This article is a brief guide to the most popular types of hydraulic fittings and connections. It is essential that you correctly identify fittings so that you can connect the right systems.
These measurements are for fitting identification using the table at the end of the page.
You will need:
- Callipers
- Thread pitch gauge
- Seat angle gauge
Step 1.
Is the fitting female or male?
- Female—the threads are on the inside of the nut and measured by the inside diameter (ID)
- Male—the threads are on the outside of the nut and measured by outside diameter (OD)
Step 2.
Is the fitting tapered or straight? If you cannot see this use callipers to measure the beginning and end of the fitting. A parallel thread will have a method of sealing, whereas taper threads seal by thread contact between the male and female ports.
- Tapered—the diameter get smaller towards the end of the female fitting or gets larger towards the end of a male fitting.
- Straight (parallel)—the threads are the same diameter end to end.
Step 3.
Determine the thread size using callipers measure the OD of the male thread or the ID of the female thread. Beware of worn threads when checking the table for an exact match.
Determine thread angle using a seat angle gauge. Match the centreline of the seat gauge with the axis of the coupling.
Step 4.
Determine thread pitch – threads per inch.
Use a thread pitch gauge to measure the threads per inch. If you do not have a thread pitch gauge, count the number of thread crests over ¼” length and multiply it by 4. For example, if there are 3.5 thread crests, multiply by 4 to get 14—the number of threads per inch. You can also use a pitch gauge against a lighted background. Measure several threads to ensure that you have the right number.
Step 5.
Check the results against the charts below.
Male Parallel Fitting Dimensions
| Thread | Threads per inch | Pitch | Outside diameter mm | Inner diameter mm |
| 1/16 | 28 | 0.907 | 7.723 | |
| 1/8 | 28 | 0.907 | 9.728 | 8.566 |
| 1/4 | 19 | 1.337 | 13.157 | 11.445 |
| 3/8 | 19 | 1.337 | 16.662 | 14.95 |
| 1/2 | 14 | 1.814 | 20.995 | 18.631 |
| 5/8 | 14 | 1.814 | 22.991 | 20.587 |
| 3/4 | 14 | 1.814 | 26.441 | 24.117 |
| 1 | 11 | 2.309 | 33.249 | 30.291 |
| 1 1/4 | 11 | 2.309 | 41.910 | 34.939 |
| 1 1/2 | 11 | 2.309 | 47.803 | 44.845 |
| 2 | 11 | 2.309 | 59.614 | 56.656 |
| 2 1/2 | 11 | 2.309 | 75.184 | 72.226 |
| 3 | 11 | 2.309 | 87.884 | 84.926 |
| 4 | 11 | 2.309 | 113.030 | |
| 5 | 11 | 2.309 | 138.430 |
Male Tapered Fitting Dimensions
| Thread | Thread per inch | Pitch | Outside diameter mm | Inner diameter mm |
| 1/16 | 28 | 0.907 | 7.723 | 6.561 |
| 1/8 | 28 | 0.907 | 9.728 | 8.566 |
| 1/4 | 19 | 1.337 | 13.157 | 11.445 |
| 3/8 | 14 | 1.337 | 16.662 | 14.95 |
| 1/2 | 11 | 1.814 | 20.995 | 18.631 |
| 3/4 | 11 | 1.814 | 26.441 | 24.117 |
| 1 | 11 | 2.309 | 33.249 | 30.291 |
| 1 1/4 | 11 | 2.309 | 41.91 | 38.952 |
| 1 1/2 | 11 | 2.309 | 47.803 | 44.845 |
| 2 | 11 | 2.309 | 59.614 | 56.656 |
| 2 1/2 | 11 | 2.309 | 75.184 | 72.226 |
| 3 | 11 | 2.309 | 87.884 | 84.926 |
| 4 | 11 | 2.309 | 113.03 | 110.072 |
| 5 | 11 | 2.309 | 138.43 | 135.472 |
BFPA Training – Indanc Academy
Industrial Ancillaries Academy runs a number of courses in partnership with the BFPA specifically designed for anyone who works with hydraulics.
Foundation Safety Course
Basic hydraulics health and safety course.
Hose Assembly Skills Course
2 day course on hose assembly procedure, including routing and installation.
Hose Integrity, Inspection and Management Course
Health and safety assessment training including risk analysis and competence.
Small Bore Tubing Integrity Course
Safety training in the manufacture of small bore tubing assemblies.
Call us on 01246 242050 or get in touch via our Contacts page to find out more details about our courses and course dates.
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