Grease Nipple & Zerk Fitting Guide: Sizes, Types & Threads

Grease nipples look simple — they're not. Four global thread standards in use, four body geometries, three materials, and a national split where the rest of the English-speaking world uses one system and Australia uses another. Get the thread wrong and you cross-thread the host casting permanently. Get the body geometry wrong and you can't fit a grease gun onto it. Get the material wrong and you snap the body installing it. This guide covers every thread standard, every body type, every blockage symptom, and the broken-nipple recovery workflow that gets you back to greasing instead of swearing.

What is a grease nipple — and is it the same as a zerk fitting?

A grease nipple is a small one-way valve threaded into a bearing housing, pivot pin, joint or moving part. When you attach a grease gun coupler to the nipple and pump the handle, fresh grease enters through the nipple and pushes old, contaminated grease out the other side of the bearing. A spring-loaded check ball seals the nipple between uses so dirt can't enter and grease can't escape.

Is a zerk fitting the same thing? Yes. "Zerk" is the US name (after Oscar Zerk, who patented the pin-style fitting in 1929). "Grease nipple" is the Australian, British, European, South African and New Zealand name. Both terms refer to the same product class. AU industrial buyers call them grease nipples; American mechanics, agricultural and 4WD enthusiasts call them zerks. The ball end where the grease gun connects is universally the same dimension across all brands and thread types — only the threaded shank that screws into the host part varies.

This guide uses "grease nipple" as the Australian standard term and flags "zerk" where US sources or AU 4WD/agricultural workshops use the import terminology. Both terms are correct.

The four thread standards every AU workshop must know

Grease nipple confusion is almost always thread confusion. There are four primary global thread standards in active use, and an AU workshop will encounter all four depending on what equipment comes through the door:

The AU reality across most workshops:

• 1/8" BSP is the default for Australian-manufactured equipment, garden machinery, trailers, and most Australian-spec plant.
• 1/4"-28 UNF appears on automotive (especially older Holden/Ford), US-imported 4WD parts, and US-built heavy plant.
• M-thread metric shows up wherever European equipment is in service — increasingly common as European trucks and construction plant expand into the AU market.
• 1/8" NPT is least common but still seen on US hydraulic and pneumatic equipment.

Workshop assortment kits like the Champion CA2415 Metric & Imperial Master Kit ($265.12), the Champion CA109 Metric Assortment Kit (105 pieces) ($88.35), the Champion CA108 Imperial Assortment Kit (113 pieces) ($105.66), and the GJ Works GKA144 144-piece Kit ($54.11) include sizes across all four standards specifically because no AU workshop avoids encountering all of them.

BSP vs NPT — the apprentice trap that destroys host castings

The two thread families also seal differently:

• 1/8" NPT is tapered — the thread wedges itself into the host casting and seals on the thread interface itself as it tightens. No gasket required.
• 1/8" BSP is parallel (BSPP — British Standard Pipe Parallel) — the thread provides mechanical grip only. Sealing requires either a copper or fibre washer under the head, a small dab of thread sealant on the thread, or a tapered seat in the host casting (BSPT variant, less common in grease nipples).

Cross-threading damage is the single most common host-casting failure in AU workshops. The fix is expensive — drill the hole oversize, tap M8 or M10, and fit an oversize Helicoil-style insert (see the Stripped Thread Repair Guide). Prevention is to identify the thread correctly before reaching for a wrench.

Body geometry — straight, 45°, 67°, 90°

The thread is one half of the selection decision. The body geometry — the angle at which the ball-end sits relative to the thread — is the other half, and it's driven entirely by access. Where there's room to swing a grease gun straight onto the nipple, a straight-body nipple works fine. Where there's no room — and on most modern equipment there isn't — you need an angled body to redirect the grease gun coupler to a usable angle.

Practical workshop rule: stock straight, 45°, and 90° in each thread size you encounter regularly. The 67° fittings are usually OEM-spec — order on demand rather than stocking. The Champion refill packs ($6.76–$12.13 each) and the multi-angle assortment kits cover the everyday workshop spectrum.

