Employee Training Bulletin #177
Pipe Thread Measurement
Doughboy and Lomart have 2 size threads on gauges
They
vary from model to model and year to year.
Either
use the parts number from your own parts list or measure gauge.
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Pipe sizes do not refer to any physical dimensions. The outside diameter of each pipe or fitting must be measured and compared to the table for size identification. For example, a 1/4" NPT pipe thread has an outside diameter of .540 inches. Each thread size has a defined number of threads per inch (TPI). The 1/4" NPT pipe thread has 18 threads per inch. Both the TPI and OD of the thread are required for positive identification of thread size because several sizes have the same TPI. |
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Male threads:
Measure the outside diameter of the large portion of the thread at
"A"; Find figure nearest this dimension in column 1 or 2 of
chart. The dimension in column 3 will be your nominal pipe thread size. |
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| Column 1 | Column 2 | Column 3 | Column 4 |
| Approx. (fraction) Inch. For quick reference only | Actual (decimal) Inch. | Pipe Thread Size | Threads per Inch |
| 19/64 | .301 | 1/16 | 27 |
| 13/32 (Aprx 3/8 inch) | .405 | 1/8 | 27 |
| 35/64 (Aprx 5/8 inch) | .540 | 1/4 | 18 |
| 43/64 | .675 | 3/8 | 18 |
| 27/32 | .840 | 1/2 | 14 |
| 1-3/64 | 1.050 | 3/4 | 14 |
| 1-5/16 | 1.315 | 1 | 11-1/2 |
| 1-21/32 | 1.660 | 1-1/4 | 11-1/2 |
| 1-29/32 | 1.900 | 1-1/2 | 11-1/2 |
| 2-3/8 | 2.375 | 2 | 11-1/2 |
| 2-7/8 | 2.875 | 2-1/2 | 8 |
| 3-1/2 | 3.5 | 3 | 8 |
| 4 | 4.0 | 3-1/2 | 8 |
| 4-1/2 | 4.5 | 4 | 8 |
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Two common pipe
thread sizes exist, the tapered National Pipe Thread (NPT) and the
straight National Standard Free-Fitting Straight Mechanical Pipe Thread (NPSM).
The NPT and NPTF threads are interchangeable with sealants such as Teflon
tape. None of the other thread standards are interchangeable. Female NPT
threads can be designated as "FPT" and male NPT threads can be
designated as "MPT." |
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| Tap size |
Threads per inch |
Drill
size w/ taper pipe reamer |
Drill
size w/out taper pipe reamer |
Straight
pipe threads |
|
1/16 |
27 |
15/64 |
D |
1/4 |
|
1/8 |
27 |
21/64 |
Q |
11/32 |
|
1/4 |
18 |
27/64 |
7/16 |
29/64 |
|
3/8 |
18 |
9/16 |
37/64 |
37/64 |
|
1/2 |
14 |
11/16 |
23/32 |
23/32 |
|
3/4 |
14 |
57/64 |
59/64 |
15/16 |
|
1 |
11.5 |
1 1/8 |
1 5/32 |
1 11/64 |
|
1 1/4 |
11.5 |
1 15/32 |
1 1/2 |
1 33/64 |
|
1 1/2 |
11.5 |
1 23/32 |
1 47/64 |
1 3/4 |
|
2 |
11.5 |
2 3/16 |
2 7/32 |
2 7/32 |