elastomer compressed in a recess, with the thread used as a clamp. What changes between the families is the thread form and the recess dimensions, so a SAE O-ring boss plug is never a substitute for a metric or a BSP O-ring plug in the same nominal size.

The 4O range in the printed chart runs from the small 5/16″X24 size up to the large 2-1/2″ SAE straight thread size, the row the chart prints as 2.1/12″X12. Each row carries its own O-ring code, its own stud length A, its own overall length L and its own hex S1, so the correct wrench and the correct seal are fixed by the part number rather than chosen at the bench.

2. 4O Specifications

2.1 Connection 1 – SAE O-ring boss male stud end, ISO 11926-3 L series

Connection 1 is a SAE straight thread O-ring boss male stud end produced to ISO 11926-3, the light duty L series of the ISO 11926 adapter family. The same stud end is published as SAE J1926-3, so a drawing marked with either designation describes this geometry. Sizes in the printed range step from 5/16″X24 through 3/8″X24, 7/16″X20, 1/2″X24, 9/16″X18, 3/4″X16, 7/8″X14, 1.1/16″X12, 1.3/16″X12, 1.5/16″X12, 1.5/8″X12 and 1.7/8″X12 to the largest printed row.

The thread is straight, with a UNF or UN pitch as printed, and it is not a tapered pipe thread. The sealing element is an O-ring carried in a groove behind the thread on the stud end, and the O-ring is listed row by row in the printed chart under the code O902 through O932.

The mating port is a SAE straight thread O-ring boss port to ISO 11926-1, published as SAE J1926-1. That port has a straight thread and a machined recess on the face that the O-ring expands into when the stud end is seated. The recess is what makes the joint seal, so port face condition belongs to the ISO 11926 adapter specification as much as the thread does.

2.2 Connection 2 – Closed end

Connection 2 is a closed end, not a port. The body ends after the hex, and the part has no second thread, no through bore and no outlet. This is the defining difference between the 4O plug and a two-port SAE O-ring boss adapter: the plug terminates a circuit, an adapter continues it.

The body hex printed as S1 is the drive and the end feature at the same time. It is the wrench point for tightening and the outermost geometry of the part. Because the body hex is turned from the same bar as the stud end, the width across flats grows with the thread size, from 12 mm at the smallest printed row to 70 mm at the largest.

A closed end also means the part contributes nothing to flow. There is no restriction to consider and no internal passage to clean, so the only surfaces that matter on this port plug during installation are the thread, the O-ring groove and the shoulder face that lands on the port recess.

2.3 Construction and Materials

The body and the stud end are machined from bar stock on CNC lathes in one piece. The stud end is turned to the L series profile, the O-ring groove is cut to the standard position behind the thread, the hex flats are milled, and the inch thread is produced to the printed designation. Thread gauges and dimensional checks are applied batch by batch against the drawing before packing.

Material options are carbon steel, 304 stainless steel, 316 stainless steel and brass. Carbon steel is the general hydraulic choice and is supplied with trivalent chromium zinc plating; zinc-nickel plating is available where a higher corrosion class is needed. Stainless steel bodies are normally supplied unplated for marine, offshore and process service where the environment attacks a plated surface.

Seal material options are NBR, FKM and EPDM. The O-ring is the pressure barrier on this joint, so the compound has to be compatible with the fluid and the temperature of the system. A SAE O-ring boss plug with the correct thread but the wrong O-ring compound will fail on the seal, not on the thread.

2.4 Pressure and Temperature

The printed dimension chart for this series carries no pressure column. Working pressure for these sizes is confirmed per order by HENGHUA engineering, based on the thread size, sealing method and material selected. The service pressure must also sit inside the pressure class of the light duty port that the plug is closing, because an L series boss is a light duty connection.

