API 6A Flange Dimensions, Pressure Ratings & Ring Gasket Chart

Goldenman API 6A Wellhead Connection Guide

Use this guide to compare API 6A oilfield flange sizes, working-pressure classes, 6B/6BX connection logic, ring-gasket selection and the Goldenman wellhead equipment that uses these high-pressure connections.

API 6A flange selection is not based on nominal bore alone. A correct connection requires the nominal bore, rated working pressure, connector type, flange or studded configuration, ring groove, gasket number, bolting, material class, temperature class and service condition to agree with the mating equipment.

Goldenman supplies API 6A wellhead equipment, including casing heads, casing spools, tubing heads, Christmas trees, FC/FLS gate valves and frac heads, together with project-specific pressure-control connections and replacement support.

Quick answer: API 6A is the product specification for wellhead and tree equipment. Goldenman wellhead products currently cover flange sizes from 2-1/16 to 21-1/4 inches and working-pressure classes from 2,000 to 15,000 psi, depending on the equipment and project configuration. Exact flange OD, bolt circle, stud size and ring-groove dimensions must be verified against the current API 6A sizing tables and the approved order-specific drawing.

Important engineering notice: This page is a procurement and compatibility guide, not a substitute for the controlled API 6A dimensional standard. Do not manufacture, modify or pressure-test a flange from a website chart alone. Use the current API Spec 6A edition, approved manufacturing drawing, connection gauges and project specification for final dimensions and acceptance.

API 6A Flange Size and Pressure Rating Chart for Oilfield Equipment

The table below is a Goldenman application matrix. It helps buyers identify which Goldenman product family commonly uses a given API 6A bore and pressure range. It is intentionally not presented as a manufacturing-dimension table: the actual flange outside diameter, bolt circle, stud pattern and ring-groove dimensions must be taken from the controlled standard and the approved drawing.

Nominal Bore / Flange Size	Goldenman Published Pressure Coverage	Typical Goldenman Equipment	Connection Notes
2-1/16 in	5K / 10K / 15K on FC/FLS gate-valve range; broader wellhead configurations are project-specific	Gate Valve, Casing/Spool Side Outlet, Christmas Tree	Available as API flanged or studded connection on selected equipment
2-9/16 in	5K / 10K / 15K on FC/FLS gate-valve range	Gate Valve, Wellhead Side Outlet, Christmas Tree	Confirm bore, pressure, ring groove and end style together
3-1/8 in	5K on the published FC/FLS gate-valve range	Gate Valve, Low-/Medium-Pressure Wellhead Connection	Do not confuse 3-1/8 in 5K with 3-1/16 in 10K/15K configurations
3-1/16 in	10K / 15K on the published FC/FLS gate-valve and frac-head outlet ranges	Gate Valve, Frac Head Side Outlet, High-Pressure Tree	A common high-pressure API 6A connection size
4-1/16 in	5K / 10K / 15K on gate valves; 10K / 15K on frac-head main bore and outlets	Gate Valve, Frac Head, Christmas Tree, Wellhead Spool	Pressure class changes the required mating connection geometry
5-1/8 in	10K / 15K on Goldenman frac-head range	Frac Head / Goat Head, High-Pressure Completion Equipment	Confirm top, bottom and side-outlet connections separately
7-1/16 in	10K / 15K on Goldenman frac-head range; also used in wellhead/spool systems	Frac Head, Wellhead Spool, Tree/Completion Interface	Large-bore pressure-control connection; verify adjacent equipment standard
11 in	Within Goldenman 2K–15K casing-head/spool product range, project-specific	Casing Head / Casing Spool	Common large wellhead top-flange size; exact pressure and flange type must be specified
13-5/8 in	Within Goldenman 2K–15K casing-head/spool product range, project-specific	Casing Head, Casing Spool, Wellhead/BOP Interface	Adjacent BOP equipment may fall under API 16A rather than API 6A
16-3/4 in	Goldenman casing-head/spool range, project-specific	Large-Bore Casing Head / Spool	Confirm casing program, suspended load and top-connection design
20-1/4 / 20-3/4 / 21-1/4 in	Goldenman casing-head/spool range, project-specific	Surface Casing Head / Large Wellhead Stack	Large-bore configurations require drawing-level confirmation

“K” means thousands of psi: 5K = 5,000 psi, 10K = 10,000 psi and 15K = 15,000 psi. Pressure availability is equipment-specific; the table does not imply that every size exists at every pressure class.

