You picked up a mystery bolt and need to identify the thread. With only TPI (threads per inch) or only metric pitch (mm between threads), you can't directly match — but a simple conversion gets you there.
The relationship
TPI × Pitch (mm) = 25.4
So if you measured 12.7 TPI, the equivalent metric pitch is 25.4 / 12.7 = 2.0 mm — that's M-something with 2.0 mm pitch.
Reverse: a thread with 1.5 mm pitch = 25.4 / 1.5 = 16.93 TPI. Closest standard imperial = 16 TPI or 18 TPI — and the difference is enough that the wrong one will cross-thread.
Common engine thread pitches
- M6×1.0 ≈ 1/4-25 (no standard imperial equivalent — don't mix)
- M8×1.25 ≈ 5/16-20 (close but not interchangeable)
- M10×1.5 ≈ 3/8-17 (no exact match — common bolt size confusion)
- M12×1.5 ≈ 7/16-17 (no exact match)
- M14×2.0 ≈ 9/16-13 (close — spark plug threads)
The takeaway: metric and imperial threads of "similar size" are never compatible. Always verify with a thread gauge before forcing a bolt.