GREASE FITTING REFERENCE

Grease Fitting Thread Sizes

Compare metric, BSP and SAE grease fitting threads, match fitting types to the correct thread size, and check near-equivalent sizes with standards-based guidance.

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GREASE FITTING THREAD BASICS

What Thread Size Is a Grease Fitting?

A grease fitting thread is the diameter and pitch of the shank that screws into the lubricated component. The designation follows the standard screw thread format: the first number is the nominal diameter, the second is either threads per inch or the pitch in millimeters. A 1/4″-28 fitting is a quarter-inch nominal diameter at 28 threads per inch. An M6×1.0 is 6 mm nominal at a 1.0 mm pitch.

THE KEY FORMAT

Nominal vs Actual

1/4″ Nominal

NOM

Written Size Is Nominal

On imperial and pipe threads, the written size is nominal, not physical. The designation descends from early twentieth-century pipe sizing, when the number referred to a bore that has since changed with wall thickness.

The nominal-versus-actual gap is the most common source of wrong-size ordering.

A 1/8″ pipe has an actual outside diameter of 0.393 in. A 1/4-18 PTF SAE short fitting, nominally a quarter inch, wants a hole of about 0.520 in.

IMPERIAL / PIPE 1/4″ · 1/8″ · 1/4-18

Written size is nominal, not physical. Confirm the actual hole diameter.

METRIC M6×1.0 · M8×1.0

More literal: an M6 usually measures 6 mm and fits a 6 mm hole, with some exceptions.

5-NAME REFERENCE

What Is a Grease Fitting Called?

These five names describe the same part, and buyers use several of them in the same week.

Zerk / Zerk Fitting

Used in North America, trade and retail. Seen in product listings, repair manuals, forums.

Grease Nipple

Used in the UK, Ireland, Australia, New Zealand, India, South Africa. Seen in supplier catalogues and engineering drawings.

Lubrication Fitting

Used in standards and engineering. Seen in SAE J534, OEM drawings, procurement documents.

Schmiernippel / Cone Lubricating Nipple

Used in Germany and Central Europe. Seen in DIN 71412 and European supplier data.

Alemite Fitting

Used in North America, legacy contexts. Seen in older equipment documentation. Search the thread designation, not the name.

THREAD SIZE REFERENCE

Grease Fitting Thread Size Chart Metric & SAE

Compare imperial, SAE, metric and British grease fitting thread sizes, match thread designation to major diameter and pitch, and identify the near-miss sizes that cause wrong-size ordering.

THREAD FORMAT Diameter × Pitch or TPI

IMPERIAL / PIPE Written size is nominal

METRIC More literal dimension

CHECK FIRST Straight vs Taper

IMPERIAL & SAE

Imperial and SAE Thread Sizes

Nominal diameter and threads per inch, with the straight or taper form and the seal method used in each application.

Thread Size	Major Diameter	Pitch / TPI	Straight or Taper	Seal Method	Typical Applications
1/4″-28 UNF	6.35 mm (0.250 in.)	28	Straight	O-ring or washer	Automotive, machinery, trailers
1/4″-28 SAE-LT	6.35 mm (0.250 in.)	28	Taper	Metal-to-metal	Automotive, centralized lubrication systems
5/16″-24 UNF	7.94 mm (0.3125 in.)	24	Straight	O-ring or washer	Construction, high-vibration machinery
3/8″-24 UNF	9.53 mm (0.375 in.)	24	Straight	O-ring or washer	Heavy industrial, high-load bearings
1/8″-27 NPT	10.3 mm (0.405 in.)	27	Taper	Thread sealant	Hydraulic systems, lawn equipment, power tools
1/4″-18 NPT	13.7 mm (0.540 in.)	18	Taper	Thread sealant	High-flow industrial, heavy hydraulics

ISO METRIC · DIN 71412

Metric Thread Sizes

Metric fittings are more literal than imperial and pipe threads. An M6 usually measures 6 mm and fits a 6 mm hole, though some German and British standard sizes are exceptions.

Thread Size	Major Diameter	Pitch	Straight or Taper	Seal Method	Notes
M6×1.0	6.00 mm	1.0 mm	Straight	O-ring or washer	Light-duty European equipment
M8×1.0	8.00 mm	1.0 mm	Straight	O-ring or washer	General European machinery
M8×1.25	8.00 mm	1.25 mm	Straight	O-ring or washer	Commonly associated with DIN 71412
M10×1.0	10.00 mm	1.0 mm	Straight	O-ring or washer	Fine pitch, vibration-prone joints
M10×1.5	10.00 mm	1.5 mm	Straight	O-ring or washer	Heavy machinery
M12×1.5	12.00 mm	1.5 mm	Straight	O-ring or washer	Heavy equipment

BRITISH & WHITWORTH

British and Whitworth Thread Sizes

BSP and BSPT fittings share a 55° thread angle but differ in form. Confirm BSPP or BSPT before ordering, because a taper fitting will not seal in a straight port.