How to identify an unknown grease nipple

An unmarked nipple from an unknown source — pulled out of a customer's vehicle, found in a parts bin, or stripped off a serviced asset — needs identifying before you order a replacement. The systematic approach:

1. Look at the thread profile. If the threaded shank is visibly conical (wider at the head, narrower at the tip), it's NPT (tapered). If parallel, it's one of BSP, UNF, or metric.
2. Measure the thread diameter with calipers — the major diameter at the head end. Common diameters:
   • 1/8" BSP: 9.7mm major diameter
   • 1/4" BSP: 13.2mm major diameter
   • 1/4"-28 UNF: 6.35mm major diameter
   • 1/8" NPT: 10.2mm major diameter at the head, tapering down
   • M6×1: 6.0mm major diameter
   • M8×1: 8.0mm major diameter
   • M10×1: 10.0mm major diameter
3. Count the threads per inch (TPI) or measure the pitch. A thread gauge is the fastest tool. 28 TPI on a 9.7mm parallel thread = 1/8" BSP. 27 TPI tapered = 1/8" NPT. 1.0mm pitch on an 8mm parallel thread = M8×1. The Metric vs Imperial Fasteners Guide covers thread identification in more detail.
4. Check the hex flats. 1/8" BSP commonly has 10mm or 14mm hex flats. 1/4"-28 UNF has 11/32" (8.7mm). Metric M6/M8/M10 typically has 8mm, 10mm, or 12mm. This is a confirming check, not a primary identifier.
5. Test-fit a known nipple from the assortment kit as a final confirmation before installing the replacement.

The Workshop Buddy Metric Grab Kit (M6×1 to M10×1.5) ($33.33) and Imperial Grab Kit (1/8"-27 to 1/4"-28) ($33.33) are designed specifically for this — both kits include a test-fit selection across the common AU sizes for ~$67 total.

Materials — mild steel, stainless steel, brass

Grease nipple body material drives corrosion resistance, strength, and price:

The most common installation mistake is over-torquing a brass-body nipple. Brass bodies have roughly one-third the yield strength of steel and snap at the thread root when over-tightened. Practical Machinist forum consensus: torque brass to 8 Nm max (feel: a snug hand-spanner pull, not a wrench pull). Steel and stainless tolerate 20-30 Nm comfortably.

For coastal Australia, marine industry, and food-grade processing, stainless 316/A4 is the only sensible specification. The premium over mild steel is modest (Champion SSCN29 stainless 25-pack at $76.44 vs CN29 mild steel equivalents at ~$58.88) and corrosion-driven failure costs vastly more than the upfront fitting cost.

Button head fittings — high-pressure / hard-to-reach

The standard grease nipple has a small ball-end designed for a standard push-on grease gun coupler. Button head fittings are an alternative geometry with a larger, flatter head designed for a dedicated button-head coupler. Two scenarios drive button-head selection:

1. Hard-to-reach applications — the larger flat head accepts a coupler at a wider range of angles without slipping off. Excavator stick pins, articulated dump truck joints, and large agricultural equipment commonly use button heads on points where the operator can only approach the fitting from off-axis.
2. High-pressure greasing — button heads handle higher line pressures (10,000+ psi capable) without coupler blow-off, making them the standard on assets serviced with high-pressure grease guns. See the just-published Grease Gun Guide for the high-pressure gun selection and the 10,000 psi vs 5,000 psi vs hand-pump rating breakdown.

Button head fittings require a matching button-head coupler — a standard grease gun coupler won't grip a button-head fitting. Workshops that service mining or large construction plant typically keep both styles of gun ready, or use a Macnaught Lock-N-Lube-style universal coupler that accommodates both.

AIMS stocks button head fittings in standard thread sizes — search /collections/grease-nipples for button head variants. Compatible button-head couplers and high-pressure grease guns are available in /collections/grease-guns alongside the Macnaught universal Lock-N-Lube range. For lubrication oil dispensing (engine oil, gear oil, hydraulic oil) rather than grease, see the Oil Pump & Drum Pump Guide covering Macnaught BOP battery, OILMASTER and R-SERIES air-operated drum pumps. Specialty heavy-mining button heads sourced on request through our supplier network. Contact us for OEM-spec mining/excavator button head fittings.

Header blocks and central greasing

Central greasing systems consolidate multiple grease points into a single accessible location via a header block — a manifold with one input port and multiple output ports that route grease to individual bearings through small-bore tubing.

The Alemlube Header Block 5 Outlet Fittings and Grease Nipples 6135 ($103.28) is a complete 5-point central greasing manifold. Mount the block in an accessible workshop location, run small-bore copper or nylon line from each outlet to the target bearing, and you can service five hard-to-reach points from one easy-access nipple instead of climbing onto equipment to reach each one individually.

