Choosing a port and a matching metric stud fitting can be confusing. While there are two popular types of metric parallel threaded ports, there are many different sealing methods for the mating male threaded connections. This post should clear up the confusion by explaining the differences in these connections, as well as some of the common misconceptions.
The two popular metric ports are ISO 6149-1 and DIN 3852-1. DIN-3852-1 is the same as ISO 9974-1. The ISO 6149-1 port style has a gland area at the top of the threads for accommodating an O-ring. The DIN 3852-1 / ISO 9974-1 port style, on the other hand, has threads coming all the way up to the surface of the port. Of course, both ports have metric threads and, in most cases, the same thread size and pitch for a given size fitting. All of which adds to the confusion.
In order to get a better understanding, let’s get into more details on both metric ports.
As mentioned, the ISO 6149-1 port style has a gland area at the top of the threads for accommodating an O-ring.
Figure 1 – ISO 6149-1 gland area
To help users identify this port style, the ISO 6149-1 standard specifies that this port is to be clearly identified by having any of the following:
Figure 2 – Examples of ISO 6149-1 port identifying markers
However, some manufacturers ignore these standard requirements, contributing to the confusion for users. The purpose of these marks is to help with identification. If this portion of the standard is ignored, it can be very difficult to spot the difference. For example, the SAE ORB port (per SAE J1926-1 or ISO 11926-1) and ISO 6149-1 port look exactly alike at a quick view. The difference is that one has metric threads while the other has UN/UNF threads.
The adapter fitting for connecting to the ISO 6149-1 port can only be of two styles, and each should also be clearly identified. If it is a fixed stud (non-adjustable) end, then there should be an identification groove on the port end as shown in Figure 3a. See an example of a O-ring Face Seal (ORFS) fitting with an ISO 6149-1 port end.
Figure 3a– ISO 6149-1 fixed stud end identifying marker
If it is an adjustable style fitting, then the locknut would have a turned diameter, facing the washer, as shown in Figure 3b.
Figure 3b – ISO 6149-1 adjustable stud end identifying marker
These identification marks will still be clearly visible, even once the fitting is fully assembled. When servicing is required and replacement parts are needed, these identifications become extremely important. Both the non-adjustable and adjustable styles use a single, round cross section O-ring and, when properly assembled, the O-ring is squeezed down into the gland of the port thereby creating a very effective seal. Fig 4 shows how these two styles of fittings connect to the port.
Figure 4 – ISO 6149-1 combinations
The second port style is referred to as DIN 3852-1 or ISO 9974-1. This is generally referred to as a spotface seal because sealing takes place on the surface of the port. The threads on this style come all the way to the surface as seen in Figure 5. There is a 45-degree chamfer for cleaning up sharp threads, but the chamfer does not play any role in the sealing process.
Figure 5 – DIN 3852-1 / ISO 9974-1
The common issue for this style of port is the chamfer being made too large. The correct dimensions for this type of seal can be found in Table 1 of DIN 3852-1 / ISO 9974-1. When in doubt, get your calipers out! A chamfer that is too large will take away valuable surface area, that would prevent the sealing element from being properly compressed. There are various styles of sealing for the mating male fitting of this style port. Some use an elastomer seal, while others may use a metal to metal seal. The styles are differentiated by letters in the pertinent standards. While each of them serves their own purpose, some are more reliable than others. There is significant explanation needed to detail why some styles are better than others. These will be outlined in a separate post soon.
The types of sealing styles shown in DIN-3852-1 / ISO 9974 are below:
1) Type E - For fixed stud ends only. These use a trapezoidal-sectioned elastomeric seal which is embedded into a groove that’s cut into the stud end of the adapter fitting. See an example of an O-ring Face Seal (ORFS) fitting with a Type E port end.
2) Type G and Type H – utilizes an O-ring and retaining ring for fixed and adjustable stud ends. See an example of a 37-degree flare fitting with a Type H port end.
3) Type D – Applies to fixed studs only. These styles utilize a seal with an elastomer bonded to the ID of a metal ring
4) Type A – Applies to fixed studs only. This style utilizes a soft metal seal, typically made of a soft material such as copper
5) Type B - Applies to fixed studs only. This style uses a cutting face seal as a direct metal to metal seal between the adapter fitting and the face of the port. A slight ridge of material on the fitting coins the surface of the port to create the seal
As stated earlier, some of these use an elastomer for sealing, while others are metal to metal seals. Regardless, they are all interchangeable in the DIN 3852-1 / ISO 9974-1 style metric port. Figure 6 summarizes different fitting styles that can be used with this port.
