GREASE FITTING REFERENCE

Grease Fitting Size Chart

Match a grease fitting to the tapped hole by thread standard and pitch — 1/4″-28 SAE-LT, 1/8″-27 NPT, M6 × 1.0, M8 × 1.0 and BSP. Compare metric, SAE, NPT and BSP threads side by side.

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FITTING SIZE BASICS

How to Identify a Grease Fitting Thread

A grease fitting size is determined by two things: the thread standard and the thread pitch. If a fitting does not start by hand, stop — verify the standard before applying a wrench.

THE KEY DISTINCTION

Pipe Size vs Bolt Size

1/8″ NPT

Measure the Crest Diameter

A pipe-thread measurement will not match its nominal number, while a bolt-thread measurement will. Calipers across the thread crests give the true outside diameter.

The smaller number is the larger fitting.

A 1/8″ NPT fitting measures about 0.405″ across the threads, while a 1/4″-28 fitting measures 0.250″. Nominal pipe size is not actual diameter.

1/4″-28 SAE-LT 0.250″ · 6.35 mm

Bolt-size thread. The number is the actual thread diameter.

1/8″-27 NPT ≈0.405″ · 10.29 mm

Pipe-size thread. The number is a nominal trade designation.

5-STEP CHECK

How to Measure a Fitting Thread

Three checks identify almost any thread in under a minute. Confirm the standard before you order or install.

Check for Taper

Run a thumbnail or straight edge down the threads. If the diameter narrows toward the tip, it is a tapered thread — NPT or BSPT. If it stays parallel, it is a straight thread — UNF, metric or BSPP.

Measure the Pitch

Use a thread pitch gauge. Count threads per inch for imperial threads (28 TPI = 1/4″-28; 27 TPI = 1/8″ NPT) or measure millimeter spacing for metric threads (1.0 mm = M6 × 1.0, M8 × 1.0, M10 × 1.0).

Measure the Crest Diameter

Calipers across the thread crests give the true outside diameter. Remember that a pipe thread will not match its nominal number, while a bolt thread will.

Compare Against the Standard

Match pitch and crest diameter against the size chart. The same three checks work in reverse for a fitting that is still in hand but unlabeled.

Stop If It Does Not Start by Hand

If a fitting does not start by hand, stop. Verify the standard before applying a wrench. The cost of a cross-threaded boss is a re-tap or a replacement component, not a fitting.

MOST COMMON (US) 1/4″-28 SAE-LT

MOST COMMON (METRIC) M6 × 1.0

INDUSTRIAL / AG 1/8″-27 NPT

UK / AUSTRALIA 1/8″ BSP

THREAD STANDARD LOOKUP

Grease Fitting Thread Sizes

A grease fitting — also called a Zerk fitting in North America or a grease nipple in the UK and Australia — is sized by thread standard and pitch, not by “small, medium, large.”

Thread Standard	Common Sizes	Pitch (TPI / mm)	Region / Typical Use
SAE-LT	1/4″-28 · 3/16″-32 · 5/16″-24	28 · 32 · 24 TPI	North America — automotive, light truck, general equipment
NPT / NPTF	1/8″-27 · 1/4″-18	27 · 18 TPI	North America — construction, agricultural, industrial
Metric (ISO)	M6 × 1.0 · M8 × 1.0 · M8 × 1.25 · M10 × 1.0 · M10 × 1.5 · M12 × 1.5	1.0 – 1.5 mm	Europe, Asia — European machinery, modern automotive
BSP / BSPP / BSPT	1/8″ (G1/8) · 1/4″ (G1/4)	28 · 19 TPI	UK, Australia, Asia — general industrial equipment

THE PIPE-SIZE TRAP A “pipe” thread and a “bolt” thread with the same number are not the same size.

A 1/8″ NPT fitting measures about 0.405″ across the threads, while a 1/4″-28 fitting measures 0.250″. The smaller number is physically the larger fitting.

BOLT THREAD 1/4″-28

≠

PIPE THREAD 1/8″ NPT

Cross-threading these together damages the boss and scrapes out the threads in the equipment.

SHAPE SELECTION

Grease Fitting Types by Shape

Thread size tells you whether the fitting fits the hole. The shape tells you whether it works in the space around the hole.