The use cases:

• Heavy plant with multiple buried bearings — agricultural machinery, conveyors, large fans
• Confined-space machinery where bearings sit behind guards, panels, or in unsafe-access positions
• Production-line equipment requiring scheduled multi-point greasing on shutdown — central greasing reduces the per-cycle labour from 30 minutes (climbing, removing guards, accessing each point) to 5 minutes (one technician at the manifold)

For workshops servicing assets with more than 5–6 grease points, full automatic central greasing systems (electric pumps + distribution manifolds + timer controllers) become viable. Header blocks are the manual-pump entry tier.

How to install a new grease nipple correctly

Whether replacing a broken nipple or fitting a new one to a custom assembly, the workflow:

1. Confirm the existing thread (if replacing). Wrong thread = host casting damage. See the identification section above.
2. If drilling and tapping a new hole: drill to the tap drill size for your chosen thread. Common drill/tap pairs: 1/8" BSP = 8.8mm drill / R 1/8" tap. 1/4"-28 UNF = 5.5mm drill (#3 letter drill) / 1/4"-28 UNF tap. M6×1 = 5.0mm drill / M6×1 tap. M8×1 = 7.0mm drill / M8×1 tap. The Tap & Die Guide covers tapping technique.
3. Tap straight — perpendicular to the surface. An off-axis tap creates an off-axis fitting that the grease gun coupler won't engage cleanly.
4. Clean the new thread with compressed air or a thread-clearing pick to remove swarf. Swarf inside the thread will gall the nipple as you fit it.
5. Apply thread sealant or PTFE tape on parallel threads (BSP, UNF, metric). NPT tapered threads typically don't need sealant. See the Thread Locking & Sealing Guide for sealant selection — Loctite 577 or 567 for high-pressure applications, PTFE tape for general workshop work.
6. Hand-thread the nipple in for at least 3 full turns. If you feel resistance in the first 2 turns, stop — wrong thread or cross-threading. Back out, check, re-confirm.
7. Wrench-tighten to 20-30 Nm for steel/stainless, 8-10 Nm for brass. A small open-ended spanner is the right tool — not a socket wrench, which over-torques easily.
8. Test with the grease gun — attach the coupler, pump 2-3 strokes. Grease should enter without back-pressure. Disconnect cleanly. Wipe excess grease off the fitting and surrounding surface.

For high-vibration applications (excavator pins, agricultural drawbar pivots, rotary equipment) add a small dab of Loctite 243 medium-strength threadlocker on the thread before installation. This prevents the nipple from backing out under vibration without preventing future removal.

Why won't my grease nipple take grease? — 6 diagnostic steps

A grease nipple that won't accept grease is the #1 maintenance frustration. The systematic diagnostic:

1. Confirm the grease gun itself is working. Disconnect the coupler from the nipple and pump the handle — grease should emerge from the coupler. If not, the gun is the problem (empty cartridge, blocked feed, air-locked priming — see the Grease Gun Guide diagnostic).
2. Confirm the coupler is fully engaged. A loose coupler engagement vents grease around the connection rather than into the nipple. Push the coupler firmly onto the nipple ball — you should hear or feel a slight click. If the ball end is dirty, contaminated grease prevents seal. Wipe clean and re-try.
3. Test the nipple ball valve with a small nail. A 16-gauge nail or pick depressed against the ball should move freely. If the ball is stuck, the internal mechanism is fouled — see the unblocking section below.
4. Check for back-pressure. If the grease gun handle gets harder and harder to pump but no grease moves into the nipple, the back-pressure is downstream — the bearing cavity itself may be full of contaminated grease that won't compress further, or the bearing's relief path is blocked. Solution: continue moderate pressure, allow time for the bearing to relieve, or in extreme cases dismantle and clean.
5. Verify the bearing has a grease path. Some sealed bearings appear to have a grease nipple but the bearing internal cavity is fully sealed — the nipple is decorative or non-functional. Common on small budget machinery imported from low-cost markets. Check the equipment service manual.
6. Check for contaminated grease in the nipple. If old hardened grease has solidified inside the nipple body itself (not the bearing cavity), grease physically can't move through. See the unblocking section.