Temperature capability follows the O-ring material as a material fact. NBR covers approximately -30°C to +100°C, FKM covers approximately -20°C to +200°C, and EPDM covers approximately -40°C to +120°C. Carbon steel with zinc plating suits general hydraulic service, and 304 or 316 stainless steel is used where corrosion resistance is the deciding factor.

The thread is not the variable that limits the joint. A straight thread carrying an O-ring sees the load evenly along the engagement, so the practical limits come from the O-ring compound, the condition of the port recess and the correct seating of the shoulder.

2.5 Part Number and Dimension Table

The table below is reproduced from the printed dimension chart for this series, exactly as printed.

Two printed anomalies are carried through exactly as they appear on the chart. The 4O-10 row prints A as 2.7, while the neighbouring rows print 11.1 and 15.1; the printed figure is 2.7 and should be confirmed with the supplier before it is quoted. The 4O-32 row prints the thread as 2.1/12″X12, which is a printing error in the source catalogue (see section 6).

What each column means:

• PART NO. – the full part number to quote, built from the 4O prefix and the dash size code.
• E – connection 1, the SAE straight thread O-ring boss stud end, printed as nominal size and threads per inch, for example 9/16″X18.
• O-RING E – the O-ring that belongs on the E end of that row, printed as the O-ring code O902 through O932.
• A – the stud length from the end face of the stud to the shoulder or hex face, in millimetres.
• L – the overall length of the plug, in millimetres.
• S1 – the width across flats of the body hex, in millimetres.

Dash size and thread chart for the printed range:

3. How to Read a 4O Plug Part Number

The 4O plug part number has two parts. The prefix 4O identifies the family: a blanking plug with a SAE O-ring boss L series stud end to ISO 11926-3. The dash code identifies the size, and its two digits correspond to the SAE boss dash number rather than to a millimetre value.

Read the printed row 4O-06 as a worked example. The prefix 4O places the part in the SAE O-ring boss plug range. The dash code 06 gives a stud end thread of 9/16″X18, an O-ring of O906 on that end, a stud length A of 9.9 mm, an overall length L of 17.5 mm and a body hex S1 of 17 mm. A buyer with a size 6 SAE O-ring boss port to blank can order 4O-06 directly and know the wrench size before the hydraulic plug arrives.

Dash numbers follow the SAE sizing convention, so the same dash number appears across the ISO 11926 adapter family: -04 is a 7/16″ boss, -06 is a 9/16″, -08 is a 3/4″ and -12 is a 1.1/16″ boss. That consistency is useful when a manifold carries a mix of SAE O-ring boss ports, because one dash number maps to one boss size across the range.

Two rows need attention when the part number is written on a purchase order. The dash 10 row prints A as 2.7, so that dimension is confirmed with the supplier rather than assumed. The dash 32 row is the largest size in the printed range and its printed thread cell contains an obvious typographical error, which is dealt with in section 6. Orders for either row are confirmed against the drawing.

4. Installation and Fitment

A 4O plug seals on the O-ring, and the thread only clamps. That single rule explains every installation requirement for a SAE adapter fitting. Thread sealant, PTFE tape or a bonded washer must never be used instead of the O-ring on a SAE O-ring boss stud end, because the seal has to form in the port recess, not in the thread.

Assembly steps:

1. Confirm the port is a SAE straight thread O-ring boss port to ISO 11926-1 and that the dash size on the drawing matches the part number.
2. Clean the port thread and the recess, and remove any old sealant, swarf or damaged O-ring material from the recess. A scored recess will leak regardless of the part fitted.
3. Fit a new O-ring of the correct code and compound into the stud end groove, and check that it is seated fully and not twisted.
4. Lubricate the O-ring and thread lightly with clean system fluid so the elastomer slides into the recess instead of rolling out of the groove.
5. Start the plug by hand, keeping the body square to the port, and turn it until it is finger tight.
6. Tighten with a wrench that fits the body hex S1 for that row, bringing the plug down until the shoulder lands evenly on the port face. Torque is judged in words, not quoted as a figure: the plug is seated firmly with a steady pull and no forcing, and the body must not be driven past the point where the shoulder stops against the port.