Shape	Angle	Use It When	Common Applications
Straight	0°	Direct, unobstructed access to the lubrication point	General machinery, bearings, U-joints
Angled	45°	Reduced overhead clearance or awkward access	Steering joints, suspension, tight assemblies
Right-angle	90°	Side access on a flat surface	Tie-rod ends, pivot points, gearbox fill points
Flush / low-profile	0°	Low clearance where a protruding fitting gets knocked off	Steering linkages, rotating assemblies
Drive-in	n/a	Unthreaded or sheet-metal holes; no tapping required	Light equipment, thin-wall housings

NOT INTERCHANGEABLE

Metric, SAE, NPT and BSP Fittings Cannot Be Swapped

The differences are physical, not cosmetic. Thread angle, pitch and sealing method all change between standards — and a fitting that starts by hand can still fail to seal.

ANGLE

NPT / Metric 60°

→

BSP 55°

PITCH

1/8″ NPT 27 TPI

→

G1/8 BSP 28 TPI

SEAL

Tapered Metal-to-metal

→

Parallel O-ring / washer

NOMINAL VS ACTUAL

Crest Diameter Reference

Bolt-size threads measure close to their nominal number. Pipe-size threads do not — the nominal figure is a trade designation, not a diameter.

Nominal	Thread Type	Crest Diameter
1/4″-28	SAE-LT — bolt thread	0.250″ · 6.35 mm
3/16″-32	SAE-LT — bolt thread	0.1875″ · 4.76 mm
1/8″-27	NPT — pipe thread	≈0.405″ · 10.29 mm
1/4″-18	NPT — pipe thread	≈0.540″ · 13.72 mm
M6 × 1.0	Metric — bolt thread	6 mm
M8 × 1.0	Metric — bolt thread	8 mm
G1/8	BSP — parallel pipe thread	≈0.383″ · 9.73 mm

MEASURE FIRST When in doubt, measure the crest diameter.

Calipers across the thread crests give the true outside diameter. Compare it against the nominal number — if the two do not line up, you are looking at a pipe thread.

Nominal 1/8″

Actual 0.405″

A “1/4″ bolt thread measures 0.250”, while a “1/8″ pipe thread measures about 0.405”. The smaller number is the larger fitting.

VERIFY BEFORE YOU SPECIFY If a fitting does not start by hand, stop.

Verify the standard before applying a wrench. The cost of a cross-threaded boss is a re-tap or a replacement component, not a fitting. For any compliance or material claim, confirm the certificate and test report for the specific SKU and market.

SOURCING-GRADE SPECS

Material, Plating & Sealing. Size Gets It In. Material Keeps It There.

Size and shape get the fitting into the hole. Material and plating determine how long it survives there — and those are the specifications a sourcing buyer asks about after the sizing question is answered.

SIZE ≠ SERVICE LIFE

The Fitting That Fits May Not Be the Fitting That Lasts

A zinc-plated carbon steel fitting is the standard workhorse. A stainless fitting costs more and should be specified deliberately — for corrosion, washdown or chemical exposure — not as a default.

SIZE & SHAPE Fits the Hole Thread standard, pitch, angle

→

MATERIAL & PLATING Survives the Service Corrosion, washdown, pressure

Confirm the certificate and test report.

A “stainless” specification should name the grade and the standard it is tested against — for the specific SKU and market.

Material & Plating

MATERIAL

Zinc-plated carbon steel covers most indoor and general outdoor machinery at the right price point. Stainless (typically 304) is specified where corrosion or washdown is a factor — food processing, marine, chemical. Nickel-plated and brass fittings appear in lower-pressure and specialized applications.

Zinc-Plated Steel Stainless 304 Nickel Brass

Ball-Check Construction

INTERNAL

The ball, spring and seat are the working parts. A heat-treated ball and a spring that holds consistent preload are what keep grease going in and contamination out.

Heat-Treated Ball Spring Preload Valve Seat

Sealing Method

SEALING

Confirm whether the fitting seals on a taper or on an O-ring / washer, because it changes what a customer needs to have on hand at installation. Tapered threads seal metal-to-metal; parallel threads seal on an O-ring, washer or sealing face.

Taper Seal O-Ring Washer Sealing Face

PRACTICAL RULE Specify the material for the service, not for the catalog page.

OUSHKE reports a 0.2% defect rate on its own batch testing before shipment, and every batch is thread-gauged — the company-published figure, presented as such. For contractual or due-diligence use, request the underlying inspection records for your SKU.

FINAL CHECK Confirm the SKU

BULK SOURCING

Sourcing Grease Fittings in Bulk

Grease fittings are a small-ticket, high-frequency line item. They move in volume, they carry low unit value, and they punish inconsistent supply — a customer who can’t get the right fitting in a kit is a customer who goes elsewhere.

CORE STOCK

Build the Line Around the Sizes That Move

1/4″-28 + M6

The two sizes that cover most replacement work are 1/4″-28 SAE-LT and M6 × 1.0. Add 1/8″-27 NPT and 1/8″ BSP for broader industrial and regional coverage.

