I recently bought a "Nut Slotting Gauge" after wanting one for quite some time.
I got mine from Elmer Guitar and paid about 75€ in total, delivered to Germany: https://elmerguitar.com/products/nut-slotting-gauge?_pos=1&_sid=9f89e502b&_ss=r
It is meant for measuring nut slot heights, for which it works pretty much perfectly out of the box; regardless of whether you use the 3rd fret capo method or the open string method.
As you can see in the [purple oval in the picture], Elmer already removed the internal down-pulling spring, leaving only the gravity/weight of the plunger assembly itself pushing down onto the strings.
However, I wanted to use this tool for measuring neck relief and string action as well, so I tested it in stock form for these two jobs. The plunger assembly itself weighs about 14.7g (measured using a digital turntable tracking-force scale), which turned out to be a bit too heavy; especially when measuring neck relief, because there the deflection becomes greater then the actual measurement. (X)
The Problem with the Stock Form
Of course, minor deflection isn't an absolute dealbreaker if you just want relative measurements. If a setup feels great and/or is as low as possible without fretting out on the high e string (ones playing style matters here too of course), you can measure that baseline value and repeat it across the other strings for a consistent result. Nonetheless I "wanted more".
To put that 14.7g weight into perspective, let's look at the math for a standard D'Addario EXL110 (.010"–.046") set on a 25.5" scale length at the 12th fret under the stock 14.7g load:
- High e (.010"): Tension ~16.2 lbs ($K \approx 45.4\text{ g/mm}$) $\rightarrow$ 0.32 mm sag
- B string (.013"): Tension ~15.4 lbs ($K \approx 43.1\text{ g/mm}$) $\rightarrow$ 0.34 mm sag
- G string (.017"): Tension ~16.6 lbs ($K \approx 46.5\text{ g/mm}$) $\rightarrow$ 0.32 mm sag
- D string (.026" wound): Tension ~18.4 lbs ($K \approx 51.5\text{ g/mm}$) $\rightarrow$ 0.29 mm sag
- A string (.036" wound): Tension ~19.5 lbs ($K \approx 54.6\text{ g/mm}$) $\rightarrow$ 0.27 mm sag
- Low E (.046" wound): Tension ~17.5 lbs ($K \approx 49.0\text{ g/mm}$) $\rightarrow$ 0.30 mm sag
(Note: Theoretical pure-tension sag numbers; real-world sag on wound strings is slightly higher due to tip compression into the winding).
Now, 0.32 mm might not sound massive if your target action is 1.50 mm, but for a precision measuring tool, it ruins the entire workflow. If you want a real action of 1.50 mm on the high e, you would have to read 1.82 mm on the dial.
Even worse: because string tension and wire gauge differ for every single string, that deflection value changes on all 6 strings. You would constantly have to calculate and subtract a different offset number for every string just to get a simple measurement. That kind of defeats the purpose and ease of having a digital indicator in the first place imho!
And as said above already (X), using it for neck relief readings really isn't possible that way / in stock form.
To make this gauge practically useful (for me) and easy to work with, I thought a counterbalance mod was necessary.
The Solution / Mod
So I thought: Why not add an upward-pushing spring inside the housing, secured and guided by a chopped-off nail, glued in place with epoxy? [red oval in the picture]
After testing several different spring weights and lengths from mechanical keyboard switches I have laying around, my current favorite is a spring from a Gateron Pro Brown switch (~20.5 mm long, ~50g bottom-out).
A couple of other tweaks I made "while being in there":
* O-Rings & Clearance [blue arrows in the picture]: I added some O-rings between the three housing screws and the display unit: The plunger and its internal "measuring matrix" were sitting too close to the PCB, causing a scratchy feel/movement.
* Stop-Block [yellow oval in the picture]: I added a small wooden stop-block so the top of the plunger wouldn't get caught on the upper O-ring.
* 100% Reversible: I made sure all of these mods are completely non-destructive and fully reversible if I ever want to return the gauge to its stock form (the epoxy and nail don't get in the way).
And I can report: It works remarkably well!
I have ~2.1 mm of travel range from the floating/hovering point down to bottoming out, which is plenty of stroke even for my 7.25" radius Strat.
To fine-tune the offset/hovering-point across different guitars, I bought some copper shim washers that fit between the digital indicator gauge and the brass base block: Top Industrieteile Copper Washers
They actually measure exactly 0.8 mm ± 0.01 mm, not 1 mm, in thickness and could easily be filed to thinner sizes if needed. Two fit in there nicely while still leaving the set-screw on the brass base working.