Common fitment errors and their symptoms:

• No O-ring on the stud end: the port weeps along the thread and the leak continues no matter how hard this hydraulic plug is tightened.
• Sealant used instead of the O-ring: the plug stops short of full engagement, the shoulder never seats on the recess, and the joint leaks.
• Wrong dash size or wrong series: the stud end enters the port but bottoms out early, or the thread engages with a slack feel and never holds.
• Damaged or dirty port recess: the O-ring cannot seal on a scored face, so seepage appears immediately after assembly.
• Over tightening with the wrong wrench: the hex corners round off and the plug has to be removed by other means.

Reuse practice: the steel body may be reused if the thread, the shoulder and the groove are undamaged, but the O-ring is replaced at every removal.

5. Applications and Industries

The 4O plug blanks SAE O-ring boss ports on machines that are built in variants, shipped with spare ports, or repaired in the field. Application tags for the series are hydraulic systems, water jetting, oil and gas, industrial equipment, construction machinery, agriculture, marine and material handling.

Four scenarios show where the part earns its place. On agricultural equipment, a tractor shipped without an auxiliary hydraulic option has its SAE boss ports closed with these plugs, and the dealer removes a plug when the option is fitted in the field. On construction machinery, a valve block with more ports than the standard circuit uses is blanked at the factory, which protects the internal passages from contamination during storage and transport. In industrial equipment, a hydraulic power unit that shares one manifold across several machine models is configured by fitting or removing plugs rather than by stocking different manifolds. In oil and gas service, a stainless steel body with an FKM O-ring blanks a boss on a skid that sees washdown, weather and a wide temperature range.

The series also suits maintenance work. When a gauge, a test point or a diagnostic line is removed from a SAE boss port, the correct dash size port plug restores the port to a sealed condition without disturbing the manifold.

6. Replacement and Cross-Reference Guide

Cross-reference for this series is organised by standard and sealing method, never by brand. The correct replacement part is identified by the thread, the seal type and the seat geometry, and by nothing else.

Measure these three things:

1. The thread. Measure the outside diameter of the stud end and count the threads per inch, or read the dash size from the port drawing. A 9/16″X18 marking is a size 6 SAE straight thread, and the pitch of 18 threads per inch distinguishes it from the 9/16″X18 metric or BSP families that share the nominal diameter but not the thread form.
2. The seal. Confirm that the port has a machined recess on its face and that the part carries an O-ring in a groove behind the thread. The O-ring code printed for each dash size, O902 through O932, must match the port series.
3. The seat and the hex. Check the stud length A and the overall length L against the port depth, and check the body hex S1 so the correct wrench is on hand.

Two cross-reference points are specific to this chart. First, the L series and the S series SAE boss stud ends are different: ISO 11926-3 is the light duty series used here, and the heavy duty SAE boss stud end has a longer stud and a different groove position. The two are not interchangeable in the same port even though both are SAE straight thread O-ring boss parts.

Second, the largest row needs a decision before ordering. The 4O-32 row prints the thread as 2.1/12″X12, which is a typographical error in the source catalogue; the dash 32 SAE O-ring boss thread is 2-1/2″-12, and that is the thread the row represents. Buyers who need the dash 32 size should confirm the row with the supplier, and confirm the port recess dimensions against the drawing at the same time, so the correct part is quoted for a large boss port.

A third point applies across families rather than within one. A SAE adapter will not seal a metric ISO 6149 port or a G (BSPP) ISO 1179 port, because the thread angle, the pitch and the recess differ. Matching the nominal diameter is not enough, so match the standard first, then the size.

