What Is a Bushing? Reducer Fitting

Quick Answer

A pipe bushing is a small forged fitting used to reduce the size of a threaded or socket weld connection. It has a larger male thread (or plain end) on the outside that screws into a larger fitting or port, and a smaller female thread (or socket) on the inside that accepts a smaller pipe. Bushings are manufactured per ASME B16.11 in sizes NPS 1/4 to NPS 4 and are available as hex bushings or flush (round) bushings.

Bushings provide an inline size reduction within a single connection point. They are the simplest and most compact way to transition between pipe sizes in threaded and socket weld piping systems.

Hex Bushing vs Flush Bushing

Feature	Hex Bushing	Flush Bushing
External shape	Hexagonal head	Round (cylindrical)
Installation	Wrench flats for tightening	Requires internal wrench or press fit
Protrusion	Hex head protrudes from connection	Sits flush with mating surface
Use case	General piping, accessible locations	Flush-mount applications, tight spaces
Material visibility	Hex allows stamped markings	Limited marking space

ASME B16.11 Specifications

Property	Detail
Standard	ASME B16.11
Size range	NPS 1/4 x 1/8 to NPS 4 x 3
Pressure classes	2000#, 3000#, 6000#
Connection	Male NPT (outside) x Female NPT (inside)
Materials	ASTM A105 (CS), A182 F304/F316 (SS), A182 F11/F22 (alloy), A350 LF2 (low-temp)
Manufacturing	Forged and machined

Bushing Dimensions

Size (in x in)	Male Thread OD (mm)	Female Thread ID (mm)	Overall Length (mm)
1/2 x 1/4	21.3	13.7	25.4
3/4 x 1/2	26.7	21.3	28.6
1 x 3/4	33.4	26.7	34.9
1-1/2 x 1	48.3	33.4	41.3
2 x 1-1/2	60.3	48.3	47.6

Bushing vs Reducing Coupling

Factor	Bushing	Reducing Coupling
Design	Male (outside) x Female (inside)	Female x Female
Gender	Converts female port to smaller female	Joins two male pipe ends
Pieces for reduction	1 (inserts into existing fitting)	1 (requires both pipes to thread in)
Typical use	Reducing a port in a tee, valve, or flange	Inline pipe size change
Flow path	Short, abrupt reduction	Longer, slightly smoother
Accessibility	Must access the port opening	Must access both pipe ends

A common scenario: a 1-inch threaded tee needs a 1/2-inch branch. A 1 x 1/2 bushing threads directly into the tee branch outlet. No additional fittings are needed.

Applications

• Reducing ports in fittings: Tees, crosses, and elbows with oversized outlets
• Valve connections: Adapting valve ports to smaller piping
• Instrument connections: Reducing header outlets to instrument tap sizes
• Vent and drain assemblies: Adapting larger fittings to small-bore vent/drain valves
• Fire protection: Reducing sprinkler header outlets per NFPA standards

Limitations

Bushings have practical limits:

• Maximum size reduction: One or two pipe sizes per bushing (e.g., NPS 1 to NPS 3/4 or NPS 1/2). Larger reductions require stacked bushings, which is poor practice
• Flow restriction: The abrupt diameter change increases turbulence and pressure drop
• Strength: Bushings are not designed for bending loads; they handle axial and pressure loads only
• Not for BW piping: Bushings are exclusively threaded or socket weld fittings. For buttweld size reductions, use reducers or swage nipples

Read the full guide to pipe fittings