S-GN Series Grease Nipples (Grease Fittings / Zerk Fittings)

Frequently Asked Questions: Grease Nipples and Grease Fittings

What is a grease nipple and what is it used for?

A grease nipple (also called a zerk fitting, grease fitting or lubricating nipple) is a small one-way valve threaded into a bearing housing, joint or pivot point to allow pressurised grease delivery from a grease gun. The internal spring-loaded ball check valve opens under grease gun pressure (3,000 to 12,000 PSI) and closes when the coupler is removed, sealing the lubrication point against dirt and moisture. Every piece of rotating or sliding machinery with serviceable bearings uses grease nipples: automotive chassis joints, tractor pivot points, excavator pins, conveyor bearings and industrial equipment.

What is the difference between SAE and metric grease fittings?

SAE fittings use imperial thread sizes (1/4"-28 UNF, 1/8" NPT, 5/16"-32 UNF) while metric fittings use millimetre-based threads (M6x1, M8x1, M10x1, M12x1.5). They are not interchangeable because the thread pitch and diameter differ. North American equipment predominantly uses SAE threads. European, Asian and much modern agricultural machinery uses metric. Australian and UK equipment often uses BSP (British Standard Pipe) threads. Always identify the existing thread before ordering replacements; forcing the wrong thread damages the tapped hole permanently.

How does the ball check valve inside a grease nipple work?

The ball check is a spring-loaded steel ball seated against a machined bore inside the nipple body. When the grease gun coupler engages and applies pressure, the ball lifts off its seat, opening a passage for grease to flow through the fitting and into the bearing cavity. When the coupler is removed and pressure drops, the heat-treated spring pushes the ball back onto its seat, creating a seal that prevents dirt, water and debris from entering the lubrication point. This one-way mechanism is why grease stays inside the bearing between service intervals.

What thread sizes are most common for grease nipples?

The three most common grease fitting threads worldwide are 1/8" NPT (North America), 1/4"-28 UNF (North America, SAE standard) and M6x1 metric (Europe, Asia). In the UK and Australia, 1/8" BSP is standard. For larger equipment, M8x1, M10x1 and 1/4" NPT are common. STAR manufactures all standard thread sizes from M6 through M16 in metric, and 1/8" through 3/8" in imperial (UNF, NPT and BSP). Measure the existing fitting with calipers or a thread gauge before ordering replacements.

When should I use a 90° grease nipple instead of straight?

Use a 90° fitting when the grease point faces a wall, frame rail, housing or adjacent component that completely blocks straight-line coupler access. The 90° bend routes the coupler engagement point perpendicular to the thread axis, allowing the grease gun to connect from the side. Common applications include king pins behind steering knuckles, suspension bushings against frame rails, and pivot points inside enclosed housings. If partial obstruction exists but straight access is still possible with effort, a 45° fitting provides a compromise angle.

What is the difference between NPT and BSP grease fittings?

NPT (National Pipe Thread) is a tapered thread standard used in North America, while BSP (British Standard Pipe) is used in the UK, Australia, Asia and much of Europe. Both are 1/8" nominal size for grease fittings, but the thread angle differs: NPT uses a 60° thread angle while BSP uses a 55° angle. They are not interchangeable. Cross-threading an NPT fitting into a BSP hole (or vice versa) creates a poor seal and damages the threads. Always verify your equipment's thread standard before ordering.

How do I fix a blocked or clogged grease nipple?

Remove the fitting with a wrench, then clear the blockage using a thin wire or needle pushed through the ball check from the thread end. If hardened grease is the cause, soak the fitting in solvent (mineral spirits or brake cleaner) for 10-15 minutes to dissolve the blockage. Test by pressing the ball with a pin; it should move freely against spring pressure. If the ball is seized or the spring is broken, replace the fitting entirely. Before reinstalling, also clear the grease passage in the equipment using a wire or compressed air to ensure grease can reach the bearing.

Can I use any grease gun coupler on any grease nipple?

