What a 1/4-28 UNF Grease Nipple Is
A 1/4-28 UNF grease nipple is a short, straight-thread lubrication fitting with a nominal quarter-inch diameter and twenty-eight threads per inch. The UNF designation means Unified National Fine, a thread form used widely on industrial equipment where a compact, closely spaced thread is needed. The fitting body contains a spring-loaded ball check that admits grease under pressure and closes to keep contaminants out. This page covers the standard short body version, which is the common reference part for general lubrication points. For recessed or hard-to-reach fittings, the extended-reach long version is covered on a separate page.
Maintenance teams often specify this nipple by its thread callout alone, but the body style and material matter just as much. The short body works where the grease coupler can seat directly on the fitting without obstruction. A straight thread means the nipple seals on a mating shoulder or sealing washer, not by thread interference like a tapered pipe thread. That distinction is the root of most ordering mistakes, so the identification steps later in this page are worth reading before you buy.
How the Ball Check Works
The ball check inside the nipple is a small sphere held against a seat by a spring. Grease pressure from the coupler pushes the ball back, opening a path into the bearing or joint. When the coupler releases, the spring returns the ball to its seat and blocks the return flow. This one-way action keeps lubricant in and dirt, water, and abrasive particles out between service intervals.
The check also gives a positive feel at the grease gun, because the operator must overcome spring force before flow begins. A stiff or blocked ball usually means hardened grease or damage inside the fitting. Replacing the nipple is faster than trying to clean the check, especially on a short body part where internal access is limited.
Specifications
The declared values below are the reference set for the standard short 1/4-28 UNF grease nipple. Use them for drawing checks, incoming inspection, and replacement comparison. The thread is straight, not tapered, and the fitting relies on a sealing face at the base of the hex or body shoulder.
| Specification | Value |
|---|---|
| Thread | 1/4-28 UNF |
| Nominal diameter | 1/4 inch, 6.35 mm |
| Threads per inch | 28 TPI |
| Pitch | 0.907 mm |
| Hex size | 9/32 inch or 7 mm |
| Working pressure, carbon steel | 6,000 PSI |
| Working pressure, stainless | 8,000 PSI |
| Body styles | Straight 0 degree, 45 degree, 90 degree, flush |
| Overall length range | 0.75-1.25 inch, 19-32 mm |
Working pressure ratings are for the fitting body and check under normal grease service. They are not a system pressure rating for the machine. Always match the nipple material to the lubrication environment and the base material of the housing.
Threads It Is Commonly Confused With
The 1/4-28 UNF thread is often mixed up with 1/4 NPT, 1/4 BSPP, and M6x1.0 because the nominal sizes sound close. The differences are easy to find with a caliper and a thread gauge. Measure the outside diameter first, then count threads per inch or check pitch, and finally look at whether the thread is straight or tapered.
| Thread form | TPI or pitch | Measured OD | Verdict |
|---|---|---|---|
| 1/4-28 UNF | 28 TPI, 0.907 mm pitch | 6.35 mm | Not interchangeable |
| 1/4 NPT | 18 TPI | About 13.6 mm | Not interchangeable |
| 1/4 BSPP | 19 TPI | About 13.2 mm | Not interchangeable |
| M6x1.0 | 1.0 mm pitch | 6 mm | Not interchangeable |
A 1/4 NPT fitting is tapered and visibly larger, so it cannot thread into a straight UNF hole without damage. A 1/4 BSPP fitting is also larger and uses a parallel thread with a different pitch. The M6x1.0 is only 0.35 mm smaller than the UNF outside diameter, which makes it the most dangerous near-miss because it may start by hand but will bind and strip.
For a wider reference on grease fitting threads and dimensions, see the grease zerk size chart. If you need the metric side of the range, the metric grease fittings hub groups the ISO and DIN options. The M6 grease nipple page covers the near-miss thread in detail.
Standard and Extended Bodies
This page is the reference for the standard short 1/4-28 UNF grease nipple. The short body keeps the fitting close to the housing and works where the coupler has clear access. Overall length across the straight, 45 degree, 90 degree, and flush styles stays within the 0.75-1.25 inch range, or 19-32 mm.
Extended-reach versions are a different product built for recessed fittings, deep bosses, or guarded lubrication points. They use the same 1/4-28 UNF thread but add a longer neck or tube. If your coupler cannot reach the nipple, use the long quarter-inch grease fittings page instead of forcing a standard short body into service.
The adjacent size in the same family is the 5/16 inch nipple, which shares many body and material options. Compare it on the 5/16 grease nipple page. For conical head variants, the conical head grease nipples page shows the head form differences.
Choosing Body Style and Material
Body style follows access. A straight 0 degree nipple works when the grease coupler approaches head-on. A 45 degree or 90 degree nipple moves the coupler connection away from an obstruction such as a guard, bracket, or nearby shaft. A flush nipple sits nearly flat and suits low-clearance points where a protruding fitting would be struck or worn away.
Material choice follows the environment. Carbon steel is the standard for indoor equipment with clean, dry conditions. Stainless steel is the better choice for washdown areas, outdoor exposure, or any point where corrosion would seize the check or damage the thread. Hand-start the nipple for two full turns, then wrench it to full engagement, and do not over-tighten a straight thread fitting against its sealing face.
If you are not sure which body style or material fits your maintenance plan, request a selection review with your thread and access details. The full range of short body options is listed in the grease nipples category.
Frequently Asked Questions
What is a 1/4-28 UNF grease nipple used for?
It is used to deliver grease into a bearing, bushing, or joint through a straight fine thread. The fitting accepts a standard grease coupler and closes after lubrication.
What does 1/4-28 UNF mean on a grease fitting?
The 1/4 refers to a nominal quarter-inch thread diameter, and 28 means twenty-eight threads per inch. UNF stands for Unified National Fine, a straight thread form.
Is a 1/4-28 UNF grease nipple the same as 1/4 NPT?
No, they are not the same. The UNF thread is straight and measures 6.35 mm, while 1/4 NPT is tapered and measures about 13.6 mm.
Is a 1/4-28 UNF grease nipple the same as 1/4 BSPP?
No, the two threads are different. The UNF thread has 28 TPI, while 1/4 BSPP has 19 TPI and a larger outside diameter.
Is a 1/4-28 UNF grease nipple the same as M6x1.0?
No, they are close but not interchangeable. The M6x1.0 thread is 0.35 mm smaller in outside diameter and uses a 1.0 mm pitch.
What pressure rating does a 1/4-28 UNF grease nipple have?
Carbon steel bodies are rated for 6,000 PSI working pressure. Stainless steel bodies are rated for 8,000 PSI working pressure.
Should I choose carbon steel or stainless steel for this nipple?
Choose carbon steel for dry indoor equipment with low corrosion risk. Choose stainless steel for washdown, outdoor, or humid environments.
Which body style do I need for my grease point?
Choose straight for head-on coupler access, angled for obstructed points, and flush for very low clearance. Match the style to the coupler path.
Will a standard grease coupler fit a 1/4-28 UNF nipple?
Yes, a standard coupler fits the nipple head on straight, 45 degree, and 90 degree bodies. Flush bodies may need a narrow or specialty coupler tip.
Can I order 1/4-28 UNF nipples in bulk?
IsoHiTech Lubrication Systems, founded in 2023, engineering since 2006, builds and supports every unit. Email inquiry@isohitech.com for specifications and a quotation.
For volume orders and scheduled deliveries, contact the sales team for a quote.