1/4″-28 SAE-LT

M6 × 1.0 Metric

1/8″-27 NPT

1/8″ BSP

High-frequency core sizes

A distributor stocking a basic fitting kit will sell through 1/4″-28 and M6 × 1.0 faster than the rest of the range combined.

REGIONAL COVERAGE

Match the Thread Standard to the Market

Thread standards differ by region, and so do the fittings your customers expect to find in the box. Carry the standard the equipment in each market actually uses.

NORTH AMERICA NPT / SAE 1/8″-27 · 1/4″-28

UK / AUSTRALIA BSP G1/8 · G1/4

WHY IT MATTERS

A Zerk fitting in North America is a grease nipple in the UK and Australia — and the thread standards differ with the region. The chart covers both because the market does.

MARKET COVERAGE

Start With the Sizes That Cover the Bulk of Replacement Work

A core program concentrates on 1/4″-28 SAE-LT and M6 × 1.0. Add NPT and BSP as the market requires rather than carrying every standard at the same depth.

1/4″-28 Core — US M6 × 1.0 Core — Metric 1/8″ NPT Industrial 1/8″ BSP UK / AU

FACTORY-DIRECT ADVANTAGE

What a Factory-Direct Supply Changes

OUSHKE manufactures the fittings, not just resells them, and consolidates them with the grease guns and fittings range, the wider air tools line, and four other tool categories.

ONE SUPPLIER

One Source for the Program

Fittings, grease guns and the air tools that drive them come from the same factory, under the same inspection standard, on the same purchase order.

Consolidated Order

SPECIFICATION CONTROL

Bring the Spec You Want to Sell

Because the tooling is in-house, thread, plating and packaging changes don’t wait on a third party’s production calendar. Specify the product, not just the one in stock.

In-House Tooling

PRIVATE LABEL

Your Brand on the Box

Branding, packaging and set configuration are adjustable — a fitting assortment under your own label rather than someone else’s catalog number.

Your Label

OPENING-ORDER STRUCTURE

Test the Category Before You Commit to the Full Program

Customization carries the usual trade-offs: a private-label or nonstandard thread raises MOQ and lead time. That is stated plainly, because a buyer who understands the trade-off negotiates better than one who discovers it later.

STEP 01

Small Opening Order

STEP 02

Prove Sell-Through

THEN NEGOTIATE From Real Sales Data Not from a catalog estimate

REQUEST A SAMPLE FIRST Prove the spec before you commit to a full program.

Send your fitting specification and target quantity through the grease guns and fittings range for a factory-direct quote — or request a sample first. Confirm the certificate, test report and SKU before any compliance claim.

QUOTE INPUTS Spec · Quantity · Market

Frequently Asked Questions (FAQs)

Find answers to common questions about our hand tools, their care, and usage. If you need additional information, our customer support team is ready to assist you.

Ask an expert

What size grease fitting do I need?

Match the thread standard and pitch of the tapped hole. For most US automotive and light equipment, that is 1/4″-28 SAE-LT; for metric machinery, M6 × 1.0 or M8 × 1.0. If you can’t identify the thread, measure the pitch and crest diameter with a pitch gauge and calipers, or test-fit a few common sizes.

Is 1/4-28 the same as 1/8 NPT?

No. 1/4″-28 is a bolt-size thread measuring 0.250″ across the crests; 1/8″ NPT is a pipe-size thread measuring about 0.405″. The 1/8″ NPT fitting is physically larger, despite the smaller nominal number, and the two are not interchangeable.

Are metric and SAE grease fittings interchangeable?

No. They differ in thread angle (60° for metric and NPT, 55° for BSP) and pitch, and they seal differently. A metric fitting in an SAE hole will bind or fail to seal. Never force a fitting that does not start by hand.

How do I measure a grease fitting to find its size?

Three checks: confirm whether the thread is tapered or parallel, measure the pitch with a thread pitch gauge (TPI for imperial, millimeters for metric), and measure the crest diameter with calipers. Compare against the size chart above.

What is the difference between a Zerk and a grease nipple?

They are the same component under different regional names — “Zerk fitting” in North America and “grease nipple” in the UK and Australia. The thread standards differ by region (NPT/SAE in North America, BSP in the UK and Australia), which is why the chart covers both.

What is the most common grease fitting size?

1/4″-28 SAE-LT in North America and M6 × 1.0 in metric regions. A basic replacement kit should carry both, plus 1/8″-27 NPT for industrial and agricultural equipment.