Standard hydraulic couplers and 4-jaw couplers fit all standard-head grease nipples regardless of thread size, because the coupler grips the external head diameter which is standardised. Button head (flush) fittings require a dedicated button-head coupler or adapter because the head sits below the surface. Pin-type fittings require a pin-type coupler. Always match the coupler type to the nipple head style. Thread size does not affect coupler compatibility; it only determines which tapped hole the fitting screws into.

What causes a grease nipple to break off during service?

The most common cause is over-torquing during installation, which creates stress fractures in the neck. Other causes include corrosion weakening the body (especially in outdoor or marine environments), impact damage from adjacent moving parts, and fatigue from repeated high-pressure greasing cycles on a fitting that was already cracked. To prevent breakage: install to the correct torque (hand-tight plus 1/4 turn for tapered threads), use anti-seize on the threads in corrosive environments, and replace any fitting that shows visible corrosion or deformation.

How often should grease fittings be replaced?

Replace grease nipples when the ball check no longer seals (grease leaks back out after the coupler is removed), when the head is deformed and the coupler cannot grip properly, or when corrosion has compromised the body. In normal service, a quality heat-treated fitting lasts 3-5 years or longer. In harsh environments (marine, mining, chemical exposure), inspect fittings annually and replace any that show corrosion, deformation or failed ball checks. Always replace fittings during major equipment overhauls as preventive maintenance.

What is DIN 71412 standard for grease nipples?

DIN 71412 is the German industrial standard that specifies dimensions, materials and performance requirements for grease nipples used in automotive and industrial applications. It covers straight (Form A), 45° (Form B) and 90° (Form C) configurations. STAR grease nipples conform to both DIN 71412 and SAE J534, ensuring compatibility with equipment manufactured to either standard. The DIN standard is widely referenced in European, Asian and Middle Eastern equipment specifications.

Should I use stainless steel or brass grease nipples for marine applications?

303 stainless steel is the standard choice for marine environments because it resists saltwater corrosion without relying on a sacrificial zinc coating that eventually wears through. Brass is suitable for chemical environments where even stainless steel may corrode (certain acids, chlorine-heavy washdown). For boat trailers, marine winches, davits and deck equipment that see regular saltwater exposure, stainless steel fittings eliminate the corrosion-related ball check failures that cause standard zinc-plated fittings to seize within 6-12 months in coastal conditions.

How do I measure the thread size of a grease nipple?

Use calipers to measure the major (outside) diameter of the thread, then use a thread pitch gauge to determine the pitch (threads per inch for imperial, or millimetre pitch for metric). Common measurements: 1/4"-28 UNF measures approximately 6.35 mm diameter with 28 threads per inch. M6x1 measures 6 mm diameter with 1 mm pitch. M8x1 measures 8 mm diameter. M10x1 measures 10 mm diameter. If the fitting is still installed, measure the hole diameter and match to a thread chart. STAR provides a grease fitting size chart with our assortment kits.

How do I install a grease nipple correctly?

Clean the tapped hole with a wire brush or tap to remove debris and old thread sealant. For tapered threads (NPT), apply a thin layer of PTFE tape or pipe sealant to the male threads. Thread the fitting in by hand until finger-tight, then tighten with a wrench: 1/4 turn past finger-tight for tapered threads, or to the specified torque for parallel threads. Do not over-tighten; excessive torque cracks the fitting body or strips the tapped hole. After installation, connect a grease gun and pump until fresh grease appears at the bearing seal to confirm the grease passage is clear.

What are the risks of high-pressure grease injection injury from a grease fitting?

Grease injection injury occurs when high-pressure grease (3,000 to 12,000 PSI) penetrates the skin, typically through a pinhole leak in a hose or at a fitting connection. The entry wound appears minor but grease spreads through tissue planes causing severe inflammation, infection and potential amputation if not treated within hours. Never place fingers over a grease nipple while someone else operates the gun. Never test for leaks by running a hand along a pressurised hose. If injection occurs, seek emergency medical treatment immediately and inform staff it is a high-pressure injection injury.