Custom fittings – non-standard reducers, special-angle elbows, forged blocks, manifolds or fittings with project-specific bores – are made to a drawing rather than straight from ASME B16.9 or B16.11. The drawing becomes the contract. If it is unclear, out of date or not formally approved, the result is usually a rejected part at inspection. This guide explains what the drawing should contain, how revisions should be controlled and what should be approved before material is cut.
What a Manufacturing Drawing for a Custom Fitting Should Show
- All critical dimensions with tolerances – including end-to-end or centre-to-end lengths, bores, wall thickness at the thinnest point and any angles. Where a tolerance is not shown, state which standard's tolerances apply (for example "unless noted, tolerances per ASME B16.9").
- Material – the full specification and grade (for example ASTM A182 F316L or ASTM A234 WPB), plus any supplementary requirements such as impact testing or hardness limits.
- End preparation – bevel angle and root face for buttweld ends, socket depth for socket weld ends, or thread type for threaded ends.
- Surface finish and heat treatment where they matter for service.
- Marking requirements – what must be stamped or stencilled and where.
- Drawing number, revision letter and revision date in the title block.
Controlling Drawing Revisions
Most problems with custom fittings come from more than one revision of the same drawing being in circulation. A simple discipline avoids this:
- One revision is current. The purchase order should quote the drawing number and revision explicitly, for example "DWG-1234 Rev C".
- Every change raises the revision. Even a tolerance change should not be made on an existing revision. A revision table on the drawing should say what changed and why.
- Changes after order go in writing. If the buyer changes a dimension after the order, the new revision is sent formally and its effect on price and delivery is confirmed before work continues.
- Superseded revisions are withdrawn. Old copies should be marked "superseded" so they are not used on the shop floor or at inspection.
What to Approve Before Manufacture
Before forging or machining starts, the buyer and supplier should agree the following. For many projects this is formalised as a drawing approval or "approved for construction" step.
- The drawing itself – the supplier may return the buyer's drawing with its own manufacturing notes, or issue its own drawing for approval. Either way, one approved revision should exist.
- Inspection and test plan (ITP) – which checks are done (dimensional, visual, PMI, hydrotest, NDT such as dye penetrant or ultrasonic), at what stage, and which are witness or hold points for the buyer or a third-party inspector.
- Material source – the raw material and its certificate, so traceability from heat number to finished part is clear.
- Documents to be delivered – for example an EN 10204 3.1 certificate, dimensional report against the approved drawing revision, and NDT reports.
Why This Matters at Inspection
At final inspection, the part is checked against the approved drawing revision and the ITP. A dimensional report that refers to a different revision, or a feature that was never toleranced, is a common cause of disputes. Agreeing these items up front protects both the buyer and the manufacturer, and usually shortens delivery because questions are resolved before production rather than after.
Checklist for Your Custom Fitting Enquiry
- Drawing with number and revision, and all critical tolerances
- Material specification and grade, with supplementary requirements
- Quantity and required delivery
- Testing and NDT requirements, and any third-party inspection
- Documentation required with the shipment
For standard items, see our fitting RFQ guide and how to read an EN 10204 3.1 MTC.
