DIN 4-Bolt Hydraulic Flanges: Form R, Form S, and Fit

DIN 4-bolt hydraulic flanges are international flange connections used where a flanged head, O-ring, and four-bolt port pattern are required. DIN refers to the German standards body, and these flanges are often compared with SAE Code 61 and SAE Code 62 styles.

The basic connection uses an unthreaded female port with four bolt holes at the corners. The male flanged head aligns with those holes and includes a recessed groove for an O-ring. The O-ring seals against the flat port face while the bolts hold the flange head to the mating flange or port surface.

How the DIN 4-Bolt Connection Seals

The seal is made at the O-ring face, not by thread interference. That makes the condition of the groove, O-ring, and flat port face important. The bolts provide the clamping load that keeps the flange head seated against the port.

Because the connection depends on both sealing geometry and bolt alignment, the flange should not be selected only by hose size or outside appearance. The bolt pattern, pressure class, and mating port have to match.

Form R and Form S Pressure Classes

DIN 4-bolt flanges are commonly discussed in two pressure groups. Form R is similar to SAE Code 61. Form S is the heavier-duty series and is similar to SAE Code 62.

Depending on the flange standard and size, either metric or inch bolts may be encountered. DIN 20066 4-bolt flanges are also commonly compared with JIS B 8363 and SAE J518 patterns, so replacement work should confirm the actual standard instead of relying on the general DIN label.

Code 61 and Code 62 Comparison

Code 61 flanges are commonly associated with working pressures up to about 3,000 PSI. Code 62 is the high-pressure series and is commonly associated with ratings up to about 6,000 PSI in all sizes.

Those pressure classes are not interchangeable simply because the parts look similar. A replacement flange should match the form, bolt pattern, pressure rating, O-ring size, port face, and bolt type used by the system.

Replacement Checks

Before replacing a DIN 4-bolt flange, confirm whether the connection is Form R or Form S, whether the system references DIN 20066, JIS B 8363, SAE J518, Code 61, or Code 62, and whether the bolts are metric or inch. The correct part should seat cleanly without filing, drilling, or forcing the holes into alignment.