These extended-reach grease fittings use a 1/4″-28 UNF-2A straight thread with hydraulic-type head dimensions, lifting the coupler head out of recessed pockets, past guards, and out of deep counterbores where a standard-length fitting cannot be reached. This page covers only these long imperial fine-thread versions. Metric machines: M10 grease nipples. 1/8 inch tapered pipe thread in 316 stainless: the 316 straight nipple page. Full family: the lubrication fitting category.
Specifications
| Item | Specification |
|---|---|
| Thread | 1/4″-28 UNF-2A straight, 28 TPI, 60° thread form |
| Type | Hydraulic type |
| Head diameter | 0.375″ (9.5 mm) |
| Drive | 0.125″ (3.2 mm) internal hex, Allen key installation |
| Working pressure | 5,000 PSI (345 bar) standard; up to 10,000 PSI (689 bar) heavy-duty |
| Proof pressure | typically 1.5x working |
| Installation torque | 8 – 12 ft-lbs (11 – 16 Nm) into cast iron or steel; reduce 20 – 25% into aluminum |
| Minimum engagement | 5 full threads, about 0.18″ (4.5 mm) |
| Materials | zinc plated SAE 1215 carbon steel, passivated; 316 stainless steel; brass C36000 / C46400 |
| Temperature | -40°C to +120°C carbon steel; -40°C to +200°C 316 with FKM seal; -50°C to +200°C brass |
| Check valve | hardened steel ball, HRC 58 – 62; NBR seal standard, FKM optional |
| Grease | NLGI #0 – #3, #2 most common |
Four Overall Lengths
| Overall length | Thread length | Typical recess |
|---|---|---|
| 0.90″ (22.9 mm) | 0.43″ (10.9 mm) | shallow pockets behind thin guards |
| 1.00″ (25.4 mm) | 0.50″ (12.7 mm) | general recessed points |
| 1.08″ (27.4 mm) | 0.55″ (14.0 mm) | deeper counterbores |
| 1.25″ (31.8 mm) | 0.63″ (16.0 mm) | deepest pockets, maximum standoff |
Measure recess depth, guard thickness, and coupler approach space, then take the shortest shank that clears everything; a longer shank bends if struck. At least 5 full threads, about 0.18″ (4.5 mm), must engage.
Materials and Seals
Zinc plated SAE 1215 carbon steel, passivated, suits dry indoor machinery, -40°C to +120°C. The 316 stainless version runs -40°C to +200°C with an FKM seal for wet areas and synthetic ester greases. Brass C36000 / C46400 covers -50°C to +200°C for non-sparking assemblies. All three share a hardened steel ball check valve, HRC 58 – 62, against dirt and backflow.
Installation Notes
Drive the fitting with a 0.125″ (3.2 mm) Allen key; standard-length fittings use an external hex of 5/16″ or 7/16″ across flats. Torque to 8 – 12 ft-lbs (11 – 16 Nm) into cast iron or steel, reduced by 20 – 25% in aluminum. Never use PTFE tape or anaerobic sealant: the straight thread seals by thread form and torque, and shredded compound migrates into the lubrication circuit.
UNF Is Not NPT
A 1/4″-28 UNF fitting and a 1/4 NPT fitting share a nominal name and nothing else. UNF is straight, 28 TPI, 60° form; NPT is tapered at 18 TPI, and they never interchange. Forcing one into the other strips the port.
Applications
Typical installations: machine tool guideways and spindle housings, conveyor bearing blocks, textile machinery, printing and packaging equipment, factory automation frames.
Honest Limits Before You Order
Three boundaries apply. First, 1/4″-28 UNF and 1/4 NPT are not interchangeable; a wrong guess ruins the female thread. Second, the standard grade tops out at 5,000 PSI (345 bar), and a hand gun on a stiff point can exceed that, so high-pressure points need the heavy-duty grade at up to 10,000 PSI (689 bar), confirmed before ordering. Third, no PTFE tape or anaerobic sealant; sealing relies on thread form and torque. Need more standoff than 1.25″ (31.8 mm)? Request a quote with your recess dimensions.
Frequently Asked Questions
When do I need a long 1/4″-28 UNF grease fitting instead of a standard one?
Pick a long shank when the point sits in a recessed pocket, behind a guard, or inside a counterbore the coupler cannot reach. Open positions do fine with standard lengths; metric equipment goes to the M10 page, 1/8 inch tapered pipe thread to the 316 page.
How do I choose between the four overall lengths?
Measure the recess depth, guard thickness, and coupler approach space, then take the shortest length that clears everything. Reserve 1.25″ (31.8 mm) for genuinely deep pockets; longer shanks bend more easily.
Why do these fittings use an internal hex?
A 0.125″ (3.2 mm) Allen socket keeps the head compact at 0.375″ (9.5 mm) where a long shank leaves little room. Standard-length fittings carry an external hex of 5/16″ or 7/16″ across flats.
Will my grease gun coupler fit the head?
The head is a 0.375″ (9.5 mm) hydraulic-type cone, which standard four-jaw hydraulic couplers grip. The real constraint is clearance: the coupler must enter the pocket and approach straight.
How do I tell a 1/4″-28 UNF thread from a 1/4 NPT thread?
Count the pitch: UNF is 28 TPI, NPT is 18 TPI. Then check the form: UNF is straight, NPT tapers. Both are called 1/4 inch but never interchange; cross-fitting destroys the port.
Which material should I pick?
Zinc plated SAE 1215 carbon steel, passivated, covers dry indoor machinery. Choose 316 stainless with an FKM seal for wet areas, heat to +200°C, or synthetic ester greases. Brass suits non-sparking assemblies from -50°C.
What torque should I use, and does the housing material matter?
Torque to 8 – 12 ft-lbs (11 – 16 Nm) into cast iron or steel. In aluminum, reduce by 20 – 25% to protect the softer female thread, and engage at least 5 full threads, about 0.18″ (4.5 mm).
Can I use PTFE tape or thread sealant on the straight thread?
No. A straight UNF thread seals by thread form and torque. PTFE shreds and sealant fragments migrate into the lubrication circuit and can block the fitting; fix thread or torque instead.
Which greases work with these fittings?
NLGI #0 – #3 all work; #2 is the common choice. The standard NBR seal suits petroleum-based greases; for synthetic ester greases, order the FKM seal option.
Who is behind this page and how do I get a quote?
IsoHiTech Lubrication Systems, founded in 2023, engineering since 2006, supplies lubrication fittings, pumps, and system components. Send thread, length, and material requirements to inquiry@isohitech.com, or send your drawing for a quotation.