Thread Size	Major Diameter	Thread Angle	Straight or Taper	Notes
1/8″-28 BSPT	9.7 mm	55°	Taper	Partially enters 1/8″-27 NPT and will not seal
1/4″ BSP	13.2 mm (nominal)	55°	Straight	Confirm BSPP or BSPT before ordering
1/8″ BSP	9.7 mm (nominal)	55°	Straight	British-influenced markets

NEAR MISS Why These Sizes Do Not Line Up

Metric and imperial thread systems were developed independently. There is no conversion table that makes a 1/4″-28 into a metric equivalent, because 28 threads per inch is not a metric pitch.

Imperial 1/4″-28

Metric No Equivalent

The sizes that look closest are the ones that cause the damage. Confirm the standard and the thread form before matching a number.

WHY THE FORM MATTERS

The Same Thread Size Can Be Straight or Taper

A 1/4″-28 UNF and a 1/4″-28 SAE-LT share the same nominal diameter and pitch but differ in form. One seals with an O-ring or washer; the other seals metal-to-metal. Choose the wrong one and the joint leaks.

1/4″

UNF Straight

→

Seal O-ring

1/4″

SAE-LT Taper

→

Seal Metal

1/8″

BSP Straight

≠

BSPT Taper

VERIFY BEFORE YOU ORDER A nominal thread size is a starting point, not a guarantee of fit.

Different standards can specify different hole diameters for the same written thread. Confirm which standard applies to your equipment, and check the straight or taper form, before you match a number. A supplier catalogue may list the same part under a different name, so search the thread designation rather than the fitting name.

IDENTIFY YOUR THREAD

Tapered vs Straight Threads, and How to Identify Yours in 3 Steps

Tapered threads seal by wedge engagement. Straight threads seal on a face, using an O-ring, a bonded washer or a copper gasket at the base of the fitting. A straight thread with no sealing washer will leak no matter how tight it is.

FIRST THING TO CHECK

Tapered vs Straight: The First Thing to Check

Check for taper first, because it changes what you are looking for and what you need to order with it. Tapered threads generally need a thread sealant and no washer. Straight threads need a sealing washer at the base.

TAPERED Wedge Seal Metal-to-metal along the flanks

→

STRAIGHT Face Seal O-ring, bonded washer or gasket

Tapered: NPT, NPTF, BSPT, SAE-LT.

Straight: UNF, ISO metric, BSPP. The two do not interchange, and a straight thread without its sealing washer will leak.

Check for Taper

FORM

Measure the shank diameter at the first full thread and again near the shoulder. A difference greater than about 0.005 in. indicates a tapered thread. Treat the figure as an indicative check rather than a tolerance.

First Thread Shoulder 0.005 in.

Measure the Major Diameter

DIAMETER

Measure across the thread crests with a caliper. Record it in millimeters, which compares more easily against a metric table. A clean 6.00 mm reading points to metric. A fractional reading such as 0.250 in. points to imperial.

Metric

Imperial

Fractional

Match the Pitch

PITCH

Use a thread pitch gauge. Do not estimate. A one-thread-per-inch difference between NPT and BSPT is not visible to the eye, and it is the difference between a seal and a leak.

Pitch Gauge NPT BSPT

TWO FALLBACKS IF YOU ARE STILL UNCERTAIN A grease fitting size identifier plate, or physical test-fitting against a known assortment.

A size identifier plate has labeled threaded holes; screw the shank in until it seats. Physical test-fitting against a known assortment is the other. Be aware that identification can stay uncertain on equipment built to mixed standards, and that replacing a fitting on a machine of unknown origin is worth a measured check rather than an assumption.

FINAL CHECK Measure, Don’t Assume

BUYER RANGE PLANNING

Which Grease Fitting Thread Sizes to Stock for Which Market

Stocking decisions follow the same rule as identification. The thread a machine uses follows where the equipment was designed, not where it is being used. A European-built machine installed in Ohio still takes metric fittings. A North American machine in Germany still takes UNF. Country of installation tells you very little. Equipment origin tells you almost everything.

MARKET 01

North America

UNF NPT

The thread families you will meet are UNF (straight), NPT / NPTF, and SAE-LT (taper). Country of installation does not change this if the equipment was designed here.

