Quick Answer: The most common grease zerk tap is a 1/4″-28 UNF tap (North America) or an M6×1.0 tap (metric machinery); larger industrial ports may need a 1/8″-27 NPT or 1/8″ BSP pipe tap. Match the grease fitting tap size to the fitting thread in the chart below, drill with the listed tap drill, and torque the fitting to spec. If the port threads are too damaged to chase, restore them with a Heli-Coil or Time-Sert insert — or skip threading entirely with a drive-in grease fitting.
Need replacement parts? See our grease fittings catalog — OEM-compatible, full specifications, fast quoting.
How Grease Fitting Threads Get Damaged
A grease fitting only works when its thread holds — locking the fitting against vibration, sealing the grease path, and keeping the ball check valve aligned with the lubrication channel. Four failure modes cause most thread damage:
- Cross-threading: Forcing a metric fitting into a UNF port — or a tapered BSPT fitting into a parallel G-thread port — destroys the first threads of both parts. Metric and UNF threads are not interchangeable; neither are tapered and parallel BSP.
- Over-torquing: Exceeding the installation torque strips the port, especially in aluminum and cast iron housings. Stainless fittings in softer metals need careful torque control to prevent galling.
- Corrosion: A rusted carbon-steel fitting seizes in the port and snaps at the shank during removal. In marine, wash-down, or coastal service, 316 stainless fittings resist this failure.
- Impact and vibration: A struck fitting tears the port threads on the way out; a loose fitting under vibration wallows the threads oversize until it falls out.
Every repair path below starts with the same question: is the port thread still good enough to chase with a tap, or does it need extraction and rebuilding? For a broader tool overview, see our grease fitting tools, removal and thread repair guide.
Grease Zerk Tap Size Chart: Fitting Thread × Tap × Drill
The correct grease zerk tap size is always the fitting’s own thread specification — never “close enough.” Identify the thread first (see the thread tester section below), then select the matching tap. Drill sizes are typical tap drills for roughly 70–75% thread engagement; verify against the tap manufacturer’s chart and the parent material before drilling.
| Fitting Thread | Grease Fitting Tap | Tap Drill (typically) | Notes |
|---|---|---|---|
| M6×1.0 | M6×1.0 taper → plug | 5.0 mm | Most common metric size; DIN 71412 |
| M8×1.25 | M8×1.25 | 6.75 mm | General machinery; M8×1.0 fine variant also exists |
| M10×1.5 | M10×1.5 | 8.5 mm | Heavy equipment; M10×1.0 fine variant also exists |
| 1/4″-28 UNF | 1/4″-28 UNF | #3 (0.213 in) or 7/32 in | The standard North American zerk thread |
| 5/16″-32 UNF | 5/16″-32 UNF | 9/32 in | Less common; check pitch gauge carefully |
| 3/8″-24 UNF | 3/8″-24 UNF | 21/64 in | Heavy-duty and button-head fittings |
| 1/8″-27 NPT | 1/8″-27 NPT (tapered pipe tap) | 21/64 in (0.328 in) | Tapered thread — tap depth sets the seal |
| 1/8″ BSP (G/BSPT, 28 TPI) | 1/8″-28 BSP tap | 8.7–8.8 mm | 9.728 mm major diameter; 55° thread form |
A grease fitting tap kit covering M6×1.0 through M10×1.5 plus 1/4″-28 and 1/8″ NPT handles most field repairs. For help identifying which thread you have, our grease zerk sizes and fitting size chart covers SAE, metric, and BSP fittings in detail.
How to Drill and Tap for a Grease Zerk: Step by Step
- Confirm the thread. Test the fitting or port with a thread tester or pitch gauge before touching a drill — tapping the wrong thread is the most expensive mistake in this job.
- Protect the mechanism. Where possible, remove the pin, shaft, or bushing behind the port so chips cannot enter the bearing cavity. Packing the tap flutes with grease traps chips and brings them out with the tap.
- Center-punch and drill. Drill square to the surface with the tap drill from the chart. Use cutting oil in steel; cast iron is typically tapped dry; aluminum typically wants a light tapping fluid.
