Threaded Boss / Bosset — BS 3799 Forged Welding Bosses
Tesco Steel & Engineering forges threaded bosses — the trade also says bosset or welding boss: a cylindrical branch fitting with a female pipe thread at one end and a 45° chamfered weld-on end at the other, fillet-welded to pipe, vessel shells or flat plate to give the parent surface a threaded connection for instruments, small-bore lines, vents, drains and sample points. Made to BS 3799-1974 in Class 3000 (⅛″–4″) and Class 6000 (⅛″–2″), threads female NPT to ASME B1.20.1 or BSPT to BS EN 10226, the base flat or contoured to the parent pipe OD, full or half length. Where its threadolet cousin owns the code-reinforced branch on process headers, the boss owns everything else a thread must sit on — in every material grade on this site: carbon steel, LTCS, the chrome-moly ladder, stainless — the F304 production photographed below — duplex and the nickel alloys. Each piece is laser-marked with grade, size, thread form, class and heat, certified to EN 10204 3.1/3.2. Dimension table in the datasheet below. ISO 9001:2015, made in Mumbai, India — exported to 50+ countries.
BS 3799-1974 · Class 3000 / 6000Threaded Boss · Bosset · Welding BossFemale NPT / BSPTNPS ⅛″–4″ · DN 6–100Flat or Contoured Base · W / W⁄2Pipe, Vessel & Plate MountingEN 10204 3.1 / 3.2 · ISO 9001:2015
ASTM A182 F304 Threaded Bosses, ½″ FNPT #3000, Heat TF21384 — Thread at One End, Weld Chamfer at the Other
What is a Threaded Boss?
The thread you weld on wherever you need it. A threaded boss (also written bosset or welding boss) is a forged, cylindrical branch fitting: a female pipe thread — NPT to ASME B1.20.1 or BSPT to BS EN 10226 — at one end, and a 45° chamfered weld-on end at the other, joined to the parent by a continuous fillet weld. Made to BS 3799-1974 in Class 3000 (⅛″–4″) and 6000 (⅛″–2″), with the base flat for plates and vessels or contoured to the parent pipe OD, in full (W) or half (W⁄2) length. It is how gauges, thermowells, vents, drains and sample points get their seat on surfaces where a saddle-based olet cannot go. We forge them in every grade from A105 to the nickel alloys, threads gauged, laser-marked and certified to EN 10204 3.1/3.2.
Also searched as: bosset, welding boss, weld-on boss, instrument boss, screwed boss, threaded branch boss, BS 3799 boss — all this page's product. Related pages: the threadolet (the MSS SP-97 code-reinforced cousin), the sockolet and weldolet, the threaded flanges that share its thread discipline, and the material pages behind every grade we forge: carbon steel, alloy steel, stainless, duplex, Monel, Inconel and Hastelloy.
Full-penetration groove weld on a contoured saddle
Reinforcement
None claimed — the designer satisfies the code where it applies
Forged into the body per MSS SP-97
Sits on
Pipe of any OD, vessel shells, dished ends, flat plate, machined pads
Run pipe, saddle contoured per size
Typical duty
Instrument bosses, gauge and thermowell seats, vents and drains on vessels and equipment
Code-reinforced threaded branches on process headers
The short version: where the connection is a pressure-piping branch under the code, take the threadolet; where a thread must sit on a vessel, a plate or any surface a saddle cannot hug, the boss is the fitting made for it.
What You're Buying in the Forging
The Weld Prep Is On the Fitting
45° ± 2.5° chamfer with a 1.6 ± 0.8 mm land, per BS 3799 — every boss arrives with a consistent groove for the fillet, so the weld is repeatable piece after piece, with no prep ground by eye at the bench.
Both Tapers, Gauged
Female NPT to ASME B1.20.1 or BSPT to BS EN 10226 — cut to the form your plant speaks and plug-gauged to its standard; the two do not interchange, so the order names one and the marking carries it.
Any Surface
Flat base for plates, shells and pads; contoured base machined to your stated pipe OD — the fitting that puts a thread where the olet family's saddles cannot go.
Two Classes, Two Lengths
Class 3000 to 4″, Class 6000 to 2″; full (W) or half (W⁄2) length — the class thickens the wall around the thread, the half length ducks under tight clearances; thread length stays per the table.