7. Ordering, Packaging and Delivery

To order, quote the series code, the full part number, the thread size, the seal material, the body material and plating, and the quantity. A complete line reads: series 4O, part number 4O-08, thread 3/4″X16, NBR O-ring, carbon steel with trivalent chromium zinc plating, quantity required. For the dash 10 and dash 32 rows, add a note that the printed dimension or thread cell is to be confirmed, and the quotation will state the confirmed figure.

Packaging supports two routes to market. Distributor orders ship in labelled cartons with the part number and the dash size printed on the outside, ready for shelf or bin storage. Direct-to-line OEM orders ship in the consumption sequence agreed with the customer, so parts can be issued to assembly without re-sorting.

Samples are available for validation, produced with the same material and seal compound as the intended production order. Lead time is confirmed per order and stated in writing on the order acknowledgement, because body material, plating and seal compound all affect the schedule.

8. FAQ

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

A SAE O-ring boss joint, such as the one closed by a 4O plug, seals on an O-ring compressed in a machined recess on the port face, and the straight thread only clamps the stud end down. A tapered pipe thread seals by interference between the thread flanks, which needs a sealant and deforms the thread itself. The two are different port preparations, and a straight thread boss cannot be sealed by wrapping the thread.

Can BSP and NPT be interchanged?

No. G (BSPP) threads to ISO 228-1 and NPT threads have different thread forms and different pitches, so they will not seal together even at the same nominal size. The same rule holds inside the O-ring families: an ISO 11926 SAE boss port needs a SAE O-ring boss stud end, an ISO 6149 metric port needs a metric O-ring stud end, and an ISO 1179 G port needs a G stud end. Read the thread designation, not the nominal diameter.

How do I identify the correct 4O size?

Read the dash size from the port or measure the stud end outside diameter and the threads per inch, then match the pair to the E column of the printed chart. Confirm that the port has a recessed O-ring seat, because that identifies a SAE O-ring boss port. Then read across the row for the O-ring code, the stud length A, the overall length L and the body hex S1.

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

NBR suits mineral hydraulic oil over approximately -30°C to +100°C and is the usual default. FKM covers approximately -20°C to +200°C and many aggressive fluids, which makes it the choice for high temperature duty. EPDM covers approximately -40°C to +120°C and is used with water, steam and many brake fluids, but it is not compatible with mineral oil. On this series the O-ring is the seal, so the compound decision is the critical one.

How is the working pressure of this fitting determined?

No pressure column is printed for this series and no per-size rating is stated on the page. Working pressure for these sizes is confirmed per order by HENGHUA engineering, based on the thread size, sealing method and material selected. The working pressure of the machine must also stay inside the pressure class of the light duty boss port being blanked.

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

Minimum order quantity is confirmed per part number and material combination with the quotation. Samples are available for validation and are made with the same material and seal compound as the production order, so the test carries over. Include the part number, the material and the seal choice in the sample request.

Why does the 4O-10 row show such a small A dimension?

The printed chart shows A as 2.7 for 4O-10 while the rows around it show 11.1 and 15.1, and the value is reproduced here exactly as printed. It appears to be a printing anomaly in the source chart, so it is flagged rather than corrected. Before quoting or manufacturing that row, confirm the stud length with the supplier.

Is the 4O-32 thread really 2.1/12 inches?

No. The 4O-32 row prints the thread as 2.1/12″X12, which is a typographical error in the source catalogue. The dash 32 SAE O-ring boss thread is 2-1/2″-12, and every other page of this range prints that thread in the conventional form. The cell is reproduced exactly as printed, and buyers ordering the dash 32 size should confirm the row so the largest boss in the range is quoted correctly.

HENGHUA manufactures the 4O plug range in SAE O-ring boss L series ISO 11926-3 geometry, from the small 5/16″X24 size to the large dash 32 boss, in carbon steel, stainless steel and brass, with NBR, FKM or EPDM seals. Send the dash size, the material, the seal compound and the quantity you need, and we will confirm the part number and the lead time for your programme.

Related series in the same family: the 4ON hollow hex plug and the 10-OG plug. 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.