Figure 6 – DIN 3852-1 / ISO 9974-1 combinations
The most important thing to remember about this style is that it seals on the spotface of the port. The styles with an elastomer seal must have a means of creating a cavity for retaining the seal material so it can seal properly. For example, looking at the O-ring and retaining ring fitting style, a cavity is created by the port, the fitting, and the metal retaining ring. This cavity helps to ensure proper compression of the O-ring, creating a leak-free connection. A common issue for this style is a missing retaining ring – it might have fallen off or wasn’t seen as being necessary. Without the retaining ring, the O-ring would be performing like a gasket, and will easily be pushed out when the system is pressurized and cause a leak.
Pro Tip: If you are replacing an O-ring/Retaining ring fitting style, check to make sure that the previous retaining ring came off with the fitting when it was removed. Retaining rings can sometimes stick to the surface, causing two rings to be placed on top of each other upon reassembly. Double retaining rings do not make for a good seal because the cavity will now be too large for properly compressing the O-ring.
As another example, the cutting face seal style is a metal to metal seal. It is often considered to be unreliable because it actually seals by coining the spotface of the port. Coining damages the sealing surface and may cause the port to have limited reusability. That said, even though it might not be as reliable as the elastomeric styles, it does have the advantage of not being limited in cases of extreme temperature conditions like the elastomeric seal.
The ISO 6149-1 and the DIN 3852-1 / ISO 9974-1 port styles have some common thread sizes. This is probably what leads to the belief that all metric fittings are interchangeable among these ports. This misconception can lead to leaking connections. Some of the more common mistakes are outlined below:
1) Using an DIN 3852-1 / ISO 9974-1 O-ring/Retaining ring style in a ISO-6149-1 port.
In the case of the DIN 3852-1 / ISO 9974-1 O-ring and retaining ring style fitting and the ISO 6149-1 fitting with O-ring, for example, the O-rings are actually different sizes. With the proper O-ring (and without the retaining ring), the O-ring and retaining ring style fitting will function in the ISO 6149-1 port. The issue is that you would now lose the traceability because there would not be any external identification mark to indicate that it is an ISO 6149-1 assembly. This may cause problems when the fitting connection requires servicing and is possibly misidentified as a result.
2) Adding an O-ring to a DIN 3852-1 / ISO 9974-1 Type E fitting for use in a ISO 6149-1 port.
The belief that an O-ring can be added to the DIN 3852-1 / ISO 9974-1 Type E style (which has a trapezoidal elastomeric seal embedded in a groove in the stud end) and then assembled into an ISO 6149-1 port. Though the threads are the same, this will not provide a reliable seal. No sealing method should ever have two elastomers coming into direct contact with each other. The two elastomer surfaces coming in contact may allow fluid to seep through.
Figure 7 - Double seal misconception example
3) Using a round cross sectioned O-ring in the place of a trapezoidal cross section O-ring in an ISO 6149 port.
Another misconception is that the DIN 3852-1 / ISO 9974 Type E style trapezoidal cross section seal is interchangeable with the round cross section O-ring. The Type E fitting is designed to accommodate the trapezoidal seal and not the round cross section O-ring. The Type E style port is designed for spotface sealing. Therefore, if a round cross sectioned O-ring was added instead of the trapezoidal elastomer, the face would not be able to compress the O-ring completely into the gland of the ISO 6149-1 port. This would cause a gap between the two sealing surfaces creating a connection that leaks.
4) Using a round cross sectioned O-ring in the place of a trapezoidal cross section O-ring in an DIN 3852-1 / ISO 9974-1 port.
This is similar to the third example above except the groove in this fitting was designed for the volume of a trapezoidal seal. Placing a round cross section O-ring in this groove would not fill the groove correctly, causing a potential leak.
There is a significant amount of knowledge that is behind how these fittings are designed to function. Understanding the content in this post should clear up some of the confusion about how these two styles of ports differ. This can also act as a quick reference piece for you and your co-workers when questions arise. With these differences now made clear, decisions about which metric fitting goes with which metric port can be made with confidence.
If you have any questions or comments, please post them and we will respond if warranted. To talk to our techConnect engineer team directly, they can be reached at Parker Tube Fittings Division, 614.279.7070. See Parker Tube Fittings Division's product line offering now.
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Contributed by Burleigh Bailey, senior project engineer at Parker Tube Fittings Division, and
Contributed by Emily Alexander, senior design engineer at Parker Tube Fittings Division
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