Specification Notes — Getting Bosses Right
Three honest notes.A fillet-welded boss is not an MSS SP-97 branch outlet: it carries no forged-in reinforcement pedigree, so where the connection is a true pressure-piping branch under B31.3, satisfy the branch rules by calculation or specify the threadolet instead. On vessels the boss is a vessel attachment: nozzle reinforcement, weld details and PWHT follow the vessel code and its documentation — specify the boss with the vessel engineer, not around them. And name the thread form explicitly: NPT and BSPT do not interchange — 60° against 55° flanks, different pitches — and a mixed joint may start by hand yet never seal; the photographed lot is marked FNPT for exactly that reason.
How Our Threaded Bosses Are Manufactured
1
Material — certified forgings in the ordered grade, heat treated as the grade demands — normalised LF2, N&T chrome-moly, solution-annealed stainless and duplex — furnace records retained; heat-number transfer at the first operation.
2
Body machining — the cylindrical body turned to the BS 3799 envelope: outside diameter D and length W (or W⁄2) for the size and class.
3
Weld-end preparation — the 45° ± 2.5° chamfer cut with its 1.6 ± 0.8 mm land; contoured bases machined to the stated parent pipe OD.
4
Thread cutting & gauging — female NPT to ASME B1.20.1 or BSPT to BS EN 10226, checked with plug gauges to the matching standard and recorded.
5
Testing & marking — PMI on alloy grades, MPI/DPT to the ordered scope, then laser marking: grade, size and thread form, class, heat number.
6
Certification & packing — EN 10204 3.1 MTC (3.2 witnessed on request); threads capped, chamfers protected, packed sea-worthy.
Where Threaded Bosses Are Used
Wherever a thread must grow out of a surface: vessel and tank shells take them for level, pressure and temperature instrument connections; skid and package builders weld them onto equipment frames and machined pads for gauges and test points; pipeline and utility crews put them on headers for vents and drains where the duty is instrument-grade; and fabricators keep a drawer of them because half of jobbing work is 'add a tapping here'. Production pieces below — the marking on each is real:
The Two Ends of the Job — Female NPT In, 45° Chamfer DownOne Boss, One Line of Truth — Grade, Size, Form, Class, Heat
Threaded Boss Dimensions — BS 3799 Quick Reference
D is the outside diameter of the boss (per class), W the overall length and L2 the minimum thread length — W and L2 are common to Class 3000 and 6000 across the shared sizes; all in millimetres per BS 3799-1974. Class 6000 runs ⅛″–2″; the 2½″–4″ sizes are Class 3000 only.
NPS (inch)
DN
D — 3000 lb (mm)
D — 6000 lb (mm)
W (mm)
L2 min. (mm)
⅛
6
16.0
22.0
38.0
6.70
¼
8
19.0
26.0
41.0
10.21
⅜
10
22.0
32.0
45.0
10.36
½
15
29.0
38.0
51.0
13.56
¾
20
35.0
45.0
51.0
13.86
1
25
45.0
60.0
51.0
17.34
1½
40
64.0
76.0
51.0
18.38
2
50
76.0
95.0
51.0
19.22
2½
65
95.0
–
51.0
28.89
3
80
110.0
–
57.0
30.48
4
100
140.0
–
64.0
33.02
The half-length variant runs W⁄2; the weld prep is the standard's 45° ± 2.5° chamfer with its 1.6 ± 0.8 mm land.Download the datasheet (PDF) for the table with the dimensional drawing and installation guidance — special lengths, counterbores and seal-weld details are machined to drawing.
How to Specify & Order a Threaded Boss
Seven elements — the thread form and the base are the two people forget:
1
Size — ⅛″ to 4″.
2
Class — 3000, or 6000 up to 2″.
3
Thread form, explicitly — NPT per ASME B1.20.1 or BSPT per BS EN 10226; the two do not interchange.
4
Base — flat, or contoured with the parent pipe size and schedule so the curvature comes machined to it.
5
Length — full (W) or half (W⁄2).
6
Material grade & certification — exactly as your specification writes it, with EN 10204 3.1 (our standard) / 3.2 witnessed; thread gauging records, MPI/DPT or NACE scope if extended.
Example: “Threaded boss to BS 3799, ½″ Class 3000, female NPT per ASME B1.20.1, flat base, full length, ASTM A182 F304/304L dual-certified, EN 10204 3.1 — 40 pieces.” Quotations normally within 24 hours with price, unit weights and delivery.
Threaded Bosses — Frequently Asked Questions
What is a threaded boss (bosset)?
