How to identify threads and gears when reverse engineering
Identify a thread by measuring the major diameter and checking pitch with a thread gauge, then match it to a standard (metric ISO, UNC/UNF, BSP, NPT). For a spur gear, count the teeth, measure the outside diameter and calculate the module (outside diameter divided by teeth plus two), then confirm pressure angle and profile against standard values before modeling.
Guilherme Rodrigues ItinoseUpdated 2 min read
Key takeaways
- Never model a thread as measured; match it to a standard.
- A thread pitch gauge settles metric vs inch in seconds.
- Gear module ≈ outside diameter / (teeth + 2) for standard spur gears.
- Standard modules and a 20° pressure angle are the usual starting point.
- Check mating parts: the pair must agree.
Threads and gears are standardized. Reverse engineering them means finding the standard that fits, not copying a worn profile.
Threads
- Measure the major diameter of the external thread or minor diameter of the internal one.
- Check pitch with metric and inch pitch gauges.
- Match to a standard: ISO metric coarse or fine, UNC/UNF, BSP (parallel or taper), NPT.
- Check form and hand: thread angle (60° or 55°) and right or left hand.
| Measured | Likely thread |
|---|---|
| Ø 9.8 mm, 1.5 mm pitch | M10 × 1.5 coarse |
| Ø 12.6 mm, 13 TPI | 1/2”-13 UNC |
| Tapered, 14 TPI, about Ø 21 mm | 1/2” NPT or BSPT (check thread angle) |
Spur gears
- Count teeth (z).
- Measure outside diameter (da).
- Calculate module: m ≈ da / (z + 2).
- Round to a standard module and check the pressure angle (20° is most common).
- Verify with span or over-pin measurement and with the mating gear.
If the numbers do not fit, the gear may be inch-based, profile-shifted or worn.
We reverse engineer threaded parts and gears to standard specifications. See reverse engineering.
When to outsource this
- You need to reproduce a gear or threaded part without drawings
- A machine mixes metric and inch hardware
When not to
- Catalog gears or fasteners you can identify and buy directly
FAQ
How do I tell metric from inch threads?
Use a pitch gauge: metric gauges read millimeters between threads, inch gauges read threads per inch. Only one will seat perfectly.
What if the module is not a round number?
Check if it is an inch diametral pitch gear, a profile-shifted gear or a worn sample. Measure over pins or span to confirm.
What about pipe threads?
Pipe threads such as BSP or NPT may be tapered. Measure diameter at both ends and check the seal type.
Can helical gears be reverse engineered?
Yes, but you also need the helix angle and hand, best measured with mating gears and dedicated tools.
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