What is a swaged nipple?

A swaged nipple (also called a swage nipple or reducing nipple ) is a male-end reducing pipe nipple — it has a larger OD male end on one side and a smaller OD male end on the other, with a smooth taper transition between them. Unlike a fitting, a swaged nipple is essentially a short piece of pipe that is swaged (mechanically reduced) at one end. Both ends are male (external thread or plain) . Swaged nipples are used to connect a larger pipe to a smaller branch or equipment nozzle without using a separate coupling + nipple assembly. Both ends may be threaded, beveled, square cut, grooved, or any combination.

A bull plug is a male-end closure fitting — it is a male-threaded or male-plain fitting used to close the end of a pipe, nozzle, or boss. Bull plugs are either hollow (open through the center, with a male thread on the outside — used as a reducing fitting) or solid (completely solid — used as a dead-end seal or dummy/support leg). In MSS SP-95, bull plugs are male-end fittings with external thread or plain end, designed to close or reduce a threaded or plain pipe opening. Square-head bull plugs have a flat square head for wrench engagement.

What is the difference between a swaged nipple and a reducer insert (MSS SP-79)?

MSS SP-95 swaged nipples have male plain or male threaded ends on both sides — they connect to the pipe externally (pipe threads into the nipple, or pipe butt-welds to the plain end). MSS SP-79 reducer inserts have a male plain end on the small side and a socket bore on the large side — the large end inserts into a B16.11 socket-weld fitting. In summary: SP-95 swaged nipple = male-to-male reducing nipple; SP-79 reducer insert = socket-to-male reducing insert for B16.11 fittings. SP-95 is for threaded or plain connections; SP-79 is specifically for socket-weld (SW) connections per ASME B16.11.

What end preparations are available for swaged nipples?

MSS SP-95 swaged nipples and bull plugs may have ends prepared in any of the following forms or combinations: Threaded (THD) : Male NPT threads per ASME B1.20.1 — the most common end prep for low-to-medium pressure service. Beveled (BW) : Plain beveled end for buttwelding — used in high-pressure and high-temperature service where a full-strength welded joint is required. Square Cut (SQ) : Plain square-cut end — for socket-welding (inserted into a socket fitting) or plain-end slip-on welding. Grooved (GRV) : Grooved end per AWWA C606 for grooved mechanical couplings — allows rapid assembly and disassembly without threading or welding. Both ends can be different preparations (e.g., threaded one end, beveled the other).

What materials are used for MSS SP-95 swaged nipples?

MSS SP-95 covers carbon steel and alloy steel swaged nipples and bull plugs. Materials: Carbon steel : steel pipe or tube per ASTM A53, A106, A333, or equivalent; fittings per ASTM A234 WPB; forgings per ASTM A105. Alloy steel : ASTM A335 (seamless alloy pipe) for high-temperature service; ASTM A182 for alloy forgings (F1, F5, F9, F11, F22, F91 for chrome-moly steels). For stainless steel and nickel alloy swaged nipples, consult the manufacturer — MSS SP-95 does not explicitly cover these materials, but they may be produced to SP-95 dimensions with appropriate material specifications. Always verify material compatibility with the piping system.

MSS SP-95 Swaged Nipples & Bull Plugs — Carbon/Alloy Steel, NPS 1/8–12

1. Scope

This standard covers dimensions, finish, tolerances, marking and material requirements for carbon and alloy steel swaged nipples and bull plugs, in sizes NPS 1/8 through NPS 12 (DN 6 to DN 300). A swaged nipple is a male-end reducing nipple with a smooth taper transition from a larger OD to a smaller OD on each end. A bull plug is a male-end closure or reducing fitting with a solid or hollow body. Both ends of these fittings may be prepared as: threaded (NPT), beveled (for buttwelding), square cut, or grooved — or any combination of these. Both concentric (same centreline) and eccentric (offset centreline) configurations are covered.

Edition: MSS SP-95-2024 (supersedes SP-95-2018) | Size Range: NPS 1/8–12 (DN 6–300) | Configuration: Concentric (same centreline) and Eccentric (offset) | End Preparations: Threaded, Beveled, Square Cut, Grooved, or any combination | Materials: Carbon steel and alloy steel | Note: Both ends may have different end preparations

2. Product Types

Product	End Type	Configuration	Function
Swaged Nipple (Concentric)	Both ends male — THD/BW/SQ/GRV	Centreline aligned	Reducing nipple; both ends on same centreline
Swaged Nipple (Eccentric)	Both ends male — THD/BW/SQ/GRV	Centreline offset	Reducing nipple; for horizontal gas/vapor lines to prevent air binding
Bull Plug (Hollow)	Male end — THD/BW/SQ	N/A	Reducing male-end fitting; or temporary end closure
Bull Plug (Solid)	Male end — THD	N/A	Dead-end seal; also used as pipe support/dummy leg
Hex Head Bull Plug	Male NPT	N/A	Closure fitting with hexagonal head for wrench engagement; reusable
Square Head Bull Plug	Male NPT	N/A	Closure fitting with square head; wrench-tightened; industrial use