A threaded boss — the trade also says bosset or welding boss — is a forged, cylindrical branch fitting with a female pipe thread at one end and a 45° chamfered weld-on end at the other. It is fillet-welded to a pipe, a vessel shell or a flat plate, and from that moment the parent surface has a threaded connection: a seat for a pressure gauge, a thermowell, a small-bore line, a vent, a drain or a sample point. Ours are made to BS 3799-1974, the screwed-fittings standard of the petroleum industry, in Class 3000 from ⅛" to 4" and Class 6000 from ⅛" to 2", with the thread cut female NPT to ASME B1.20.1 or BSPT to BS EN 10226 as specified. The base comes flat for plates and vessels or contoured to the parent pipe's OD, in full or half length, in every material grade on this site — each piece laser-marked and certified to EN 10204 3.1/3.2.
What standards cover threaded bosses?
The dimensional home is BS 3799-1974 — Steel pipe fittings, screwed and socket-welding, for the petroleum industry — which tabulates the boss's outside diameter, overall length and minimum thread length per size and class, and sets the weld preparation: a 45° ± 2.5° chamfer with a 1.6 ± 0.8 mm land. The thread itself belongs to its own standard — NPT taper to ASME B1.20.1 or BSPT to BS EN 10226, whichever the order names — gauged accordingly. The class habit runs parallel to ASME B16.11's 3000/6000 ratings for screwed fittings, and purchaser specifications sometimes cite B16.11 alongside — the production lot photographed on this page carries exactly such a marking. On the line or the vessel, the attachment is governed by the installing code: the piping code's branch-connection rules where the boss sits on pipe, and the vessel code's attachment rules where it sits on a shell — the honest-limits question below says more. Materials certify to their own standards — A105, A350 LF2, A182 — so the certificate reads like any forging certificate.
Threaded boss or threadolet — which one do I need?
Both put a female thread on a parent surface; how they attach and what they are rated to do differs, and that decides. The threadolet is an MSS SP-97 integrally reinforced branch outlet: its saddle is contoured to a run pipe, the attachment is a full-penetration groove weld, and the forged-in reinforcement satisfies the piping code's branch-compensation rules by itself — the fitting for pressure-rated branch connections on process headers. The threaded boss is simpler and more versatile: a plain cylinder, fillet-welded through its 45° chamfer, sitting as happily on a flat plate or a vessel shell as on pipe of any diameter — surfaces where a threadolet's saddle cannot go. That simplicity is also its limit: a fillet-welded boss brings no MSS SP-97 reinforcement pedigree, so where the connection is a true pressure-piping branch under B31.3, the designer must satisfy the reinforcement rules by calculation or choose the threadolet. Practical division: instrument bosses on vessels, tanks, machined bases and structures — the boss; code-reinforced threaded branches on process pipe — the threadolet, whose page on this site tells its own story.
What sizes and classes do threaded bosses come in?
Eleven sizes from NPS ⅛" to 4" (DN 6 to DN 100) — small-bore territory, because that is what threads serve. Class 3000 covers the whole range; Class 6000 is available from ⅛" to 2", the heavier body carrying a larger outside diameter at the same thread size — a ½" boss, for example, runs 29 mm OD in Class 3000 and 38 mm in Class 6000. Overall length W and minimum thread length L2 are common to both classes across the shared sizes, so the class choice changes the wall around the thread, not the stack-up. Above 2", the 2½", 3" and 4" sizes come in Class 3000 only. Two more choices complete the specification: the base — flat for plates and vessel shells, or contoured to the stated parent pipe OD — and the length, full (W) or half (W/2), the half length for tight spots and thin-wall parents where a full boss would tower. The BS 3799 dimension table on this page carries D, W and L2 for every size.
NPT or BSPT — which thread should I order?
Whichever your plant already speaks — the boss is cut to order in both. NPT, the American taper to ASME B1.20.1, dominates process industry practice worldwide and is the default on projects specified to ASME codes; its gauging and its make-up habit — sealant or PTFE tape, hand-tight plus one and a half to three turns — travel with it. BSPT, the British taper to BS EN 10226, still rules utility and marine practice in much of the Commonwealth, the Gulf and Asia, and BS 3799's own heritage sits on that side of the family. The two tapers do not interchange: threads per inch and flank angles differ — 60° for NPT, 55° for BSPT — and a mixed joint may start by hand and will not seal reliably. So the rule is simple: match the instrument or branch fitting that will screw in, state the form explicitly on the order, and we gauge to the matching standard and mark the pieces accordingly — the F304 lot photographed on this page is marked FNPT for exactly that reason. Where documents only say 'threaded', we ask rather than assume.
Flat or contoured base — and full or half length?
