Overview
The 1/4-28 UNF x 1/8 NPT grease line adaptor is a thread-conversion and extension fitting: a male 1/4-28 UNF straight thread on one end, a female 1/8 NPT tapered pipe thread on the other, and a 23 mm body in between. It is an adaptor – not a grease nipple and not a quick-connect coupler – and it exists because UNF straight threads and NPT tapered threads are incompatible and must never be forced together.
Use it when a 1/4-28 UNF port has to meet a 1/8 NPT connection, or when a recessed lubrication point needs extra reach. Its declared maximum working pressure is 10,000 PSI (690 bar). For background on thread families, see our grease fitting types guide.
Specifications
| Parameter | Specification |
|---|---|
| Male thread | 1/4-28 UNF, 0.2500″ major diameter, 28 TPI, straight thread |
| Female thread | 1/8 NPT, tapered thread, 1°47′ taper angle, 27 TPI (thread form per ASME B1.20.1; automotive fitting practice per SAE J512) |
| Overall length | 23 mm |
| Hex size | 9.0 mm across flats |
| Weight | Approx. 8 g |
| Material | Carbon steel (C45) |
| Surface treatment | Zinc plating 12-15 microns with chromate passivation |
| Max. working pressure (declared spec) | 10,000 PSI (690 bar) |
| Burst pressure (declared spec) | 15,000 PSI (1,035 bar) or higher |
| Operating temperature | -40°C to +93°C continuous |
| Tightening torque, UNF side | 15-25 ft-lbs (20-34 Nm) |
| Tightening torque, NPT side | 12-18 ft-lbs (16-24 Nm) |
| Tool | 9 mm wrench or socket; do not use impact tools |
| Sealing | NPT side: PTFE tape (2-3 wraps) or thread sealant. UNF side: straight thread, not self-sealing – use a washer or O-ring |
Applications
- Thread conversion: joining 1/4-28 UNF grease lines or ports to 1/8 NPT components in retrofit and repair work.
- Recessed points: the 23 mm extension reaches nipples set deep inside housings, guards, or frames.
- Obstacle clearance: routing a connection around shafts, covers, and adjacent hardware.
- Fleet and automotive service: chassis and driveline points with mixed thread standards.
- Industrial and agricultural machinery: centralized lubrication lines and bearing points.
Installation
Never force a mismatched thread – cross-threading damages the fitting and creates a safety hazard.
- Clean both ports and inspect the threads for damage or contamination.
- On the NPT side, apply PTFE tape (2-3 wraps) or thread sealant – the taper seals on the threads, but only with sealant in place.
- On the UNF side, fit a washer or O-ring: a straight thread cannot self-seal.
- Hand-start both ends, then tighten with a 9 mm wrench to 15-25 ft-lbs (20-34 Nm) on the UNF side and 12-18 ft-lbs (16-24 Nm) on the NPT side.
- Do not use impact tools. Check for leaks before returning the equipment to service.
If the mating component uses another thread family such as BSP, a second adaptor is required – request a quote and confirm the thread stack at order.
Maintenance
Inspect regularly during lubrication rounds: leakage at either thread, corrosion of the plating, rounded hex flats, damaged threads. Replace the fitting if any appear; there is no fixed service interval – timing depends on operating conditions, not the calendar. Renew the washer or O-ring on the UNF side whenever the joint is opened.
FAQ
What is a 1/4-28 UNF x 1/8 NPT grease line adaptor?
A small conversion fitting with a male 1/4-28 UNF straight thread on one end and a female 1/8 NPT tapered thread on the other, adding 23 mm of length. It bridges two incompatible thread systems; it is an adaptor, not a grease nipple itself.
What is the 23 mm extension for?
The extra body length moves the connection point outward, so you can reach lubrication points recessed inside housings or route a line around obstacles that would block a wrench or hose.
Why can’t I thread a 1/4-28 UNF fitting straight into a 1/8 NPT port?
UNF is a straight thread; NPT is a tapered pipe thread with a different form and pitch. Even if a mismatched pair seems to start, it will not seal and will damage both threads. This adaptor exists to make that transition correctly.
Does the NPT side need thread sealant?
Yes. Apply PTFE tape (2-3 wraps) or thread sealant. The tapered thread seals metal-to-metal only with sealant filling the thread roots – dry assembly will leak.
How does the UNF side seal?
Not on the threads. A straight UNF thread needs a washer or O-ring at the port face; plan the sealing element when you order if the port does not already have one.
What pressure can this adaptor handle?
Declared maximum working pressure is 10,000 PSI (690 bar), with declared burst pressure of 15,000 PSI (1,035 bar) or higher. In general industry practice, burst is the failure point set well above working pressure – a design margin, not permission to exceed the working rating.
What material is the adaptor made from?
Carbon steel (C45) with zinc plating 12-15 microns thick plus chromate passivation, for continuous service from -40°C to +93°C. For strongly corrosive or washdown environments, confirm suitability at order.
How do I install it?
Sealant on the NPT side, washer or O-ring on the UNF side, hand-start both ends, then a 9 mm wrench: 15-25 ft-lbs (20-34 Nm) UNF, 12-18 ft-lbs (16-24 Nm) NPT. No impact tools; check for leaks before service.
Can I use it with BSP fittings?
Not directly – BSP is a third, different thread family. You would stack a dedicated BSP adaptor, for example alongside our 1/8 BSP grease nipple, and confirm the combination at order. For metric points, see the M6 DIN 71412 grease nipple page.
How do I know if I need this adaptor?
When the two ends of a joint disagree – one side 1/4-28 UNF, the other 1/8 NPT – or when a straight connection cannot reach the point. Verify both threads before ordering, or contact our team for a quote from IsoHiTech Lubrication Systems, founded in 2023, engineering since 2006; email inquiry@isohitech.com.