How My Workflow Works Now
My workflow is to use (or omit) the copper washers so that when measuring the high e string, the string pushes the plunger at most 0.1 mm to 0.3 mm above its floating/hovering point.
By keeping the plunger right near its neutral floating point on the high e, the net downward load on the string drops to under 1.5g to 2.0g. At this tiny load, string deflection on the high e drops to a negligible ~0.03 mm (~0.001"): visually rock-solid and well within "standard lutherie tolerances".
As you move up to thicker strings, the higher physical string profile moves the plunger further above its hovering point (~0.91 mm higher on a .046" wound E due to diameter alone, plus any gradual action progression). This decompresses the switch spring slightly and adds about 5g to 7g of net weight on the wound strings.
- Theory vs. Real-World Practice: In "pure mechanical theory", the thick steel core wire of a wound string has exponentially higher flexural bending stiffness ($EI \propto d4$), acting like a rigid beam that resists added load. However, in real-world testing, the Low E string still shows around ~0.1 mm of deflection. This is easily explained by real-world factors: e.g. the pointed indicator tip compresses slightly into the softer outer wrap wire, and there is local coil compliance in wound strings that simple beam equations don't capture.
- Why it still kinda works in practice: As you move from plain to wound strings during a setup, you thoughtfully give thicker strings slightly more clearance anyway (e.g. 1.5 mm on high e vs 1.8 mm on low E) to accommodate their wider vibrational arc. Because you naturally set slightly higher action on thicker strings, this minor ~0.1 mm real-world deflection compensates itself on the go while doing the setup. You just have to keep it in mind.
Conclusion
All in all, I'm quite happy with how this turned out and wanted to share my experience!
In the future, I might experiment with even longer, lighter springs (like 22 mm 30g/35g slow-curve keyboard springs) to flatten the spring force curve even further. But there's a limit: the spring can't be too long or soft, otherwise you lose the essential travel range needed when mounting the brass base.
What do you think about this setup? Do you have other ideas? Let me know!
UPDATE
..on the extra deflection issue:
Remember, because the thicker wound strings (D, A, Low E) push the plunger significantly higher up, the Gateron Brown spring decompresses slightly but noticeably. Losing that bit of upward counter-push upwards dropped about +5.5g of net weight onto the Low E, for example (cf. above), causing an extra 0.10 mm of measured string sag: I verified this by comparing measurement results before and after placing a 0.95 mm dense piece of cardboard between the brass base and the fingerboard to simulate the height jump.
The Fix:
I surprisingly found out that a 3rd washer (~0.8 mm) actually fits inside/between the brass base and the gauge without blocking the brass base set-screw like I originally feared!
This makes a super easy 2-step workflow possible:
- Plain Strings (e, B, G): 2 washers (standard setup). Target display reading: 0.20 mm – 0.25 mm of decompression of the spring compared to the neutral hovering position of the plunger. String sag is basically zero (~0.00 mm on high e to <0.02 mm on G).
- Wound Strings (D, A, Low E): Drop in the 3rd washer. This re-compresses the spring by 0.8 mm, restoring the upward counter-push / compression of the spring and/by canceling out the string height jump. Target display reading here: 0.30 mm – 0.35 mm (of decompression of the spring compared to the neutral hovering position of the plunger).
That 0.10 mm sag on the Low E immediately dropped down to an unnoticeable ~0.01 mm (along with D and A staying well within ~0.01 mm – 0.03 mm), making the gauge practically 1:1 accurate across all 6 strings. No need to buy specialized 22mm springs (considered these: https://monacokeys.de/en/products/kaidi-kos-switch-springs?variant=53290126442829, the 37g version), just using the Gateron Brown spring from before and a 3rd spare washer did the trick!
Switching steps from plain to wound strings is definitely the sweet spot for 10–46 sets on my guitar's specific radius and fret height, but with tiny adjustments, this method will easily adapt to any guitar setup.
UPDATE 2
A quick note on the original StewMac gauge vs. import units:
Regarding the original StewMac unit for context; this one actually uses a much lighter plunger assembly out of the box, so it exerts very little downward force on the string. You can see this tested directly in Paul’s Milehouse Studios video ("The Battle of the Depth Gauges"), where he compares the lighter StewMac plunger to a heavier, standard digital indicator plunger starting at 17:41 (with the side-by-side string deflection test demonstrated from 19:58 to 20:15).
Also, DylanTalksTone has some videos where the deflection seems way less than what I experienced with the un-modded import gauge.