The base answers the surface, the length answers the space. A flat base suits what it says: vessel shells of generous radius, dished ends, flat plates, machined pads and structural members — the boss seats square and the fillet weld runs on an even land. A contoured base is machined to the stated parent pipe OD, so the chamfered end follows the pipe's curvature and the weld gap stays uniform around the joint — on small-diameter pipe this is the difference between a sound fillet and one bridging daylight; state the pipe size and schedule and the contour comes cut to it. Length is the second choice: the full W length of the BS 3799 table is the default, giving the tabulated thread engagement with room for the wrench; the half length, W/2, is the recognised variant for cramped nozzles, thin-wall parents and jacketed surfaces where a full boss stands too proud. Both carry the same thread and class. State base and length with the size — 'flat, full length' costs nothing to write and saves a drawing query later.
What materials are threaded bosses available in?
Every grade this site forges. Carbon steel A105 for general duty; low-temperature A350 LF2 for the cold bench; chrome-moly A182 F11, F22, F5, F9 and F91 where bosses sit on hot alloy circuits; stainless F304/304L and F316/316L with dual certification where heats qualify, plus stabilised F321 and F347 — the production photographed on this page is ASTM A182 F304, ½" FNPT Class 3000, laser-marked with its heat TF21384; duplex F51 and super duplex F53/F55 for chloride duty; and the nickel and copper benches — Monel, Inconel, Incoloy, Hastelloy, copper-nickel — with titanium against enquiry. The boss follows the parent metal: a vessel shell in 316L takes a 316L boss, matching weld metal and all, and dissimilar combinations are handled at the welding procedure, not by compromising the fitting. Chemistry per heat, PMI on alloy grades, EN 10204 3.1/3.2 on every lot.
How is a threaded boss installed?
Drill, seat, fillet-weld, make up — with the weld preparation already on the fitting. The hole in the parent pipe, vessel or plate is drilled or cut to suit the boss bore and deburred. The boss is seated square to the surface — a contoured base on pipe self-aligns to the stated OD — and tacked. The attachment is a continuous fillet weld around the 45° chamfer: BS 3799's 45° ± 2.5° angle with its 1.6 ± 0.8 mm land exists precisely so the fillet has a consistent groove to fill, weld after weld, without the fitter grinding a prep by eye. The thread is protected from spatter and heat while welding — a simple plug does it — and on critical work re-gauged afterwards, since welding distortion can move a thread that gauged perfectly in the shop. Make-up then follows taper-thread practice: sealant or PTFE tape, hand-tight plus one and a half to three turns, seal-welded where the specification requires. Welding discipline follows the parent material — preheat on chrome-moly, purge and low-carbon filler on stainless — and examination of the fillet follows the installing code.
What are the honest limits of a welded boss connection?
Three, all worth stating before the enquiry. First, the attachment is a fillet weld, not a full-penetration branch weld: strong, simple and fine for the instrument and small-bore duty the boss is made for, but without the MSS SP-97 integrally reinforced pedigree — where the connection is a pressure-piping branch under B31.3, the branch reinforcement rules are the designer's to satisfy, by calculation or by choosing the threadolet instead. Second, on pressure vessels the boss becomes a vessel attachment: nozzle reinforcement, attachment weld details and any PWHT follow the vessel code and the vessel's own documentation, so the boss is specified with the vessel engineer, not around them. Third, the thread's own physics: a taper thread is a sealed leak path, threads gall on stainless without anti-seize, and vibration works thread roots — the same honest limits the threadolet page walks through apply to any threaded connection, and seal welding, where specified, is done on a joint made up clean. None of this diminishes the boss; it keeps it doing the job it does well.
Is 'bosset' a brand name?
No — and that makes it the odd one out on this menu. Unlike Weldolet, Sockolet, Thredolet and their siblings, which began as Bonney Forge trademarks and turned generic with use, 'bosset' is simply one of the trade's names for the welding boss, alongside 'threaded boss', 'weld-on boss', 'instrument boss' and plain 'boss'. Different industries lean on different words — instrument engineers say boss, some piping houses write bosset, older British drawings say screwed boss — and they all mean the same forged, fillet-welded, female-threaded fitting. What we manufacture and certify are forged threaded bosses to BS 3799-1974, threads gauged to ASME B1.20.1 or BS EN 10226 — the wording our certificates carry, dimensionally interchangeable with any boss made to the same standard. If your documents write any of the names above, or simply sketch a threaded lump welded to a shell, they mean this product; name the standard, the thread form and the class, and the paperwork follows.
What is marked on each threaded boss?