Clearing a blocked grease nipple

If diagnosis points to a blocked nipple (gun works, bearing cavity has grease path, but the nipple itself won't accept grease), four field techniques in order of escalation:

1. The rag trick (Dorman Shop Press technique). Fold a thin rag (old t-shirt cotton) over the nipple. Press the grease gun coupler firmly onto the nipple through the rag. The rag creates a better seal and applies even pressure to the ball valve. Pump 3-4 strokes. Often releases mild contamination on the ball.
2. The 16d nail trick (universal forum consensus). A 16-gauge nail or pick tip depressed firmly against the centre ball of the nipple, used to physically dislodge contamination. Forum threads from Snowblower Forum, MyTractorForum, and Practical Machinist consistently report this resolves "about 90% of stuck-ball" cases. Wipe excess clean before retrying with the grease gun.
3. Penetrating oil + back-and-forth pin rotation. If the ball valve mechanism itself is seized (not just dirty), apply penetrating oil to the ball end, work the ball back and forth with a pin or nail tip until it moves freely. Wait 15-20 minutes for the penetrant to wick into the spring mechanism. Re-try the grease gun.
4. Heat application + immediate grease. Warm the nipple body briefly with a propane torch (5–10 seconds, not red-hot). The heat thins the hardened grease inside the nipple. Immediately attach the grease gun and pump while the body is still warm. The thinned grease pushes out, fresh grease pushes in. Use sparingly — repeated heat-cycling can degrade the nipple's spring temper.

If all four techniques fail, the nipple is mechanically failed (broken spring, deformed body) and needs replacement. The Grease Buster-style dedicated tool (penetrating oil reservoir + impact piston) handles severe cases without damaging the nipple body — useful where the nipple is in a position you can't easily replace.

How to remove a broken grease nipple

A snapped grease nipple — head broken off flush with the host surface, or partial thread shear leaving a stub in the casting — requires extraction. The good news: grease nipple bodies are mild steel (or stainless), much softer than hardened bolts. Extraction is usually straightforward.

1. Apply penetrating oil to the broken stub. Allow 20 minutes minimum for the oil to wick into the thread.
2. If a stub protrudes: grip with locking pliers (Vise-Grip style) and rotate counterclockwise. Many broken nipples back out at this stage without any extractor needed.
3. If flush with surface or below: centre punch the broken nipple. Drill a small pilot hole (2.5mm-3mm) into the centre. Use a screw extractor (Easy-Out) at #1 or #2 size — see the just-published Bolt Extractor Guide for the combined left-hand drill + extractor workflow.
4. For really stubborn nipples: drill the entire body out to the tap drill size for the thread, then re-tap the original thread to clean it up. The Bordo or Sutton M603S20L combined left-hand drill + extractor sets handle this in 5 minutes.
5. If the thread is damaged during extraction: follow the Stripped Thread Repair Guide — drill oversize, tap M-larger, and fit a Helicoil thread insert.

Avoid hammering or applying side-load to the broken stub. Grease nipple bodies are softer than the host casting but the host thread is the part you're trying to preserve — damaging the host thread is far more expensive than the $2 nipple.

Grease nipple caps — when dust protection matters

A grease nipple cap is a small dust cover that snaps over the ball end of the fitting. The Champion CGNC10 Grease Nipple Cap ($13.93) is the AIMS workshop default.

Caps protect the ball end between greasing intervals — without one, the nipple ball is exposed to dust, water, and chemical contamination. Buy in bulk packs from /collections/grease-nipples.

Caps matter most in three scenarios:

• Dusty environments — mining, quarrying, agriculture, construction sites. Dust accumulates on the ball end between greasing intervals, and the first grease gun stroke pushes contamination directly into the bearing. The cap blocks contamination at the source.
• High-pressure water washdown — food processing, marine, dairy. Water under pressure forces past the ball valve into the bearing cavity, displacing grease and accelerating corrosion. Caps with O-ring seals are mandatory for these applications.
• Long service intervals — equipment greased monthly or quarterly. The longer the gap between greasing, the more dust accumulates, and the higher the contamination risk per service.

For most general workshop environments — indoor maintenance, dry industrial — caps are optional. For mining, marine, food processing, agricultural and outdoor heavy plant they should be standard fitment.

Selection guide — workshop kit vs OEM-spec replacement

Two distinct purchase scenarios drive different product selection:

Scenario 1: Workshop assortment for general maintenance. You service a mix of equipment — agricultural, automotive, plant — and need ready access to common sizes. Buy assortment kits:

• Light workshop: Workshop Buddy Metric Grab Kit M6-M10 ($33.33) + Workshop Buddy Imperial Grab Kit 1/8-1/4 ($33.33). $67 covers most common AU sizes.
• Mid workshop: GJ Works GKA144 144-piece Kit ($54.11) — broader range, single case.
• Production / fleet workshop: Champion CA2415 Metric & Imperial Master Kit ($265.12) — comprehensive, includes premium stainless, fits the daily-use serviceable workshop.