- Start with a taper tap. The gradual lead keeps the tap aligned. Turn forward a half to one turn, then back off a quarter turn to break the chip. Finish through-holes with a plug tap; use a bottoming tap only at the base of blind holes.
- Respect tapered threads. NPT and BSPT taps cut a taper — stop when the tap binds and the fitting seats with roughly four full turns of engagement. Over-tapping a tapered port leaves a loose, leaking fitting.
- Clean and seal. Blow out chips, then apply PTFE tape (2–3 wraps) or medium-strength anaerobic sealant to the male thread — never on the first thread, so sealant cannot reach the check valve.
- Torque to spec. Thread in hand-tight, then wrench to the value for the thread size (list below). Test with one grease-gun stroke and confirm the check valve seals with no weeping.
Installation torque by thread size (from ISOHITECH stainless steel grease nipple specifications):
- M6×1.0: 8–12 Nm
- M8×1.25: 15–20 Nm
- M10×1.5: 20–25 Nm
- 1/4″-28 UNF: 20–27 Nm (15–20 ft-lb)
- 1/8″ BSPT: 10–15 Nm
Broken Fitting Extraction: Left-Hand Drills, Easy-Outs & EDM
A fitting that snaps at the shank leaves a threaded stub in the port. Work up this ladder, stopping at the first rung that works:
- Locking pliers — if any of the stub stands proud of the surface, grip and back it out. Penetrating oil and light hammer taps help break corrosion first.
- Left-hand drill bit — drill the stub center at low speed with a reverse-cut bit slightly smaller than the thread’s minor diameter. The counter-clockwise cutting action often backs the stub out before the hole is finished.
- Grease fitting extractor (easy-out) — drive the spiral extractor into the drilled stub and turn counter-clockwise. Use steady hand pressure only: extractors are hardened steel, and one snapped off in the stub cannot be drilled conventionally — the repair then escalates to a carbide burr or EDM.
- Weld a nut on — on steel stubs in steel housings, a MIG welder can fuse a nut to the stub; the heat also breaks corrosion. Let it cool, then back the stub out with a wrench.
- EDM — for hardened extractor or tap fragments stuck in a valuable casting, electrical discharge machining erodes the fragment without touching the parent threads.
After any extraction, chase the port with the matching tap and verify the thread before installing the replacement.
Grease Fitting Thread Tester: Identify an Unknown Thread
A grease fitting thread tester is a compact plate carrying common male and female thread posts — typically around 24 sizes across NPT, UNF, BSPT, BSW, BSF, and metric. It removes the guesswork that causes cross-threading. The procedure:
- Clean the fitting or port threads completely.
- Hand-thread the fitting into the tester’s female posts (or the tester’s male post into the port). The correct size starts smoothly and engages two to three full turns with finger pressure only.
- Never force a match. A thread that starts but binds within a turn is the wrong form or pitch — a classic trap is 1/8″ BSP (9.728 mm major diameter, 28 TPI) versus 1/8″-27 NPT, close in diameter but not interchangeable.
Thread Repair Inserts: Heli-Coil vs Time-Sert
When the port threads are stripped beyond chasing, a thread repair insert rebuilds the hole back to the original thread size — the same fitting goes back in afterward.
- Heli-Coil (wire thread insert): Drill the damaged hole oversize, tap it with the kit’s STI tap, wind the stainless wire insert in until it sits 1/4 to 1/2 turn below the surface, then snap off the driving tang. Inexpensive, widely available, and the insert threads are typically stronger than the original threads in aluminum. Budget extra inserts — some are lost to installation errors.
- Time-Sert (solid bushing): Drill, counterbore, and tap oversize, then thread the solid steel bushing in and expand its base to lock it permanently. It costs more and needs dedicated tooling, but it withstands high-cycle removal and vibration better — the right choice for ports that see frequent fitting changes or heavy shock loads.
One caution applies to both: a stainless insert paired with a stainless fitting can gall. Apply anti-seize compound to the fitting threads during final installation.
