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REF: Thread Identification
Threads Per Inch (TPI) vs Thread Pitch
Thread Pitch and Threads Per Inch are both used to measure the threading of a bolt or nut to ensure that they are going to couple together properly. 1)
If the threading of a bolt and nut are different, they will either seize or strip the threading resulting in an unusable connection.
Threads per inch, commonly abbreviated as TPI, is a term frequently used in US fasteners and is a term used to help identify how many threads are in an inch.
To determine treads per inch, an inch of the threads are measured longways and then the peaks on the fastener are counted within that inch.
For threads shorter than 1 in, you can measure half an inch of threads and double the result to get the TPI.
Thread pitch is a term used in place of TPI in metric fasteners and is the distance from one thread to the adjacent thread measured longways across the thread peaks.
US Threads
US threads generally have 3 numbers although the last can be left off unless required.
The first number is a whole number and/or fraction, the second number is the threads per in (TPI), followed by the length in inches.
The diameter is the outside diameter of the thread peaks.
In this example of a 1/4“ bolt bolt; 1/4” x 20 x 1-1/2“ states that the app. diameter of the threads is 1/4”, the TPI is 20 and it's 1-1/2“ long.
| US Thread Pitch | |||||
| Bolt Diameter | TPI (course) | TPI (fine) | Bolt Diameter | TPI (course) | TPI (fine) |
| 1/4” | 20 | 28 | 3/4“ | 10 | 16 |
| 5/16” | 18 | 24 | 7/8“ | 9 | 14 |
| 3/8” | 16 | 24 | 1“ | 8 | 12 |
Metric Threads
Metric bolts are written down as an “M” followed by 3 numbers and again the last can be left off unless required.
The first number is the diameter, the second the thread pitch, and the third the length in millimeters. 2)
The diameter is the outside diameter of the thread peaks.
In this example of a 10mm bolt; M10x1.25×35 states that the diameter of the threads is 10mm, the thread pitch is 1.25mm and it's 35mm long.
| Metric Thread Pitch | |||
| Bolt Diameter | Standard Thread Pitch (mm) | Fine Thread Pitch (mm) | Extra Fine Thread Pitch (mm) |
| 4mm | 0.70 | - | - |
| 5mm | 0.80 | - | - |
| 6mm | 1.00 | - | - |
| 7mm | 1.00 | - | - |
| 8mm | 1.25 | 1.00 | - |
| 10mm | 1.50 | 1.25 | 1.00 |
| 12mm | 1.75 | 1.50 | 1.25 |
| 14mm | 2.00 | 1.50 | - |
| 16mm | 2.00 | - | - |
| 18mm | 2.50 | - | - |
| 20mm | 2.50 | - | - |
| 24mm | 3.00 | - | - |
Brake Line Threads
The most common sizes for brake line connections are 3/8”-24, 7/16“-24, 1/2”-20, 5/8“-18, 10mm-1.0 and 12mm-1.0. 3)
3/8” x 24 vs M10 x 1.0mm
3/8“ x 24 TPI equates to a thread pitch of 0.0416666666666667” or 1.05833333mm.
3/8“ x 24 stated in metric units equates to 9.525mm x 1.05833333mm
So a 3/8” fitting is slightly smaller in diameter than the 10mm fitting but each thread is .058mm farther apart than a 10mm fitting's threads.
| Conversion | ||
| 3/8“ x 24 | 9.525mm Dia. | 1.05833333mm Thread Pitch |
| M10 x 1.0 | 10mm Dia | 1.0 Thread Pitch |
A 3/8” x 24 UNF American nut will thread very easily and loosely into a 10mm x 1.0 thread fitting but will most likely strip the threads when you tighten it down. 4)
You can start a 10mm x 1.0 threaded nut into a 3/8“ x 24 UNF thread fitting but it will immediately begin stripping the threads.
It will catch the first thread, making you think that it will go in, but it will destroy the threads as you tighten it.
