What Bearing Grease Nipples Do
A bearing grease nipple is a small check valve mounted in a housing port. It lets a grease gun push lubricant into the bearing cavity, then the internal ball reseats to keep dirt and moisture out. The fitting is chosen by the housing port thread, not by the bearing bore size, because the port is the fixed mounting point.
When grease enters under gun pressure, the ball check opens against its spring. When pressure stops, the ball returns to its seat and blocks the passage. This simple action protects the bearing from contamination while allowing regular relubrication without disassembly.
For a full range of body shapes and thread sizes, browse the grease nipple category before selecting a specific size page.
Head Styles and Coupler Matching
The head style dictates which grease gun coupler you need. A ball head takes the standard jaw coupler found on most manual and pneumatic guns. Button heads and flush heads use their own dedicated couplers, so verify your gun tip before ordering.
Ball heads are the most common choice for general bearing lubrication. Button heads sit lower and work where a protruding ball could snag. Flush heads sit nearly flat and are used on guarded or recessed ports where clearance is minimal.
For conical head designs that follow the DIN-style form, see the conical head grease nipples page. For angled body options, review the angled bodies page.
Thread Systems and What Fits What
Metric, UNF, and tapered pipe threads are not interchangeable. A metric port requires a metric nipple, a UNF port requires a UNF nipple, and a tapered pipe port requires a tapered pipe nipple. Mixing thread systems will damage the port or leak under pressure.
Parallel threads such as metric and UNF seat on a sealing face or a bonded washer and take no thread sealant. Tapered pipe threads, often called NPT, seal by thread deformation and take PTFE tape at two to three wraps. Never use thread-locking compounds on any grease nipple.
| Thread | Typical Use | Where to Buy It |
|---|---|---|
| M6 | Small bearing housings and compact blocks | See the M6 size page |
| M8 | General industrial bearing units | See the M8 size page |
| M10 | Larger pillow blocks and flanged housings | See the M10 size page |
| M12 | Heavy housings and large bearing cavities | See the M12 size page |
| 1/4-28 UNF | Inch-pattern equipment and some hydraulic components | See the UNF size page |
For smaller ports, the M4 size page covers the compact option. The M6 size page lists the common small-port fitting. The M8 size page covers mid-range bearing units. The M10 size page handles larger housings. The M12 grease fitting page covers heavy ports. The UNF size page covers inch-thread ports. JIS coarse variants exist and are a separate page, so check the JIS variant page if your port is not standard metric fine pitch.
Drive Angles and Access
Drive angle describes the direction the grease enters relative to the fitting body. Straight fittings have a zero-degree path and work best when the gun approaches directly in line with the port. Angled fittings at forty-five or ninety degrees solve access problems where a straight gun cannot reach.
Choose a straight fitting when the port is open and the gun has a clear approach. Choose a forty-five degree fitting when the gun must come from a shallow side angle. Choose a ninety degree fitting when the gun must approach perpendicular to the port axis, such as inside a machine frame.
Angled bodies are covered in detail on the angled bodies page linked earlier. Straight and angled options both appear within each thread size page listed above.
Identifying an Unknown Fitting
Measure the thread outside diameter and pitch before ordering a replacement. A metric fine pitch of one point zero is typical for M6, M8, M10, and M12 grease nipples, but confirm per port because some housings use coarse variants. A thread gauge or caliper will separate metric from inch threads quickly.
Compare the measured diameter against known values. M6 measures near six millimeters, M8 near eight, M10 near ten, and M12 near twelve. A 1/4-28 UNF thread measures near six point three five millimeters with twenty-eight threads per inch.
Do not force grease through a damaged nipple. Replace a fitting with a crushed ball, worn hex, or stripped thread rather than risking port damage or poor sealing.
Installation Torque and Care
Torque guidance for parallel metric threads is modest. M6 and M8 fittings are typically tightened to eight to twelve newton meters. M10 fittings are typically tightened to twenty-five to thirty-five newton meters.
Use a wrench on the fitting hex and avoid overtightening. Parallel threads seal on a face or washer, so extra torque does not improve sealing. Tapered pipe threads need only enough torque to compress the PTFE tape at two to three wraps.
If you need help matching a fitting to a housing port, request a fitting recommendation with your measured thread details.
Frequently Asked Questions
Which thread size do I need for my bearing housing?
Measure the port thread outside diameter and pitch with a caliper and thread gauge. Compare those measurements to the metric and inch values listed in the thread table above.
What is the difference between a ball head, button head, and flush head?
A ball head protrudes and uses a standard jaw coupler. Button and flush heads sit lower and require their own dedicated couplers.
Will my grease gun coupler fit every grease nipple?
No, coupler fit depends on head style. A standard jaw coupler fits ball heads, while button and flush heads need matching couplers.
Should I choose a straight, forty-five degree, or ninety degree fitting?
Choose straight when the gun approaches in line with the port. Choose forty-five or ninety degrees when access is angled or perpendicular.
How does the ball check inside a grease nipple work?
Grease gun pressure pushes the ball off its seat and lets lubricant flow through. When pressure stops, the ball reseats and blocks dirt entry.
Are metric and inch grease nipple threads interchangeable?
No, metric and inch threads are not interchangeable. Mixing them will damage the port or leak under pressure.
How do I identify a grease fitting already installed in a machine?
Measure the thread outside diameter and pitch before removing the fitting. Compare those values to the table in the thread systems section.
How tight should a bearing grease nipple be installed?
M6 and M8 fittings take eight to twelve newton meters. M10 fittings take twenty-five to thirty-five newton meters.
Why will grease not go through a nipple during lubrication?
The ball check may be stuck, the passage blocked, or the coupler not fully seated. Replace a damaged nipple rather than forcing grease through it.
Where can I order bearing grease nipples in each thread size?
IsoHiTech Lubrication Systems, founded in 2023, engineering since 2006, builds and supports every unit. Email inquiry@isohitech.com for specifications and a quotation.
Use the size page links above for M4, M6, M8, M10, M12, and UNF options, or ask for a quote on your required thread size.