What Dimensions Define an API 6A Flange?

When engineers search for “API 6A flange dimensions,” they are usually trying to identify more than one measurement. A complete dimensional check normally includes the following fields.

Dimensional Field	Why It Matters	Typical Procurement Risk
Nominal Bore	Defines the nominal flow opening and connection family	Ordering by bore alone ignores pressure class and flange type
Rated Working Pressure	Determines the pressure class of the connection and equipment	Same bore at different pressure ratings is not automatically interchangeable
Flange Outside Diameter	Confirms external geometry and installation envelope	Clearance conflict or wrong mating flange
Bolt / Stud Circle	Controls bolting alignment	Stud holes do not line up even when bore appears correct
Number of Studs / Holes	Part of the pressure-rated bolting pattern	Incorrect bolting pattern prevents assembly
Stud Diameter and Length	Required for the specified connection and stack geometry	Wrong bolt material, diameter or engagement can invalidate the assembly
Ring Groove Type / Number	Defines the metal ring-joint sealing interface	Wrong ring cannot seal correctly
Flanged or Studded Configuration	Changes how the equipment is assembled and its stack envelope	Replacement equipment cannot be installed as expected

Example: “4-1/16 in 10K” is not a complete purchase description. It identifies a nominal bore and working-pressure class, but the buyer still needs to confirm the API connector type, flanged or studded construction, ring groove and gasket, stud arrangement, material class, temperature class, PSL/PR requirement and service condition.

API 6A Pressure Ratings: 2K, 3K, 5K, 10K and 15K

Oilfield buyers often use abbreviated pressure classes. The following conversion table is useful when an RFQ mixes psi and MPa units.

Oilfield Pressure Class	Working Pressure, psi	Approx. MPa	Goldenman Product Examples
2K	2,000	13.8	Selected casing-head and spool configurations
3K	3,000	20.7	Selected wellhead/spool configurations
5K	5,000	34.5	FC/FLS gate valves, wellhead equipment
10K	10,000	69.0	FC/FLS gate valves, frac heads, high-pressure wellhead equipment
15K	15,000	103.4	FC/FLS gate valves, frac heads, high-pressure wellhead equipment

Pressure class is not a universal interchangeability code. Two components both marked 10,000 psi can still have different bores, end connections, ring grooves, material classes or governing equipment standards.

API 6B vs 6BX Flanges: What Buyers Need to Know

API 6A includes 6B and 6BX flanged and studded connector families. These names describe controlled connector systems; they should not be treated as generic pipe flanges. The correct connector family depends on the API 6A size and rated working pressure specified for the equipment.

6B Connector Family

Check the Exact API Size and Pressure Class

The 6B designation is only one part of the connection definition. Mating parts still require matching nominal size, rated working pressure, ring groove, bolting and equipment drawing.

6BX Connector Family

Do Not Assume Same Bore Means Same Connection

6BX connections use their own controlled geometry and matching ring-joint arrangement. An apparently similar bore does not make a 6B and 6BX connector interchangeable.

Compatibility rule: Never force a 6B/6BX decision from bore size alone. Confirm the marked connection, pressure class, ring-groove type and approved drawing. If the upper and lower connections differ, a purpose-designed adapter flange or spool may be required.