Scenario 2: OEM-spec replacement. You've identified an exact fitting type, size, and geometry, and need replacement-spec only:

• Bulk replacement (10-25 of one type): Champion CN-series 25/packs ($11.82–$76.44), perfect for fleet servicing where you replace nipples on a schedule.
• Single replacement: Alemlube individual refills ($2.55–$3.50) — buy one of the exact spec.
• Refill packs for trade box: Champion Refill packs ($6.76–$12.13) — 10-piece packs in straight, 45°, 67°, 90° for replenishing your trade box.
• Marine / food-grade replacement: Champion SSCN-series stainless 316/A4 ($35.95–$76.44 per pack) — non-negotiable for these environments.

AIMS supply — Champion, Alemlube, Lubemate, Workshop Buddy, GJ Works

AIMS stocks 160+ unique grease nipple SKUs across five vendors in /collections/grease-nipples — the deepest grease nipple supply position in AU industrial distribution outside the specialty lubrication trade. Vendor profile:

Browse the complete range at /collections/grease-nipples. For grease guns, couplers and high-pressure equipment to use with these fittings see /collections/grease-guns and the Grease Gun Guide. For lubricants and grease selection see /collections/greases and the Grease Types & Selection Guide.

Honest scope: AIMS doesn't stock Tecalemit (UK premium specialty), Lincoln Industrial (US specialty branded), or Stauff (German central greasing). These specialty brands are available through our supplier network on request — contact us if you need a specific brand-spec replacement for European or US heavy plant.

Common mistakes — failure modes table

Frequently Asked Questions

What is a grease nipple and what does it do?

A grease nipple is a small one-way valve threaded into a bearing housing, pivot pin, or moving part. When a grease gun coupler attaches and pumps grease through, the nipple's internal spring-loaded ball valve opens to admit fresh grease and closes between strokes to prevent grease escape and dirt ingress. The nipple allows scheduled relubrication without dismantling the bearing.

Is a zerk fitting the same as a grease nipple?

Yes — same product, different name. "Zerk" is the US term (after Oscar Zerk, who patented the design in 1929); "grease nipple" is the Australian, British, European and New Zealand term. Both refer to the same one-way valve fitting. AU industrial buyers use "grease nipple"; 4WD and agricultural buyers from US-influenced supply chains sometimes use "zerk." The product is identical.

What thread size is a standard grease nipple in Australia?

1/8" BSP is the dominant standard on Australian-manufactured equipment, garden machinery, trailers, and AU-spec plant. 1/4"-28 UNF is common on automotive (especially older Holden/Ford) and US-imported 4WD. M-thread metric (M6×1, M8×1, M10×1) appears on European-built equipment. 1/8" NPT shows up on US hydraulic equipment. AU workshops typically stock all four standards because all four appear regularly.

How do I tell BSP from NPT grease nipples?

1/8" BSP has 28 threads per inch with a parallel shank (cylindrical, same diameter top to bottom). 1/8" NPT has 27 threads per inch with a tapered shank (visibly cone-shaped, wider at the head than the tip). A thread gauge confirms TPI in seconds. They look almost identical — 1/8" BSP at 28 TPI vs 1/8" NPT at 27 TPI — and cross-threading damages the host casting. Hand-thread the nipple three turns minimum; if you feel resistance, stop and re-verify.

What's the difference between metric and imperial grease nipples?

Imperial (1/8" BSP, 1/4"-28 UNF, 1/8" NPT) measures in inches and TPI. Metric (M6×1, M8×1, M10×1) measures in millimetres and pitch in millimetres. The two systems are not cross-compatible — an M6×1 nipple won't fit a 1/4"-28 UNF hole even though they're similar sizes. Always match metric-to-metric and imperial-to-imperial. Champion CA2415 master kit and similar assortment kits include both systems for AU workshops servicing mixed equipment.

Are all grease nipple ball ends the same size?

Yes. The ball end where the grease gun coupler attaches is universally the same dimension across all standard grease nipples regardless of thread type or angle. A standard grease gun coupler fits a 1/8" BSP nipple, a 1/4"-28 UNF nipple, a M6×1 nipple, and a 1/8" NPT nipple identically. The thread is purely how the nipple fits into the host casting; the coupler end is global standard. Button head fittings are the exception — they have a larger flat head and require a dedicated button-head coupler.

