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Use these charts to match a tube size to the right tube nut, or to identify a fitting you've removed. The charts come from the AGS Pro Fitting Center Fitting Guide (C-2690).
Standard UNF (inch) tube nut threads
| Nominal tube size | Thread O.D. | Threads per inch | Thread |
|---|---|---|---|
| 3/16" | 3/8" | 24 | 3/8-24 |
| 1/4" | 7/16" | 24 | 7/16-24 |
| 5/16" | 1/2" | 20 | 1/2-20 |
| 3/8" | 5/8" | 18 | 5/8-18 |
These are the standard pairings. Some vehicles use a larger nut on a smaller line (for example a 7/16-24 or 1/2-20 nut on 3/16" line), so always measure the threads too.
Common metric tube nut threads
| Application | Tube size | Thread O.D. | Pitch | Thread |
|---|---|---|---|---|
| Japanese | 3/16" | 10 mm | 1.0 mm | M10 x 1.0 |
| European | 3/16" | 10 mm | 1.0 mm | M10 x 1.0 |
| GM | 6 mm | 12 mm | 1.0 mm | M12 x 1.0 |
NPT (pipe) threads
NPT threads are tapered and seal on the threads themselves. You'll find them on brass fittings, adapters and some valves, not on brake tube nuts.
| Nominal pipe size | Thread O.D. | Threads per inch | Thread |
|---|---|---|---|
| 1/8" | 0.405" | 27 | 1/8" NPT |
| 1/4" | 0.540" | 18 | 1/4" NPT |
| 3/8" | 0.675" | 18 | 3/8" NPT |
| 1/2" | 0.840" | 14 | 1/2" NPT |
How to measure a fitting
- Tube O.D.: measure the outside of the tube with calipers. 3/16" = 4.75 mm, so a 4.75 mm tube is 3/16".
- Thread O.D.: measure across the outside of the threads with calipers.
- Pitch: use a thread pitch gauge. Inch threads are counted in threads per inch (24, 20, 18). Metric threads are measured in millimeters between threads (1.0, 1.25, 1.5).
- Flare type: check whether the line uses an inverted double flare or a bubble flare. See Brake Line Flare Types.
Inch and metric threads can be close in size (for example 3/8-24 and M10 x 1.0). Never force a tube nut. If it doesn't thread in easily by hand, it's the wrong thread.
Stocking a shop or store? The fitting cabinet program and fitting assortments cover the most common sizes.
Source: AGS C-2690 Pro Fitting Center Fitting Guide.