Self-Tapping Grease Fittings: When They Can Replace Tapping
Self tapping grease fittings — also sold as a self tapping grease zerk or self tapping grease nipple — carry a modified thread with cutting edges that form their own thread as they are driven into a plain drilled hole. They are a legitimate field fix in the right conditions:
- Good candidates: aluminum, cast iron, and thin mild-steel sections; emergency repairs where no tap is on the truck; stripped ports that can accept the next size up.
- Poor candidates: hardened steel, high-pressure circuits, and heavily vibrating mounts. The formed thread holds less than a cut machine thread, and repeated removal enlarges the hole.
When you need a thread-free installation with a real pressure rating, a drive-in grease fitting is the engineered alternative: a serrated shank presses into a smooth drilled hole (3/16″, 1/4″, or 5/16″) and holds working pressures up to 8,000 PSI — designed for exactly this situation, damaged or oversized holes where a threaded fitting will not hold. Like self-tappers, it is a one-time installation; for a permanent, serviceable port, tapping for a standard threaded fitting remains the better engineering choice.
Frequently Asked Questions
What size tap do I need for a grease fitting?
Most North American grease fittings use a 1/4″-28 UNF tap; metric machinery typically uses M6×1.0, M8×1.25, or M10×1.5; larger industrial ports may be 1/8″-27 NPT or 1/8″ BSP. Identify the thread with a thread tester or pitch gauge first, then match the tap — see the chart above.
What drill size should I use for a 1/4-28 grease zerk tap?
Typically a #3 drill (0.213 in) or a 7/32 in drill, giving roughly 70–75% thread engagement in steel. Verify against the tap manufacturer’s chart, and adjust slightly for very soft or very hard parent materials.
Are grease zerk fittings self tapping?
Standard zerks are not — they require a tapped hole. Self tapping grease zerk variants with cutting threads do exist for aluminum, cast iron, and thin steel. For hardened steel, high pressure, or frequent service, tap a proper thread or use a drive-in fitting.
How do you remove a broken grease fitting?
Use locking pliers if the stub is exposed. Otherwise, a left-hand drill bit often backs the stub out while drilling. A grease fitting extractor (easy-out) is the next step — apply steady hand pressure, because a snapped extractor usually means carbide-burr grinding or EDM. Welding a nut to the stub also works on steel. Always chase the threads with a tap afterward.
Can a stripped grease fitting hole be repaired?
Yes. First try chasing the damaged threads with the matching tap. If they are too far gone, install a Heli-Coil wire insert or a Time-Sert solid bushing to restore the original thread size. Alternatively, drill the hole smooth and press in a drive-in grease fitting for a thread-free repair.
Shop ISOHITECH Grease Fittings
Once the port is tapped, chased, or rebuilt, fit it with a quality threaded fitting:
| Product | Thread Options | Pressure Rating | Best For |
|---|---|---|---|
| Straight Grease Fittings (Zerk Nipples) | M6×1.0–M12×1.75, 1/4″-28 UNF | 8,000 PSI (550 bar) | Freshly tapped ports; DIN 71412 standard sizes |
| Stainless Steel Grease Nipples | Metric M6–M12, UNF, 1/8″–1/4″ BSPT, G1/8″ | 100 bar (1,450 PSI) | Marine, food-grade, and corrosive environments |
| 1/8″ BSP Grease Nipple | 1/8″ BSP, 28 TPI | 10,000 PSI (689 bar) | BSP-threaded ports on imported machinery |
| Drive-In Grease Fitting | Thread-free (3/16″–5/16″ shank) | Up to 8,000 PSI | Damaged or oversized holes — no tapping required |
Browse the full range in our grease nipples and fittings category. ISOHITECH supplies aftermarket-compatible components; brand names such as Lincoln, Alemite, and SKF are used for compatibility reference only — ISOHITECH is not an authorized OEM dealer.
Not sure which thread your machine uses, or need fittings matched to a repair batch? Contact us for a quote — send a photo of the fitting or port and our team will identify it.