1/4″ UNF

1/4″ SAE-LT

1/8″ NPT

1/8″ PTF

Most commonly reordered

1/4″-28 in both UNF and SAE-LT is widely described as the most common standard grease fitting size in North American equipment.

MARKET 02

Germany, Central and Northern Europe

The thread families you will meet are ISO metric per DIN 71412. An M6 usually measures 6 mm and fits a 6 mm hole, though some German and British standard sizes are exceptions.

COMMON M6 ×1.0

COMMON M8 ×1.0

ALSO SEEN

M8×1.25, M10×1.0, M10×1.5 and M12×1.5. The pitch matters as much as the diameter, so confirm both before ordering.

MARKET 03

UK, Ireland, Australia, New Zealand, India, South Africa

The thread families you will meet are BSP and BSPT. Confirm BSPP or BSPT before ordering, because a taper fitting will not seal in a straight port.

1/8″ BSP 1/8″-28 BSPT 1/4″ BSP Confirm BSPP or BSPT

OTHER MARKETS

Japan, Korea and Older British / European Equipment

Two further groups appear often enough to plan for, and both require confirmation against the machine’s manual rather than an assumption.

JAPAN & KOREA

JIS B 0202 Series

Often interchangeable with metric in practice, but not guaranteed. Confirm against the equipment before assuming an ISO metric fitting will seat.

Confirm Before Ordering

OLDER BRITISH & EUROPEAN

Whitworth and BSF

Legacy thread families still found on older equipment. Confirm against the machine’s manual rather than matching by eye.

Legacy

MARKET PATTERN

1/4″-28 Leads North America

Widely described as the most common size in North American equipment and the size most often reordered. Treat it as a pattern, not a guarantee.

Reorder Leader

A METHOD, NOT A PERCENTAGE

Setting Your Opening Order From Your Own Customer Base

There is no universal stock ratio, and any supplier who quotes you one is describing their own customer base rather than yours. Derive it instead.

STEP 01 · LIST

Equipment Your customers run

STEP 02 · WEIGHT

Origin Dominant market

START WITH Sizes That Wear Fittings are consumables. Reorders justify depth.

THE HONEST TRADE-OFF A range built for one market will not serve another.

Broad coverage across UNF, metric and BSP surfaces more demand and carries more SKUs, more tooling and more inventory. Depth in one system converts better per SKU and limits you to one customer base. Carry the mating pair where a market straddles two systems, and test before you commit depth: an opening order that proves sell-through on four sizes beats a container that proves nothing on forty.

RANGE INPUTS Market · Origin · Depth

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 thread size are most grease fittings?

In North America, 1/4″-28 is the most common standard grease fitting size, in both the straight UNF and the tapered SAE-LT form. In European equipment, M6×1.0 and M8×1.25 are the sizes you will meet most often. There is no single global answer, because the thread follows the equipment’s country of design.

How do I know if my grease fitting is metric or SAE?

Measure the shank across the thread crests with a caliper. A clean metric reading such as 6.00 mm or 8.00 mm points to a metric fitting. A fractional reading such as 0.250 in. or 0.3125 in. points to imperial. Confirm with a thread pitch gauge before ordering replacements, because diameter alone does not separate 1/4″-28 UNF from a metric fitting of similar size.

Is 1/4-28 the same as M6?

No. 1/4″-28 UNF measures 6.35 mm across the threads; M6×1.0 measures 6.00 mm. They differ by 0.35 mm in diameter and by pitch, roughly 0.907 mm against 1.0 mm. An M6 will often start by hand in a 1/4″-28 port and then cross-thread it. They are close but not interchangeable.

What is the difference between 1/8″ NPT and 1/8″ BSP?

One thread per inch and 5 degrees of thread angle. NPT is 1/8″-27 at 60°; BSPT is 1/8″-28 at 55°. Both are tapered. A BSPT fitting will partially enter an NPT port and will not seal, because the flank angles do not match.

How do I measure a grease fitting thread without a gauge?

Measure the major diameter across the thread crests with a caliper, and check for taper by comparing the diameter at the first full thread against the diameter near the shoulder. That narrows the size, but it will not reliably separate pitches that differ by one thread per inch. For that, use a pitch gauge, a printed size identifier, or test-fit against a known assortment.

Why does my grease fitting thread in partway and then stop?

Two likely causes. If the fitting is the wrong thread family, it engages a few turns and then binds, which is the classic 1/4″-28 and M6 pattern. If it is the right family but a tapered thread, it may simply be reaching the depth at which the taper wedges. Do not force it further. Continued tightening on a mismatched thread strips the port.