Application Deep-Dive
Wellhead Spool Machining — Materials, Tolerances, and API 6A Adjacent Practice
Wellhead spools carry the connection between wellhead components — casing head, tubing head, adapters. They see pressure loading, sometimes sour service, and always require the documentation trail that API-adjacent work demands. This guide covers materials, tolerances, and the process.
What a wellhead spool is
A spool is a flanged component between two wellhead sections — used to add length, provide port access, or adapt between different flange sizes/ratings. Typically machined from forgings; flanges on both ends with a bore matching adjacent components.
Common in wellhead assemblies, tree stacks, and BOP-adjacent equipment. Rated for the pressure class of the wellhead (2K, 3K, 5K, 10K, 15K psi).
Materials
| Alloy | Where used |
|---|---|
| 4140 pre-hardened | Standard-service wellhead spools; most common baseline |
| F22 (2.25Cr-1Mo) | Higher-strength wellhead components; some sour-service |
| F91 (9Cr-1Mo-V) | High-temperature or advanced applications |
| 17-4 PH H1150 | Corrosion-resistant applications |
| Inconel 718 | Sour service (H2S) or extended-life critical applications |
| Super Duplex 2507 | Chloride-heavy applications |
Machining scope
- Flange faces (both ends) — API 6A ring-groove machining; typical BX or R-series ring grooves. Flatness and surface finish per API spec.
- Through-bore — Concentric to flange faces; typically ±0.0005" ID tolerance depending on rating.
- Ring gasket grooves — Per API 6A dimensions (BX-152, R-46, R-53, etc.). Surface finish critical for sealing.
- Bolt-circle patterns — Stud holes per API 6A pattern for the pressure/size class.
- Optional port machining — Threaded or flanged access ports if design calls for them.
Tolerances and documentation
Wellhead-spec parts require API-adjacent documentation: material traceability by heat and lot, first-article inspection with CMM data, certificate of conformance. Some customers require full API 6A monogram; others accept ISO 9001 with API-adjacent practice.
Tolerances follow API 6A dimensional specs for ring grooves and flange geometry. Bore concentricity: ±0.005" TIR typical; face flatness: 0.002" TIR; ring groove surface finish: 32 μin Ra.
How B&R machines wellhead spools
Typical process: forging arrives with MTR, PMI verification, rough turning to remove scale, heat-treatment coordination if required, finish turning of bore and OD, milling of ring grooves and flange faces, drilling of stud holes and any port geometry, CMM verification of critical dimensions, certificate of conformance issued with delivery.
Machines: Fadal VMC 4020 or YCM TV-188B for milling operations depending on part size; Hwacheon Hi-Tech 400, Mega 100, or Samsung SL-45A/65A for turning depending on part diameter and length. See our machine white papers for platform details.
Frequently asked questions
Where can I get wellhead spools CNC machined in Texas?
B&R Productions in New Waverly, TX machines wellhead spools regularly. Direct number for RFQs and rig-down work: (936) 291-7827.
How long does wellhead spools machining typically take?
Depends on quantity and material availability. Standard work: 1–3 weeks. Emergency turnaround: same-day to 3 days when material is on the shelf. Call to discuss your specific job.
What material is standard for wellhead spools?
4140 pre-hardened for standard-service wellhead work. F22 for higher-strength applications. 17-4 PH or Inconel 718 for corrosion-resistant or sour service. Choice depends on service environment and rating.
Do you machine to full API 6A?
We machine to API 6A-adjacent tolerances routinely; contact us for current API-monogram-license status if the finished part must carry the API stamp.
Can you machine a spool from customer-supplied forging?
Yes — routine. Provide the forging + MTR + print; we handle rough machining, coordination with heat-treat if required, finish machining, inspection, and documentation.
What's typical lead time for wellhead spools?
2–4 weeks depending on quantity and material availability. Forging release lead time (if we're sourcing) adds 4–6 weeks. Emergency single-piece work faster when material is on shelf.