Why won't my grease nipple take grease?

Six possible causes: (1) grease gun itself is empty or air-locked, (2) coupler not fully engaged on the ball, (3) ball valve in nipple is stuck with contamination, (4) bearing cavity is full and back-pressure prevents further grease entry, (5) bearing is internally sealed and the nipple is decorative/non-functional, or (6) hardened grease has solidified inside the nipple body itself. Diagnose in that order — gun → coupler → ball → cavity → bearing path → nipple body.

How do I unblock a grease nipple?

Four techniques in escalating order: (1) the "rag trick" — fold a thin rag over the nipple and press the coupler through it for better seal; (2) the "16d nail" trick — depress the ball valve with a nail or pick tip to dislodge contamination (works 90% of stuck-ball cases per forum consensus); (3) penetrating oil plus gentle back-and-forth ball-pin rotation; (4) heat the nipple body briefly with a propane torch then immediately attach the grease gun. If all four fail, replace the nipple.

Can I remove a broken grease nipple without damaging the casting?

Yes, usually. Grease nipple bodies are mild steel — much softer than hardened bolts. Apply penetrating oil, wait 20 minutes. If a stub protrudes, grip with locking pliers and turn counterclockwise — many back out at this stage. If flush or below: centre punch, drill a 2.5-3mm pilot hole, use a #1 or #2 screw extractor (Bordo or Sutton Easy-Out). See the Bolt Extractor Guide for the full extraction workflow. Replace any nipple after removal — don't reinstall used.

What's a button head grease nipple and when do I use one?

A button head fitting has a larger, flatter head designed for a dedicated button-head coupler. Used in two scenarios: hard-to-reach applications where the coupler needs to engage from off-axis (excavator pins, articulated joints, large agricultural equipment), and high-pressure greasing applications (10,000+ psi capable) where standard couplers can blow off under pressure. Button heads require a matching coupler — a standard grease gun coupler won't work. Common on heavy plant and mining equipment.

Should I use stainless or brass grease nipples outdoors?

Stainless 316/A4 — every time. Stainless handles marine salt-spray, food-grade washdown, chemical exposure, and outdoor weather. Brass tarnishes and has low torque tolerance (snaps at 8-10 Nm). Mild steel zinc-plated works for dry indoor workshops but rusts in marine, coastal, or wet environments within 6-12 months. The Champion SSCN-series stainless 316/A4 range costs modestly more than mild steel — a few extra dollars per pack — and prevents corrosion-driven failure that costs orders of magnitude more.

What torque should I use to install a grease nipple?

Steel and stainless steel: 20-30 Nm — a firm hand-spanner pull with a 10mm or 14mm open-ended spanner. Brass: 8-10 Nm only — a snug spanner pull, not a wrench pull. Brass snaps at the thread root if over-torqued. Always use an open-ended spanner, not a socket wrench, which over-torques easily. Hand-thread three turns minimum first — if resistance during initial threading, you have the wrong thread and need to stop.

Why does my grease gun coupler keep popping off?

Three common causes: (1) coupler not fully engaged on the ball — push firmly until you feel a slight click; (2) ball end of the nipple is contaminated or damaged — wipe clean or replace the nipple; (3) line pressure is too high for a standard coupler (over 5,000 psi) — switch to a button head fitting + button-head coupler, or use a Lock-N-Lube-style locking coupler that mechanically grips the ball rather than relying on spring tension. See the Grease Gun Guide for coupler selection.

Do I need a thread sealant on a grease nipple?

Parallel threads (1/8" BSP, 1/4"-28 UNF, M6×1, M8×1, M10×1) need either PTFE tape, a copper/fibre washer, or anaerobic thread sealant (Loctite 577 or 567) to seal — the parallel thread itself doesn't seal against grease pressure. Tapered threads (1/8" NPT) seal on the thread interface itself and don't strictly need sealant, though many workshops apply PTFE tape as a precaution. Always seal parallel threads; tapered threads are optional.

What's the difference between 45° and 90° grease nipples — does it matter which I use?

Yes — angle determines access. A 45° fitting redirects the ball end 45° from the thread axis; a 90° fitting redirects it 90° (perpendicular). On a tight bearing where the grease gun can only approach from a specific angle, the wrong fitting geometry means the coupler can't engage. Workshop assortment kits include straight, 45°, and 90° in each thread size precisely because access varies. Check the access angle before ordering, or stock all three angles to handle any situation.