API Ring Gaskets: R, RX and BX Selection Logic

API oilfield flanged connections use metal ring-joint sealing systems. The buyer must match the ring type and ring number to the actual flange groove. A ring with the wrong profile or number is not an acceptable substitute simply because it appears to fit.

Ring Type	Selection Basis	Buyer Must Confirm	Common Error
R	Matching API ring-joint groove and connection specification	Ring number, material, groove condition and service	Ordering only by nominal flange size
RX	Matching RX-compatible ring groove and specified flange system	Exact ring number and flange pressure class	Assuming every R groove accepts the same RX ring
BX	Matching BX groove in the specified high-pressure connector system	BX number, material, flange type and groove condition	Substituting a visually similar ring

For a replacement gasket request, provide the existing ring number if legible, the equipment type, nominal bore, pressure class and clear photos of the ring groove. For sour or corrosive service, also provide the required material and H₂S/CO₂ conditions.

Where API 6A Flanges Are Used in Goldenman Oilfield Equipment

The commercial value of an API flange chart is not the flange itself—it is identifying the equipment that must connect correctly around the flange. Goldenman uses API pressure connections across the following wellhead product groups.

Equipment	Goldenman Published Range	Why the Flange Matters	Product Link
Casing Head / Casing Spool	2-1/16–21-1/4 in connection range; 2K–15K product family	Connects wellhead stages, casing hangers, side outlets and pressure-control equipment	View Casing Heads & Spools
FC / FLS Gate Valve	2-1/16–4-1/16 in; 5K–15K	End connection must match the tree, spool or manifold	View FC/FLS Gate Valves
Frac Head / Goat Head	4-1/16–7-1/16 in main bore; 10K–15K; 3-1/16 and 4-1/16 in side outlets	Top, bottom and side connections must match frac tree and flowline interfaces	View Frac Heads
Christmas Tree / Wellhead System	Project-configured API 6A assembly	Multiple pressure-containing components must have compatible bores, pressure ratings and end connections	View Wellhead Equipment

What if the two connections do not match?

When the upper and lower equipment have different nominal sizes, pressure classes or connector standards, the solution may require a purpose-designed adapter flange, crossover connector, casing spool or drilling spool rather than a simple “same-size” flange.

View Drilling Spool & Adapter Flange

How to Select an API 6A Flange Connection

Identify the equipment. State whether the connection belongs to a casing head, tubing head, Christmas tree, gate valve, frac head, adapter or another pressure-control component.
Confirm nominal bore. Use the equipment marking or approved drawing rather than visual estimation.
Confirm rated working pressure. Do not assume a 5K and 10K connection with the same bore have the same geometry.
Confirm connector family. Determine the applicable API flange/studded type and whether the mating equipment follows API 6A or another equipment standard.
Confirm ring groove and gasket. Record the ring type and exact ring number.
Confirm bolting. Verify stud diameter, material, quantity, length and required engagement.
Define service conditions. Include temperature class, material class, PSL/PR requirements and standard or H₂S service.
Check the complete stack. Confirm adjacent valve, spool, tree and BOP interfaces before approving the final drawing.

How to Identify an Existing API Oilfield Flange for Replacement

A replacement request is much faster when the buyer sends both markings and measurements. For used equipment, do not rely on paint color, outside appearance or an old quotation alone.

Send Goldenman these measurements and photos:

• Equipment type and manufacturer

• Nameplate and API markings

• Nominal bore

• Working-pressure marking

• Flange outside diameter

• Bolt/stud circle

• Number of stud holes

• Stud diameter

• Ring groove photo

• Existing gasket number

• Face-to-face / stack height

• Upper and lower mating equipment

Common API 6A Flange Selection Mistakes

Ordering by Bore Only

“4-1/16 flange” is incomplete. Pressure, connector family, ring groove and service requirements still have to be defined.

Treating 3-1/8 and 3-1/16 as the Same

These nominal sizes appear in different pressure configurations. Do not round one to the other during replacement purchasing.

