A Practical Guide to Orbital Hydraulic Motor Port Standards

When replacing an orbital hydraulic motor, most buyers naturally start with the displacement, mounting dimensions, shaft, and torque requirements. These are important, but there is another detail that often causes problems during installation: the hydraulic ports.

A motor can have the correct displacement and mounting dimensions and still not be a direct replacement if the port threads or sealing method are different.

This is particularly common when replacing a motor on existing equipment. The original motor may use G/BSPP ports, while the replacement is supplied with Metric or SAE ports. The difference may not be obvious until the hose fitting is installed.

For this reason, the port configuration should be confirmed before the motor is ordered.

Three Common Port Standards

The three port systems most commonly encountered on orbital hydraulic motors are G/BSPP, Metric, and SAE/UNF.

At first glance, some of these sizes can look very similar. For example, G1/2, M20×1.5, and 7/8-14 UNF are all common hydraulic connection sizes, but they are based on different thread systems and should not be treated as interchangeable.

G / BSPP

G threads, commonly referred to as BSPP (British Standard Pipe Parallel), are widely found on hydraulic equipment in Europe, Asia, and other international markets.

The thread is parallel rather than tapered. Depending on the port design, sealing is normally achieved with an O-ring, bonded seal, or sealing washer rather than by the thread itself.

Typical sizes include:

G1/4

G3/8

G1/2

G3/4

G1

For example, a motor may be configured with G1/2 main ports and a G1/4 case drain port.

Metric Threads

Metric ports are also widely used, particularly on equipment designed around metric hydraulic components.

Common sizes include:

M14×1.5

M16×1.5

M18×1.5

M20×1.5

M22×1.5

The notation is relatively straightforward. In M20×1.5, "20" refers to the nominal thread diameter in millimeters, while "1.5" is the thread pitch.

Metric hydraulic connections can use different sealing arrangements, so identifying the thread size alone is not always enough. The sealing method and port design should also be checked.

SAE / UNF

SAE thread connections are particularly common on hydraulic equipment in the United States and Canada.

Typical UNF sizes found on hydraulic components include:

3/4-16 UNF

7/8-14 UNF

1-1/16-12 UNF

O-ring Boss (ORB) connections are commonly used with this type of straight thread connection.

For example, 7/8-14 UNF indicates a nominal 7/8-inch thread with 14 threads per inch.

The important point is that the thread designation alone does not tell the whole story. The fitting and sealing arrangement must also match the motor port.

Main Ports and Case Drain Ports

An orbital motor normally has two main working ports, commonly identified as A and B, and some models also have a case drain port.

The A and B ports carry the main hydraulic flow and working pressure. Reversing the direction of flow allows the motor to rotate in the opposite direction.

The case drain has a different purpose. It allows internal leakage oil from the motor housing to return to the reservoir and helps control pressure inside the motor housing.

Since the flow through the case drain is normally much lower than the main working flow, the drain port is generally smaller than the A/B ports.

For example:

Main ports: G1/2
Case drain: G1/4

This is a common configuration on certain orbital motor models.

One point is worth mentioning here: there is no universal rule that defines one specific drain-port size for every main-port size.

A G1/2 main port may be paired with a G1/4 drain port on one motor series, while another manufacturer's design may use a different combination. The same applies to Metric and SAE configurations.

The drain port should therefore be specified separately when ordering a motor.

Do Not Assume That Different Port Standards Can Be Interchanged

This is one of the most common mistakes when replacing hydraulic motors.

A G1/2 fitting cannot simply be replaced by an M20×1.5 fitting because the dimensions appear similar. Likewise, a 7/8-14 UNF connection should not be treated as equivalent to either of them.

The following four items need to match:

Thread type + thread size + sealing method + fitting design

A mismatch can result in damaged threads, poor sealing, oil leakage, or the need for an additional adapter.

In some applications an adapter is perfectly acceptable. For a direct replacement, however, using the same port configuration as the original motor is usually the simpler solution.

Port Standards by Market

There is no strict rule that one country uses only one hydraulic thread standard. Equipment manufacturers often use components from different suppliers, and imported machinery can bring another port system into the local market.

As a general reference:

Europe: G/BSPP and Metric are both common

China and other Asian markets: Metric and G/BSPP are widely used

United States: SAE/UNF connections are common

Canada: SAE/UNF is widely encountered, particularly on North American equipment

Other markets: G/BSPP, Metric, and SAE can all be found depending on the equipment and its origin

For replacement motors, the existing hydraulic system is a better reference than the country where the machine was sold.

What Should Be Checked Before Ordering?

Before placing an order, it is worth confirming the complete port configuration rather than simply asking for a "G1/2 motor" or an "SAE motor."

At minimum, check:

Main port standard and size

Case drain standard and size

Sealing method

Port position

Existing hose and fitting specifications

A technical drawing or a clear photo of the original motor ports can also be useful when the exact specification is uncertain.

Final Thoughts

Port compatibility is a small detail, but it can make a big difference when replacing an orbital hydraulic motor.

The safest approach is straightforward: match the replacement motor to the existing hydraulic circuit whenever possible.

Check the A/B ports, case drain, thread standard, thread size, sealing method, and port position before confirming the order. This can eliminate unnecessary adapters and make installation much easier once the motor reaches the machine.