FASTENER IDENTIFICATION TOOL
Grease Fitting Thread Identifier
Identify a grease fitting thread from three measurements: outside diameter, thread pitch, and taper. Enter those values and the tool returns the standard (1/4-28 UNF, 1/8 NPT, M6, M8, or another), plus the tap-drill size and sealing method.
SIMPLE AND ACCURATE
How to Measure a Grease Fitting Thread
Three steps, in order. Do them in this order because the first two remove most of the wrong answers before you get to the third.
Check Taper
vs Straight
Look at the thread along its length. A tapered thread narrows toward the tip, by only about 3 degrees.
Measure the
Outside Diameter
Measure across the thread crests with calipers. Record the actual measurement, not the expected name.
Count the
Thread Pitch
Count threads per inch for imperial, or measure crest distance in mm for metric. Record the exact pitch.
Verify by
Threading
If a fitting binds within a turn, the thread form or pitch is wrong. Back it out and re-measure.
Confirm the
Standard
Match the three values to the standard: 1/4-28 UNF, 1/8 NPT, M6, M8, or another. The tool returns the answer.
GREASE FITTING THREAD IDENTIFIER
How the Identifier Works
Enter three inputs, and the tool matches them against the reference table below.
INPUT PARAMETERS
Fitting Measurements
mm
Largest width across thread crests
mm
Crest-to-crest distance
max
The tool ranks by confidence and flags ambiguous combinations.
mm
Drill size for the matched thread.
IDENTIFIED STANDARD
Live Result
Matched Thread
1/4-28
UNF
Straight thread, 28 threads per inch. Common on automotive grease fittings.
Nominal Size
0.250 in
Threads Per Inch
28 TPI
Tap Drill
5.4 mm
Sealing Method
Washer
REFERENCE TABLE
Standard
0.250 in × 28 TPI — Straight
OD
0.250 in
Outside diameter
TPI
28
Thread pitch
FORM
Straight
Thread form
Ambiguous inputs are flagged, not guessed.
A 0.383 in outside diameter with 28 TPI matches 1/8 BSPT, not 1/8-27 NPT. The two do not interchange.
QUICK REFERENCE
Input Guide
Three measurements decide the match. Measure the outside diameter across the thread crests, count the thread pitch, and check whether the thread tapers toward the tip.
Outside Diameter
0.250 in
Thread Pitch
28 TPI
Taper
Straight
Matched Standard
1/4-28 UNF
Tap Drill
5.4 mm
Sealing
Washer
GREASE FITTING REFERENCE
Grease Fitting Thread Size Reference Chart
NOTE
Measured OD, not nominal
The table below is the full identification reference. Diameters are the measured outside diameter of the thread, not the nominal name.
Imperial Pipe
NPT & NPTF
Tapered
Imperial Machine
UNF & SAE
Straight & tapered
Metric
M6 · M8 · M10
ISO metric
British
BSP & BSPT
Parallel & tapered
| Thread Size | Outside Diameter | TPI | Sealing | Typical Use |
|---|---|---|---|---|
| 1/8-27 NPT | 0.405 in (10.3 mm) | 27 | Taper + sealant | Industrial lube points, hydraulics, lawn equipment |
| 1/4-18 NPT | 0.540 in (13.7 mm) | 18 | Taper + sealant | High-flow industrial, heavier hydraulics |
| 3/8-18 NPT | 0.675 in (17.1 mm) | 18 | Taper + sealant | Large bearings, heavy equipment |
| 1/2-14 NPT | 0.840 in (21.3 mm) | 14 | Taper + sealant | Large industrial machinery |
| Thread Size | Outside Diameter | TPI | Sealing | Typical Use |
|---|---|---|---|---|
| 1/4-28 UNF | 0.250 in (6.35 mm) | 28 | Straight, O-ring or washer | General North American zerk standard |
| 1/4-28 SAE-LT | 0.250 in (6.35 mm) | 28 | Taper, self-sealing | Automotive, centralized lube systems |
| 5/16-24 UNF | 0.3125 in (7.94 mm) | 24 | Straight, O-ring or washer | Construction, high-vibration machinery |
| 3/8-24 UNF | 0.375 in (9.53 mm) | 24 | Straight, O-ring or washer | Heavy industrial, button-head fittings |
| Thread Size | Outside Diameter | Pitch | Typical Use |
|---|---|---|---|
| M6 × 0.75 | 6.0 mm | 0.75 mm | DIN 71412 fittings |
| M6 × 1.0 | 6.0 mm | 1.0 mm | Most common metric; light European equipment |
| M8 × 1.0 | 8.0 mm | 1.0 mm | Fine variant |
| M8 × 1.25 | 8.0 mm | 1.25 mm | General machinery |
| M10 × 1.0 | 10.0 mm | 1.0 mm | Fine variant; trucks, industrial pivots |
| M10 × 1.5 | 10.0 mm | 1.5 mm | Heavy equipment |
| Thread Size | Outside Diameter | TPI | Sealing | Typical Use |
|---|---|---|---|---|
| 1/8 BSPT | 0.383 in (9.73 mm) | 28 | Taper | British and some Asian-built equipment |
| 1/8 BSPP | 0.383 in (9.73 mm) | 28 | Parallel, washer | British and some Asian-built equipment |
!
