How to identify a thread
Measure the outside diameter of a male thread at its largest point, or the inside diameter of a female port at the opening. Round to the nearest sensible figure and hold it.
Then get the pitch. On inch threads count the crests over one inch; on metric measure the distance from one crest to the next. A thread pitch gauge does both in seconds and removes the guesswork.
Check for taper by laying a straight edge along the thread, or by eye across the crests. A tapered thread narrows visibly over its length.
Diameter plus pitch plus taper narrows it to one or two candidates. If two remain, the thread angle decides, and that needs a gauge or a good profile comparison.
The thread systems compared
These seven cover almost everything on industrial equipment. The parallel-versus-tapered column and the angle column are what actually separate them.
| System | Angle | Form | Sealed by | Where |
|---|---|---|---|---|
| UNC / UNF | 60° | Parallel | Not a seal; clamp load | Inch fasteners |
| Metric coarse / fine | 60° | Parallel | Not a seal; clamp load | Metric fasteners |
| NPT | 60° | Tapered 1:16 | Thread flanks plus sealant | US pipe |
| NPS | 60° | Parallel | Gasket or O-ring at the face | US pipe, mechanical joints |
| BSPP (G) | 55° | Parallel | Bonded washer or O-ring | UK, EU, Asia pipe |
| BSPT (R) | 55° | Tapered 1:16 | Thread flanks plus sealant | UK, EU, Asia pipe |
| JIC 37° | 60° | Parallel | 37° flare cone at the seat | Hydraulics |
The pairs that catch people out
BSP and NPT at 1/2 and 3/4 inch share 14 threads per inch and near-identical diameters. Only the 55 against 60 degree angle separates them, and they will start together before binding.
JIC and ORFS look similar at a glance but seal completely differently: JIC on a 37 degree cone, ORFS on an O-ring in a flat face. They do not interchange.
JIC and SAE 45 degree flare share thread sizes at some dash numbers but differ in cone angle. A 37 degree cone in a 45 degree seat contacts on a line and weeps.
Metric fine and UNF overlap closely at some sizes. M12 x 1.25 and 1/2-20 UNF are close enough that a bolt will start and then gall solid a few turns in.
Common thread sizes and pitches
For quick identification, these are the diameters and pitches you meet most often on machinery.
| Thread | OD (mm) | OD (in) | Pitch / TPI |
|---|---|---|---|
| M6 x 1.0 | 6.0 | 0.236 | 1.0 mm |
| M8 x 1.25 | 8.0 | 0.315 | 1.25 mm |
| M10 x 1.5 | 10.0 | 0.394 | 1.5 mm |
| M12 x 1.75 | 12.0 | 0.472 | 1.75 mm |
| 1/4-20 UNC | 6.35 | 0.250 | 20 TPI |
| 5/16-18 UNC | 7.94 | 0.3125 | 18 TPI |
| 3/8-16 UNC | 9.53 | 0.375 | 16 TPI |
| 1/2-13 UNC | 12.70 | 0.500 | 13 TPI |
| 1/2-20 UNF | 12.70 | 0.500 | 20 TPI |