Manufacturer, material grade, size and thread form, class and heat number — laser-marked on the cylindrical body, which offers the boss's one generous flat for legible marking. The production lot photographed on this page reads exactly: 'ASTM A182 F304 / BOSS 1/2" FNPT / ASME B16.11 X 1/2" BW / 3000LB / TF21384 INDIA' — grade first, then the product with its thread size and form (female NPT), the referenced fitting standard with the weld end, the pressure class, and the heat number with origin. That heat number is the working end of the marking: it ties each piece to its EN 10204 certificate, its ladle chemistry and its heat-treatment record, and it is what stores check before a boss disappears into a vessel package. PMI confirms alloy grades match the mark before dispatch, and project-specific marking — tag numbers, PO references, vessel item numbers — is applied to order and recorded on the documents.
What testing and certification come with threaded bosses?
The forging's full paper trail, plus the thread's own check. Chemistry and mechanicals per heat on the EN 10204 3.1 MTC — 3.2 witnessed on request — with heat-treatment condition and furnace records where the grade demands them: normalised LF2, N&T chrome-moly, solution-annealed stainless and duplex. Thread gauging to the ordered form's standard — ASME B1.20.1 plug gauges for NPT, BS EN 10226 for BSPT — with the gauging recorded. PMI verifies every alloy piece. NDT to the ordered scope: MPI or DPT of the machined body and chamfer, hardness testing for sour service, with NACE MR0175 / ISO 15156 compliance available on request. Dimensional inspection reports to BS 3799 — D, W, L2 and the 45° chamfer with its land, because on this fitting the weld prep is part of the specification. Third-party witness or review is arranged on request; state the agency and scope on the enquiry and the documentation follows the material.
Where can I find threaded boss dimensions?
In the datasheet on this page — a two-page PDF with the BS 3799 dimension table for all eleven sizes, ⅛" through 4", reproduced on this page for quick reference. Three dimensions do the work: D, the outside diameter of the boss, tabulated separately for Class 3000 and Class 6000 — the class changes the metal around the thread, from 16 mm versus 22 mm at ⅛" up to 76 mm versus 95 mm at 2"; W, the overall length, common to both classes at each size, from 38 mm to 64 mm across the range; and L2, the minimum thread length, from 6.7 mm at ⅛" to 33 mm at 4". All in millimetres per BS 3799-1974. The half-length variant runs W/2; contoured bases follow the stated parent pipe OD without changing the tabulated envelope; and the weld prep is the standard's 45° ± 2.5° chamfer with a 1.6 ± 0.8 mm land. Drawing-specific variants — special lengths, counterbores, seal-weld lips — are machined to order against the same certification.
What details are needed to get an accurate threaded boss quotation?
Seven elements plus commercial terms: (1) size — ⅛" to 4"; (2) class — 3000, or 6000 up to 2"; (3) thread form — NPT to ASME B1.20.1 or BSPT to BS EN 10226; write it explicitly, because the two do not interchange; (4) base — flat, or contoured with the parent pipe size and schedule so the curvature comes machined to it; (5) length — full (W) or half (W/2); (6) material grade, exactly as your specification writes it — A105, A350 LF2, A182 F304/304L dual-certified, F316L, duplex F51; (7) certification and NDT — EN 10204 3.1 (our standard) or 3.2 witnessed, thread gauging records, plus MPI/DPT or NACE scope where extended — then quantity and destination. Send it to sales@tescosteel.com or through the inquiry form; quotations normally return within 24 hours with price, unit weights and delivery, and common carbon steel and stainless combinations usually quote from stock.
Who manufactures threaded bosses in India?
Tesco Steel & Engineering is an ISO 9001:2015 certified manufacturer based in Mumbai, India, forging threaded bosses (bossets, welding bosses) to BS 3799-1974 — female NPT or BSPT, Class 3000 in NPS ⅛" to 4" and Class 6000 to 2", flat or contoured bases, full and half lengths, with threads gauged to ASME B1.20.1 or BS EN 10226. Materials span our whole forge: carbon steel A105, LTCS A350 LF2, chrome-moly A182 F11 through F91, stainless 304/316/321/347 families — the F304 production photographed on this page — duplex and super duplex, and the nickel alloys against enquiry. PMI verification, laser marking tying each piece to its heat, and EN 10204 3.1/3.2 certification on every lot. The same works forges the complete MSS SP-97 olet family — weldolets, sockolets, elbolets, threadolets, nipolets, latrolets, sweepolets and flangeolets — so a branch-connection enquiry of any kind lands on one bench. Exported to more than 50 countries; datasheet with the BS 3799 dimension table downloadable on this page.
