API 6A Type 6B 5,000 psi Weld Neck Flange — Dimensions & R/RX Ring Numbers

API Specification 6A (aligned with ISO 10423) is the American Petroleum Institute's specification for wellhead and Christmas tree equipment. Among much else it defines the two flange families used on wellheads and trees — Type 6B and Type 6BX — with rated working pressures from 2,000 up to 20,000 psi. This page covers the Type 6B weld neck flanges rated 5,000 psi, the top rating of the 6B family.

Unlike an ASME class number, an API 6A rating is the actual rated working pressure: a 5,000 psi (≈345 bar) flange is designed to operate at that pressure within its temperature class. 5,000 psi is the highest rating in the Type 6B family — above it, API 6A moves entirely to the Type 6BX design with its pressure-energized BX gaskets.

6B is API 6A's ring-joint flange family in the tradition of the ASME RTJ: it seals on an R or RX metal ring gasket seated in a machined groove, and after makeup a standoff gap remains between the flange faces — the gasket, not the faces, carries the joint compression. Type 6B covers rated working pressures of 2,000, 3,000 and 5,000 psi in the regular wellhead bores.

Type 6B (2,000–5,000 psi) uses R or RX ring gaskets with a standoff gap between the faces after makeup. Type 6BX (2,000–20,000 psi) uses the pressure-energized BX gasket and makes up face-to-face, giving a stiffer, more compact joint — and it is the only type specified at 10,000 psi and above. At 5,000 psi both exist: 6B in the regular bores on this page, 6BX in its own size family.

Both fit the same flange groove, which is why the chart column reads 'R or RX'. The R ring is the classic oval or octagonal cross-section, compressed by bolt load. The RX ring is a pressure-energized redesign of the same nominal number — its asymmetric profile uses internal pressure to improve the seal, at the cost of a taller standoff. An RX-24 replaces an R-24 in the same groove without re-machining.

Because a 6B joint compresses the ring rather than the faces, a deliberate gap remains between the raised faces after correct makeup — closing it fully would mean the gasket is over-compressed or the wrong ring is fitted. Crews use the standoff to judge makeup, and it is why re-tightening a leaking 6B joint has limits: once the ring has yielded, it should be replaced, not crushed further.

Eight sizes: 2-1/16, 2-9/16, 3-1/8, 4-1/16, 5-1/8, 7-1/16, 9 and 11 inches — the regular wellhead bore family at this rating. Ring numbers run R/RX-24 through R/RX-54 as tabulated. For larger bores or higher pressures the specification moves to the Type 6BX family.

Size is the nominal bore in inches; all other values are millimetres. O is the flange outside diameter, C the thickness, K the raised face diameter, P the ring-groove pitch diameter, E the groove depth, X the hub diameter, BC the bolt circle, N the number of studs, H the bolt hole size, Y the length through hub, A the weld-end outside diameter (matching the pipe OD) and B2 the bore. The last column gives the R/RX gasket number.

They share a dimensional heritage: the 6B ratings correspond to the classic ASME ring-joint series — 2,000 psi with Class 600, 3,000 psi with Class 900 and 5,000 psi with Class 1500 — and the R-ring numbering is common with ASME B16.20. In overlapping sizes the flanges generally mate, but API 6A and ASME B16.5 are separate specifications with their own tolerances and requirements: confirm dimensions ring-for-ring and hole-for-hole before mixing systems on a joint.

API 6A defines material classes AA through HH: AA–CC for general service (carbon, low-alloy and stainless steels) and DD–HH for sour service in line with NACE MR0175 — the class sets minimum yield, impact and hardness requirements. Typical weld neck flange materials are AISI 4130/4140 low-alloy steel, F22, 410 and 17-4 stainless, and duplex grades, selected to the well's pressure, temperature and H2S conditions.

PSL — Product Specification Level, 1 through 4 — sets the depth of quality control: material testing, NDE, weld examination, traceability and documentation increase with each level; PSL 2 and 3 are the common wellhead specifications. Temperature classes (K through Y) define the design temperature envelope and drive the impact-test temperatures for the material. Both are order-level requirements alongside the material class.

Yes, when ordered in the sour-service material classes (DD, EE, FF or HH), which invoke NACE MR0175/ISO 15156 — controlled hardness, qualified materials and impact requirements. Sour-service production equipment commonly pairs a sour material class with PSL 2 or 3; we supply accordingly with full certification.

The N and H columns give the stud count and hole size per flange — the 2-1/16 in flange takes 8 studs in 26 mm holes and the 11 in flange 12 studs in 51 mm holes — on the bolt circle BC. High-strength stud bolts with heavy hex nuts are used; even preload matters on ring joints, since the gasket rather than the faces carries the compression.

On a wellhead joint the connection must be as strong and as inspectable as the pipe or spool it terminates. The weld neck's integral tapered hub and full-penetration butt weld deliver that — the weld is volumetrically examined and the whole load path is forged, with no threads or seal welds. Threaded and other end connections are limited to the low-pressure end of the API 6A range.

On wellheads and Christmas trees for conventional wells, tubing and casing spools, production and test manifolds, and flowline tie-ins — the broad middle of surface pressure-control duty where 5,000 psi covers the shut-in pressure. Above it, equipment steps to the 6BX 10,000/15,000/20,000 psi sizes further up the stack.

The Type 6B family ends at 5,000 psi. Higher ratings — 10,000, 15,000 and 20,000 psi — exist only as Type 6BX, with pressure-energized BX gaskets and face-to-face makeup. If your project mixes ratings, note that 6B and 6BX flanges of the same nominal size do not mate: the types have different dimensions, grooves and gaskets.

We manufacture to the API 6A Type 6B dimensional requirements from certified forgings, with dimensional inspection against the specification, EN 10204 3.1 mill certificates, heat-number traceability, PMI, hardness testing (including NACE limits for sour classes) and UT/MPI examination on request. Third-party witness inspection by LR, BV, DNV, SGS or TÜV is welcomed; project-specific PSL documentation is quoted case-by-case.

Size and quantity, type (6B) and rated working pressure (5,000 psi), end connection (weld neck — with pipe/spool wall for the weld prep), gasket preference (R or RX), API 6A material class (AA–HH), temperature class, PSL level, any NACE/sour requirement, and the certification and inspection you need. Tesco Steel & Engineering is an ISO 9001:2015 certified manufacturer and exporter in Mumbai, India.