Measuring a watch spring bar correctly avoids the costliest mistake: ordering the wrong batch and having to return everything. The rule is simple but tricky: the length of the spring bar matches the lug-to-lug of the case, not the bare uncompressed bar.
Contents
- Measuring tools
- Measuring the lug-to-lug
- Measuring the diameter
- Diameter table by watch category
- Final check
- FAQ
Measuring tools
Three instruments cover 100% of the needs:
- Digital caliper 0.01 mm : the reference tool (Mitutoyo 500-181 or equivalent around €60).
- Steel ruler 0.5 mm : sufficient for the lug-to-lug but imprecise for the diameter.
- Spring bar gauge (plate drilled in 0.1 mm increments): fast workshop check.
Measuring the lug-to-lug step by step
- Remove the strap to free access to the lugs.
- Set the caliper to internal mode (jaws facing outward).
- Engage the jaws in the gap between the two lugs, against the inner walls.
- Read the value to the nearest 0.01 mm: this is the net lug-to-lug.
- Add 1.5 mm to obtain the uncompressed spring bar length (working compression + insertion into the lug holes).
A bar that is too short will drop out; a bar that is too long will refuse to engage. The functional tolerance is ±0.2 mm, never more.
NIHS 37-01 standard, paragraph 4.2
Measuring the body diameter
The body diameter of the spring bar (central tube) determines whether it fits into the lug holes. Measure with the caliper perpendicular to the axis, at the middle of the tube. Standard diameters are: 1.0 / 1.2 / 1.3 / 1.5 / 1.8 / 2.0 and 2.5 mm.
Common diameter table by category
| Watch category | Case diameter | Typical lug-to-lug | Ø usual bar |
|---|---|---|---|
| Slim dress watch | 36 mm | 18 mm | 1,5 mm |
| Daily three-hander | 40 mm | 20 mm | 1,8 mm |
| Sports diver | 42 mm | 20-22 mm | 2,0 mm |
| Chronograph | 43 mm | 22 mm | 1,8-2,0 mm |
| Military tool | 44 mm | 22-24 mm | 2,5 mm (fixed bar) |
| Vintage 1960 watch | 34 mm | 17-18 mm | 1,3 mm |
| Ladies' quartz | 28 mm | 12-14 mm | 1,0-1,2 mm |
Final check before ordering
- Measure three times and keep the average (eliminates parallax error).
- If the value falls right between two standards (e.g. 20.3 mm), order the lower value: compression makes up 0.5 mm.
- Visually check whether the lugs are straight or bevelled; bevelled lugs require short shouldered tips.
- If possible, remove the original spring bar and measure it uncompressed: this is the most reliable measurement.
FAQ
Should I measure the spring bar compressed or relaxed?
Always relaxed. The manufacturer's spec sheet states the free length; the working compression is implicit (typically 0.4 mm).
My lug-to-lug is 19.7 mm, which one do I order?
A 20 mm bar fits without issue: compression absorbs the 0.3 mm. A 19 mm would fall out at the first knock.
How do I measure without a caliper?
Place the strap on a ruler: the lug-to-lug equals the keeper width + 0.5 mm clearance. Accuracy only ±0.5 mm.
Are spring bars standardised?
Yes, by the NIHS 37-01 standard which sets the standard diameters and compressions. Chinese manufacturers comply 95% of the time.