Product

End Type

Configuration

Function

Swaged Nipple (Concentric)

Both ends male — THD/BW/SQ/GRV

Centreline aligned

Reducing nipple; both ends on same centreline

Swaged Nipple (Eccentric)

Both ends male — THD/BW/SQ/GRV

Centreline offset

Reducing nipple; for horizontal gas/vapor lines to prevent air binding

Bull Plug (Hollow)

Male end — THD/BW/SQ

N/A

Reducing male-end fitting; or temporary end closure

Bull Plug (Solid)

Male end — THD

N/A

Dead-end seal; also used as pipe support/dummy leg

Hex Head Bull Plug

Male NPT

N/A

Closure fitting with hexagonal head for wrench engagement; reusable

Square Head Bull Plug

Male NPT

N/A

Closure fitting with square head; wrench-tightened; industrial use

3. Common Reducing Combinations (Large End × Small End)

Large End NPS	Small End NPS	NPS 1/8	NPS 1/4	NPS 3/8	NPS 1/2	NPS 3/4	NPS 1
1/4	1/8	—	✓	✓	✓	✓	✓
3/8	1/4	—	—	—	✓	✓	✓
1/2	3/8	—	—	—	—	✓	✓
3/4	1/2	—	—	—	—	—	✓

Large End NPS

Small End NPS

NPS 1/8

NPS 1/4

NPS 3/8

NPS 1/2

NPS 3/4

NPS 1

1/4

1/8

—

✓

3/8

1/4

—

✓

1/2

3/8

—

✓

3/4

1/2

—

✓

Standard reducing combinations per MSS SP-95-2024. Full range up to NPS 12 available — consult manufacturer for the complete reducing combination table. Both concentric (CRE) and eccentric (ERE) configurations available for most combinations. Non-standard reductions may be available as specials.

4. Dimensional Reference (Threaded Nipple — Approx. Dimensions)

NPS	DN	Pipe OD (mm)	Approx. Wall (Sch. 40)	Thread Engagement (min.)	Standard Lengths
1/8	6	10.3	1.73	per B1.20.1	per SP-95 Table
1/4	8	13.7	2.24	per B1.20.1	per SP-95 Table
1/2	15	21.3	2.77	per B1.20.1	per SP-95 Table
3/4	20	26.7	2.87	per B1.20.1	per SP-95 Table
1	25	33.4	3.38	per B1.20.1	per SP-95 Table
2	50	60.3	3.91	per B1.20.1	per SP-95 Table
4	100	114.3	6.02	per B1.20.1	per SP-95 Table

NPS

Pipe OD (mm)

Approx. Wall (Sch. 40)

Thread Engagement (min.)

Standard Lengths

1/8

10.3

1.73

per B1.20.1

per SP-95 Table

1/4

13.7

2.24

per B1.20.1

per SP-95 Table

1/2

21.3

2.77

per B1.20.1

per SP-95 Table

3/4

26.7

2.87

per B1.20.1

per SP-95 Table

33.4

3.38

per B1.20.1

per SP-95 Table

60.3

3.91

per B1.20.1

per SP-95 Table

100

114.3

6.02

per B1.20.1

per SP-95 Table

Reference values. Actual dimensions per MSS SP-95-2024 dimensional tables. Wall thickness varies with pipe schedule. For beveled ends, OD = pipe OD and wall per the pipe schedule; bevel angle = 30° (+5°/–0°) per ASME B16.25. For grooved ends, dimensions per AWWA C606.

5. Material Specifications

Product	Material	ASTM Spec	Notes
Swaged Nipple (from pipe)	Carbon steel	A53, A106, A333, A420	Seamless or ERW pipe; WPB per A234
Swaged Nipple (from fittings)	Carbon steel	A234 WPB	Swaged from forged fittings
Bull Plug (forged)	Carbon steel	A105	Forgings for threaded closures
Alloy Steel Swaged Nipple	Alloy steel	A335 P1, P5, P9, P11, P22, P91	For high-temperature service; WP grades per A234
Alloy Steel Bull Plug	Alloy steel	A182 F1, F5, F9, F11, F22, F91	Forgings for high-temp/pressure closures

Product

Material

ASTM Spec

Notes

Swaged Nipple (from pipe)

Carbon steel

A53, A106, A333, A420

Seamless or ERW pipe; WPB per A234

Swaged Nipple (from fittings)

Carbon steel

A234 WPB

Swaged from forged fittings

Bull Plug (forged)

Carbon steel

A105

Forgings for threaded closures

Alloy Steel Swaged Nipple

Alloy steel

A335 P1, P5, P9, P11, P22, P91

For high-temperature service; WP grades per A234

Alloy Steel Bull Plug

Alloy steel

A182 F1, F5, F9, F11, F22, F91

Forgings for high-temp/pressure closures

6. Tolerances, Finish, Marking and Testing

Requirement	Specification
Thread Tolerance	NPT threads per ASME B1.20.1; gauge acceptance per B1.20.1
Bevel Angle	30° (+5°/–0°) per ASME B16.25; land inside diameter as specified
Length Tolerance	±1.5 mm for lengths ≤ 150 mm; ±3.0 mm for lengths > 150 mm (per SP-95)
OD Tolerance	±0.5 mm of nominal OD; weld-end OD tolerance per B16.25
Surface Finish	Free from fins, seams, laps; thread roots and crests clean; bevel clean and properly angled
Hydrostatic Test	Per applicable piping code; minimum 1.5× design pressure for the applicable pressure class or schedule
Marking	Per MSS SP-25: manufacturer/trademark, material designation, size (large × small NPS), end prep (THD/BW/SQ/GRV), standard (SP-95), heat number. Eccentric fittings marked "ERE" or "ECC".

