Thread Pitch Chart
SAE UNC/UNF & Metric Coarse/Fine
AI Diesel Technician's Bible™ — complete sensor thread database, thread repair procedures, tap drill reference & shop procedures.
Unlock Full Bible — $9.95Thread Pitch Chart — SAE UNC/UNF & Metric
Thread pitch reference for SAE UNC and UNF threads (threads per inch) and metric coarse and fine threads (mm pitch) for M6 through M24. Includes common diesel sensor thread sizes — NOx, O2, fuel rail.
SAE Threads — UNC & UNF
TPI = Threads Per Inch
| Size | UNC (TPI) | UNF (TPI) | Note |
|---|---|---|---|
| 1/4" | 20 | 28 | UNC most common |
| 5/16" | 18 | 24 | |
| 3/8" | 16 | 24 | |
| 7/16" | 14 | 20 | |
| 1/2" | 13 | 20 | |
| 9/16" | 12 | 18 | |
| 5/8" | 11 | 18 | |
| 3/4" | 10 | 16 | |
| 7/8" | 9 | 14 | |
| 1" | 8 | 12 |
Metric Threads — Coarse & Fine
Pitch in mm · Lower = finer thread
| Size | Coarse | Fine | Note |
|---|---|---|---|
| M6 | 1.0 mm | 0.75 mm | Sensor threads common |
| M8 | 1.25 mm | 1.0 mm | |
| M10 | 1.5 mm | 1.25 mm | Very common diesel |
| M12 | 1.75 mm | 1.25 mm | |
| M14 | 2.0 mm | 1.5 mm | Drain plugs |
| M16 | 2.0 mm | 1.5 mm | |
| M18 | 2.5 mm | 1.5 mm | O2/NOx sensors |
| M20 | 2.5 mm | 1.5 mm | |
| M22 | 2.5 mm | 1.5 mm | Fuel rail sensors |
| M24 | 3.0 mm | 2.0 mm |
Pro tip: Use a thread pitch gauge to confirm pitch before tapping — mismatched threads are the #1 cause of stripped sensor ports.
Need the full diesel sensor thread database?
The full Bible includes thread sizes for every diesel sensor port — NOx, boost, fuel rail, DEF, EGR, coolant temp, and oil pressure — organized by engine family with thread repair insert specifications.
Unlock the Full Bible — $9.95Thread Pitch — FAQ
What thread size are diesel NOx and O2 sensors?
Most diesel NOx sensors and oxygen sensors use M18 × 1.5 threads. Some older sensors use M18 × 1.5 or M12 × 1.5. Always verify with the OEM service manual for your specific engine.
What is the difference between UNC and UNF threads?
UNC (Unified National Coarse) has fewer threads per inch and is more common for general fasteners. UNF (Unified National Fine) has more threads per inch, provides better holding power in thin materials, and is used where vibration resistance is needed.
How do I identify the thread pitch on a bolt?
Use a thread pitch gauge — a set of combs that match against the threads. For metric bolts, measure the distance between thread crests in mm. For SAE bolts, count the threads per inch. A thread pitch gauge is the most accurate method.
What thread pitch are fuel rail pressure sensors on diesel engines?
Fuel rail pressure sensors on most diesel engines use M22 × 1.5 threads. Some applications use M14 × 1.5. The full sensor thread database in the AI Diesel Technician's Bible™ covers NOx, boost, fuel rail, DEF, EGR, and other diesel sensor thread sizes by engine family.
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