What a Grease Fitting Is
A grease fitting is the small metal interface that lets a grease gun push lubricant into a bearing housing, pivot pin, slide way or gearbox without opening the machine. In North America it is usually called a zerk fitting; in Europe and Asia the same part is a grease nipple. A spring-loaded steel ball sits in a drilled channel, held against a seat by a small spring, with a threaded shank below and a shaped head above.
How the Ball Check Works
Inside the fitting body sits a steel ball backed by a spring. At rest, the spring holds the ball against a conical seat, closing the channel. When the operator pushes the coupler onto the head, gun pressure overcomes the spring force and lifts the ball off its seat. When the coupler is removed, the spring pushes the ball back onto the seat, blocking dust, water and metal particles from entering the bearing cavity.
Head Styles
Head geometry decides which coupler grips the fitting and how much clearance the gun needs.
| Head style | Typical use |
|---|---|
| Straight | Standard access, gun approaches along the fitting axis |
| 45 degree | Angled access where a straight gun path is blocked |
| 90 degree | Tight clearance or recessed ports where the gun must come in from the side |
| Button head | Flat mating face for a matching button-head coupler on a low-clearance joint |
| Flush | Recessed or surface-mounted points where the fitting must sit nearly level |
| Drive-type | Pressed or driven into a prepared hole instead of threaded |
| Thread-forming | Cutting its own thread path in softer housing material |
| Remote fitting system | Moving the lubrication point away from the hot or blocked original location |
Button head fittings pair only with a button-head coupler, so see the button head grease fitting page. Coupler grip is covered on the locking grease coupler page, and protective caps on the grease nipple caps page.
Choosing the Angle
A straight fitting is the default, but only works with a clear axial path. A 45 degree fitting redirects the head partway toward the open side; a 90 degree fitting turns it a full quarter turn, suiting recessed ports. Both angled versions are collected on the angled grease nipples page, with the right-angle family at 90 degree grease nipple.
Material Options
| Material | Where it suits |
|---|---|
| Steel | Indoor machinery, protected housings, general plant use |
| Stainless steel | Wet, caustic, wash-down and outdoor exposure |
| Brass | Softer housings and light-duty or low-torque points |
Steel handles 4,500 to 10,000 PSI depending on model. Stainless steel resists corrosion where moisture or chemicals attack plated steel; see the stainless steel grease nipples page. Operating temperature runs from -29°C to +177°C.
Thread Sizes and Identification
| Thread | Family | Note |
|---|---|---|
| 1/8″-27 NPT | NPT | Tapered, seals on the thread |
| 1/4″-28 UNF | UNF | Fine pitch straight thread |
| 3/8″-24 UNF | UNF | Fine pitch straight thread |
| M6 x 1 | Metric | Straight metric thread |
| M8 x 1 | Metric | Straight metric thread |
| M10 x 1 | Metric | Straight metric thread |
| M12 x 1.5 | Metric | Straight metric thread |
Unscrew the old fitting and match it against a sample or thread gauge rather than measuring diameter alone. The metric family has its own reference page at DIN 71412 grease nipples, and individual metric points are listed on the M8 and M10 grease nipple pages. Fitting height runs 0.5″ to 1.5″, with hex across flats 7 to 17 mm depending on model.
Replacing a Worn Fitting
- Identify the thread and confirm the family before ordering.
- Remove the old fitting with the correct wrench size.
- Clean the port of old grease, sealant and particles.
- Install hand-tight plus about a quarter turn, and stop.
Frequently Asked Questions
Are zerk and grease fitting the same thing?
Yes. Zerk fitting is the common North American name, while grease fitting and grease nipple are the general terms. All describe the same threaded part with a spring-loaded ball check and a coupler-mating head.
How does the check valve inside the fitting work?
Gun pressure lifts the spring-loaded steel ball off its seat so grease passes through. When the coupler is removed, the spring pushes the ball back onto the seat.
Should I choose a conical or a button head fitting?
Conical heads are the default and mate with standard couplers. Button heads use a flat face and need a matching button-head coupler.
How do I pick between straight, 45 degree and 90 degree?
Choose the angle that lets the gun approach the head square-on with the coupler fully seated. Step to 45 degree when the axial path is blocked, and 90 degree when the point is recessed.
When is a stainless steel fitting required?
Use stainless wherever moisture, wash-down chemicals or outdoor weather would corrode a plated steel body. A corroded head prevents the coupler from sealing.
How do I identify the thread on my fitting?
Match the removed fitting against a known sample or thread gauge. Do not rely on outer diameter alone, because NPT, UNF and metric shanks are not interchangeable.
What should I do if the fitting is clogged?
High resistance at the gun usually means the ball or channel is blocked. Clean the fitting if the seat is intact, and replace it if the ball no longer seals.
How tight should a new fitting be installed?
Hand-tight plus about a quarter turn seats the thread without deforming it. Excess torque adds no sealing benefit and risks cracking a soft housing.
Does the coupler have to match the fitting?
Yes. A conical coupler grips a conical head, and a button-head coupler fits a flat button face. Pairing the wrong two lets grease escape at the seal.
Where can I get help choosing the right fitting?
IsoHiTech Lubrication Systems, founded in 2023, engineering since 2006, supplies grease fittings across steel, stainless steel and brass in straight, 45 degree and 90 degree versions and in NPT, UNF and metric threads. Send the thread size, head style, angle and material to inquiry@isohitech.com, or use the quote request form. The same contact page handles mixed orders.