Requirement

Specification

Thread Tolerance

NPT threads per ASME B1.20.1; gauge acceptance per B1.20.1

Bevel Angle

30° (+5°/–0°) per ASME B16.25; land inside diameter as specified

Length Tolerance

±1.5 mm for lengths ≤ 150 mm; ±3.0 mm for lengths > 150 mm (per SP-95)

OD Tolerance

±0.5 mm of nominal OD; weld-end OD tolerance per B16.25

Surface Finish

Free from fins, seams, laps; thread roots and crests clean; bevel clean and properly angled

Hydrostatic Test

Per applicable piping code; minimum 1.5× design pressure for the applicable pressure class or schedule

Marking

Per MSS SP-25: manufacturer/trademark, material designation, size (large × small NPS), end prep (THD/BW/SQ/GRV), standard (SP-95), heat number. Eccentric fittings marked "ERE" or "ECC".

7. Related Standards

Standard	Title
MSS SP-79	Socket-Welding Reducer Inserts (male-socket reducing insert for B16.11 SW fittings)
ASME B1.20.1	Pipe Threads, General Purpose (NPT thread)
ASME B16.25	Buttwelding Ends (bevel preparation)
AWWA C606	Grooved and Shouldered Joints (groove dimensions)
MSS SP-25	Standard Marking System for Valves, Fittings, Flanges and Unions
ASME B31.3	Process Piping (governing pressure/temperature code)

Standard

Title

MSS SP-79

Socket-Welding Reducer Inserts (male-socket reducing insert for B16.11 SW fittings)

ASME B1.20.1

Pipe Threads, General Purpose (NPT thread)

ASME B16.25

Buttwelding Ends (bevel preparation)

AWWA C606

Grooved and Shouldered Joints (groove dimensions)

MSS SP-25

Standard Marking System for Valves, Fittings, Flanges and Unions

ASME B31.3

Process Piping (governing pressure/temperature code)

8. How to Order (Per MSS SP-95)

Parameter	Example	Notes
Standard	MSS SP-95	Always specify the standard designation
Product Type	Swaged Nipple (Concentric) or Bull Plug	Concentric = CRE; Eccentric = ERE
Size (Large × Small)	NPS 2 × 1-1/2	Large end OD × small end OD
End Preparation (Large × Small)	THD × BW	THD / BW / SQ / GRV per SP-95
Wall / Schedule	Schedule 40 / Schedule 80 / XXS	Specify wall thickness matching pipe schedule
Length (Overall)	150 mm (6")	Standard lengths per SP-95 table or custom
Material / Grade	ASTM A234 WPB	Must match pipe system material

Parameter

Example

Notes

Standard

MSS SP-95

Always specify the standard designation

Product Type

Swaged Nipple (Concentric) or Bull Plug

Concentric = CRE; Eccentric = ERE

Size (Large × Small)

NPS 2 × 1-1/2

Large end OD × small end OD

End Preparation (Large × Small)

THD × BW

THD / BW / SQ / GRV per SP-95

Wall / Schedule

Schedule 40 / Schedule 80 / XXS

Specify wall thickness matching pipe schedule

Length (Overall)

150 mm (6")

Standard lengths per SP-95 table or custom

Material / Grade

ASTM A234 WPB

Must match pipe system material

Large End NPS	Small End NPS	NPS 1/8	NPS 1/4	NPS 3/8	NPS 1/2	NPS 3/4	NPS 1
1/4	1/8	—	✓	✓	✓	✓	✓
3/8	1/4	—	—	—	✓	✓	✓
1/2	3/8	—	—	—	—	✓	✓
3/4	1/2	—	—	—	—	—	✓

Large End NPS	Small End NPS	NPS 1/8	NPS 1/4	NPS 3/8	NPS 1/2	NPS 3/4	NPS 1
1/4	1/8	—	✓	✓	✓	✓	✓
3/8	1/4	—	—	—	✓	✓	✓
1/2	3/8	—	—	—	—	✓	✓
3/4	1/2	—	—	—	—	—	✓

Large End NPS	Small End NPS	NPS 1/8	NPS 1/4	NPS 3/8	NPS 1/2	NPS 3/4	NPS 1
1/4	1/8	—	✓	✓	✓	✓	✓
3/8	1/4	—	—	—	✓	✓	✓
1/2	3/8	—	—	—	—	✓	✓
3/4	1/2	—	—	—	—	—	✓