ANSI B16.1 Class 125 Weld Neck Flange — Dimensions, Drilling Chart & Weights

A forged steel welding neck flange — not cast iron — made with a flat face and drilled to the ASME B16.1 Class 125 bolt pattern used by cast-iron flanges. B16.1 itself is the grey cast iron standard, and cast iron cannot be butt-welded, so when a line needs a weld neck that bolts to a cast-iron valve, pump or AWWA fitting, the flange is produced in steel with the cast-iron drilling. Body dimensions follow ASME B16.5 Class 150 up to NPS 24 and B16.47 Series A above that.

They are tabulated in full in the two charts above: NPS 1 to 24 uses the ASME B16.5 Class 150 weld neck body, whose drilling is identical to B16.1 Class 125, and NPS 30 to 60 uses the B16.47 Series A body. A 6 in flange, for example, is 280 mm OD and 25.4 mm thick on a 241.3 mm bolt circle with eight 3/4 in bolts, at about 11 kg; a 24 in is 815 mm OD on a 749.3 mm circle with twenty 1-1/4 in bolts, at about 118 kg.

The bolt pattern is — for NPS 1 to 24 the bolt circle, hole count and hole size coincide exactly, so the two bolt together. The differences are everything else: B16.1 covers cast iron, flat faced, with much lower pressure ratings, while B16.5 covers steel, normally raised face. In practice a steel Class 150 weld neck faced flat is what the market calls a Class 125 weld neck — which is exactly what this page describes.

Because cast iron is brittle. Tightening a raised face against a flat cast-iron flange puts a bending moment on the unsupported rim and can crack it — ASME B16.5 warns against exactly this. A flat face with a full-face gasket spreads the bolt load across the entire face, so the cast-iron component sees only compression. If a raised face must mate with cast iron, B16.5 limits the joint to low-strength A307 Grade B bolting with a thin ring gasket.

The joint is limited by the cast-iron side, not the steel flange. For ASTM A126 Class B cast iron, B16.1 Class 125 allows 200 psig (13.8 bar) up to 66 °C for NPS 1-12, and 150 psig for NPS 14-24 and 30-48, derating with temperature — the full table is on this page. The steel weld neck itself carries its B16.5 or B16.47 Class 150 rating (A105: about 19.6 bar at 38 °C), so it is never the weak link in the joint.

With the standard flat face and a full-face gasket, either low-strength ASTM A307 Grade B machine bolts or A193 B7 studs may be used — the full-face gasket protects the cast iron either way. Only when a raised face bears on cast iron does ASME B16.5 restrict the joint to A307 Grade B with a ring gasket no thicker than 1.6 mm. Hole counts, hole sizes and bolt sizes per flange size are in the charts above.

ASME B16.1 tabulates Class 125 in NPS 1 to 24 continuously, then 30, 36, 42, 48, 54, 60, 72, 84 and 96. We supply NPS 1-24 and 30-60 from the charts on this page, with 72-96 against a drawing on request. In-between sizes such as 26, 28, the even sizes 32-46 and 50-58 are not B16.1 sizes; they are supplied to B16.47 Series A Class 150, which carries the drilling philosophy above NPS 24.

The thickness C and length through hub V in our charts are the published steel-flange values, which include the 2 mm raised face that B16.5 Class 150 and B16.47 Series A bodies normally carry. Machining the face flat removes it, so a flat-face flange measures 2 mm less in C and V than the chart value. Every other dimension, including the drilling, is unchanged.

A genuine quirk: ASME B16.1 specifies 2 in (50.8 mm) holes for the 1-3/4 in bolts at these two sizes, while B16.47 Series A — whose body the flange uses — drills 47.6 mm for the same bolts. We drill 47.6 mm by default, which suits both patterns, and drill the full B16.1 2 in hole when the order says so. It is the one place in the range where the two standards disagree.

Wherever steel pipe meets cast or ductile iron equipment: cast-iron gate and check valves, pump casings, strainers and expansion joints, AWWA C207 Class D and C110 ductile-iron fittings, and the fire-water, HVAC, cooling-water and effluent systems built around them. Municipal water works and the utility side of older plants are the classic home of the Class 125 pattern.

Carbon steel ASTM A105/A105N is the usual choice, with A694 F42-F65 for pipeline service and A350 LF2 for low temperature. Stainless A182 F304/L, F316/L and F321, duplex F51 and super duplex F53 cover corrosive duty, and copper-nickel 90/10 composite or solid flanges are made for seawater on request. EN equivalents such as P250GH, 1.4301 and 1.4404 are available — all with EN 10204 3.1 mill certificates.

The bore of a weld neck flange is machined to match the inside diameter of the pipe it will be welded to. The chart shows the bore for standard-weight (STD) pipe, which dominates Class 125 service, but we bore to whatever schedule you order — state the schedule or the exact ID with the enquiry so the butt weld lines up without a step.

From the weight column in our charts: a 6 inch weighs about 11 kg, a 12 inch about 36 kg, a 24 inch about 118 kg, a 36 inch about 290 kg and a 60 inch about 928 kg. These are approximate carbon-steel weights with a standard bore; the exact weight varies with the bore schedule and material.

Yes — a 2 mm raised face is available on request for joints where the mating flange is steel, and the drilling stays Class 125. Against cast iron, though, flat face with a full-face gasket is the correct and standard configuration; a raised face should only bear on a cast-iron flange under the restricted bolting rules of ASME B16.5.

Forged from certified billets, normalised or solution annealed per the material specification, dimensionally inspected to B16.5/B16.47 tolerances with the drilling template-checked against B16.1 Class 125, and marked per MSS SP-25 including the drilling and facing. Every batch ships with EN 10204 3.1 mill test certificates traceable by heat number; UT/MPI/DPT, PMI, NACE MR0175 and 3.2 third-party endorsement are available on request.

ANSI B16.1 125 lb flat face weld neck, Class 125 FF WN, 125# WNFF and "steel flange with cast-iron drilling" all describe the same product — a steel welding neck flange carrying the Class 125 cast-iron bolt pattern. ANSI B16.1 and ASME B16.1 are also the same standard; custodianship moved from ANSI to ASME decades ago but the old name persists on drawings.