Measured outside diameter, not nominal name. A 1/8 NPT fitting measures about 0.405 inch across the threads, not 0.125 inch. Record the actual measurement rather than the name stamped on the fitting.
DRILL REFERENCE
Tap Drill Sizes
Drill sizes for the most common grease fitting threads. Confirm the exact drill size against the current standard before machining a port.
METRIC THREADS
M6 · M8 · M10
M6 × 1.0
5.0 mm
M8 × 1.25
6.75 mm
M10 × 1.5
8.5 mm
IMPERIAL THREADS
1/4-28 UNF
#3 (0.213 in)
7/32
in
VS
1/8-27 NPT
21/64 (0.328 in)
11/32
in
5/16-32 UNF
9/32 in tap drill
Drill size follows the thread form, not the fitting name.
A 1/4-28 UNF straight thread and a 1/4-28 SAE-LT tapered thread share the same nominal diameter and pitch, but they require different tap drills because the seal method and thread form differ.
THREAD FAMILIES
Common Grease Fitting Thread Standards
Ten standards cover the great majority of grease fittings. They split into four families, and each family seals differently.
NORTH AMERICAN
SAE-LT & UNF
SAE-LT (1/4-28) is the standard North American grease fitting thread. It is tapered and self-sealing, which is why it dominates automotive and centralized-lubrication applications, no O-ring or washer required. UNF threads of the same 1/4-28 size are straight and seal with an O-ring or washer instead. The two look identical from the side; the difference is the taper, and the difference is why a self-sealing automotive fitting and a washer-sealed general fitting are not drop-in substitutes.
PIPE THREADS
NPT & NPTF
NPT (American Standard Taper Pipe Thread) and its dryseal variant NPTF are tapered pipe threads that seal by thread deformation plus a sealant. They appear on industrial lube points, hydraulic circuits, and lawn equipment. NPT is defined by ASME under B1.20.1; confirm the current edition before you cite a number in a specification.
METRIC
M6 · M8 · M10
Metric grease fittings follow ISO metric thread standards (ISO 261 for the general form), with M6, M8 and M10 the common sizes. They measure true to size and are the default on European-built machinery and vehicles. DIN 71412 is the German standard that governs many of these fittings.
BRITISH
BSP & BSPT
BSPT is the British tapered pipe thread, and BSPP its parallel counterpart, defined under ISO 7. The trap worth memorizing is the one between 1/8 BSP and 1/8 NPT: both measure near 0.383 inch outside diameter, but BSP is 28 TPI and NPT is 27 TPI, and the two thread forms will not interchange. If you measure 0.383 inch and the thread pitch gauge reads 28, you have BSP; if it reads 27, you have NPT.
Confirm the current edition of the applicable standard
before using these values for design or purchase
specifications. Stainless fastener grades are governed
separately.
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.
What size grease fitting do I have?
Measure the outside diameter across the thread crests, count the threads per inch (or the metric pitch), and note whether the thread tapers. Match those three values against the reference chart above. The two most common results are 1/4-28 SAE-LT (0.250 in OD, 28 TPI, tapered) and 1/8 NPT (0.405 in OD, 27 TPI, tapered).
How do I tell 1/4-28 UNF from 1/8 NPT?
Measure the diameter. 1/4-28 UNF is 0.250 inch outside diameter; 1/8 NPT is about 0.405 inch. The fitting named 1/8 is physically the larger of the two. 1/4-28 is also straight while 1/8 NPT tapers, and 1/4-28 has 28 threads per inch against 1/8 NPT's 27.
Are metric and SAE grease fittings interchangeable?
No. Metric threads (M6, M8, M10) and SAE/UNF threads (1/4-28, 5/16-24, 3/8-24) have different thread forms and pitches and will not interchange. A 6 mm fitting is not a substitute for a 1/4-28 fitting even though the diameters are close. Never force a near-match.
What is the difference between SAE-LT and UNF grease fittings?
Both are commonly 1/4-28 and identical in diameter and pitch. SAE-LT is tapered and self-sealing; UNF is straight and seals with an O-ring or washer. Choose by the port: a self-sealing tapered port takes SAE-LT, a straight port takes UNF.
What is the difference between 1/8 NPT and 1/8 BSP?
Both measure near 0.383 inch outside diameter, but 1/8 NPT has 27 threads per inch and 1/8 BSP has 28. The thread forms differ and the two do not interchange. Check the thread pitch to tell them apart.
What size drill do I use to tap a grease fitting hole?
Tap drill size follows the thread: M6 × 1.0 takes a 5.0 mm drill, M8 × 1.25 takes 6.75 mm, M10 × 1.5 takes 8.5 mm, 1/4-28 UNF takes a #3 (0.213 in), and 1/8-27 NPT takes 21/64 in (0.328 in). See the tap drill table above.