Using the Wrong Ring Gasket

Ring type and ring number must match the groove. A visually similar metal ring is not an acceptable substitute.

Mixing API 6A and General-Purpose Flanges

API wellhead connectors should not be replaced by an unrelated ASME/ANSI flange merely because the nominal bore is similar.

Ignoring the Adjacent Equipment Standard

A wellhead-to-BOP interface can involve equipment governed by different API product specifications. Confirm the complete pressure-control stack.

Copying an Old Drawing Without Wear Inspection

Used ring grooves, faces and bolting should be inspected before a replacement connection is approved for service.

Information Required for an API 6A Flange or Wellhead Connection RFQ

A complete RFQ should describe the pressure-control system, not just the word “flange.” Send the following information for dimensional review and quotation.

• Equipment type

• Nominal bore / connection size

• Rated working pressure

• Flanged or studded connection

• Upper mating connection

• Lower mating connection

• Ring groove type / number

• Required ring gasket

• PSL requirement

• PR requirement

• Material class

• Temperature class

• Standard or H₂S service

• H₂S / CO₂ conditions

• Existing drawing or part number

• Required quantity

• Third-party inspection

• Destination and delivery date

Frequently Asked Questions

What is an API 6A flange?

An API 6A flange is a pressure-rated connector used on wellhead and tree equipment manufactured within the API 6A product system. Selection requires a defined nominal size, rated working pressure and matching ring-joint and bolting arrangement.

Are all 4-1/16-inch API flanges interchangeable?

No. A 4-1/16-inch bore can appear in different pressure and equipment configurations. The rated pressure, connector family, ring groove, stud pattern and mating equipment must all be verified.

What does 10K mean on API wellhead equipment?

10K means a rated working pressure of 10,000 psi, approximately 69 MPa. It does not by itself define the bore size, flange dimensions or ring gasket.

What is the difference between 3-1/8 and 3-1/16 API connections?

They are different nominal sizes and should not be treated as the same connection. In Goldenman’s current FC/FLS gate-valve range, 3-1/8-inch appears in 5K configurations while 3-1/16-inch appears in 10K and 15K configurations.

Can a 6B flange connect directly to a 6BX flange?

Do not assume direct interchangeability. The API connector family, nominal size, pressure class, ring groove and bolting must match the approved design. A crossover or adapter may be required when the connections differ.

How do I choose between R, RX and BX ring gaskets?

Choose the ring gasket from the exact flange groove specification and ring number. Do not select from pressure alone. The flange size, connector family, groove and service material must be confirmed together.

Can an API ring gasket be selected from the old ring alone?

The old ring number is helpful, but the mating flange groove and equipment markings should also be checked. Damaged, corroded or incorrectly identified rings can lead to a wrong replacement selection.

Is an API 6A flange the same as an ANSI or ASME pipe flange?

No. API 6A wellhead connectors belong to a different equipment and pressure-control system. Do not substitute an unrelated general-purpose piping flange based only on similar nominal bore.

What should I send Goldenman for a replacement flange or spool?

Send the equipment type, nominal size, pressure class, markings, flange OD, stud pattern, ring-groove information, upper and lower mating connections, existing drawings, service conditions and clear photographs.

Need an API 6A Connection Compatibility Check?

Send Goldenman the equipment type, nominal bore, working pressure, upper and lower connections, ring-groove information and existing drawing or photos. Our team can review the connection and prepare an order-specific quotation for wellhead equipment, gate valves, frac heads, spools and adapter solutions.

Email: info@goldenman.com

Request a Technical Quote View Wellhead Equipment View Adapter Flange & Spool

Standards and data scope: This guide follows the API 6A wellhead-and-tree equipment context and Goldenman’s currently published product ranges. API 6A dimensional requirements are controlled by the current official standard and approved manufacturing documents. Goldenman product links on this page provide project-specific ranges and quotation requirements.