Description
Mitutoyo 20–50mm Holtest Three-Point Internal Micrometer Set — Series 368-903
When bore tolerances are tight and a two-point snap gauge won’t cut it, the Mitutoyo Holtest is what serious toolroom work demands. This Series 368-903 set covers the 20–50mm internal diameter range across five individual Holtest heads, each using Mitutoyo’s three-point contact system to deliver a self-centring, repeatable bore measurement that removes the operator-induced error you get with a standard two-anvil bore mic. Supplied in the original fitted timber case with three precision setting rings, this is a complete, working metrology set ready to go straight to the bench.
- Three-point self-centring contact system — eliminates cosine error. Unlike a two-anvil bore mic that relies on the operator finding true centre, the three equally-spaced carbide-tipped contacts automatically locate on the bore axis. The reading you get is the actual diameter, not a chord measurement — critical when you’re checking a reamed hole or a hydraulic cylinder bore to a H7 tolerance.
- 0.001mm graduation across the full 20–50mm range — practical toolroom resolution. Each head reads to one micron, which is meaningful resolution for checking interference fits, press fits, and bearing housings where a couple of thou either way determines whether a part is scrap or a pass. The thimble and vernier barrel are clean and legible on all five heads.
- Five individual heads covering 20–50mm in stepped ranges. Rather than a single instrument stretched across the full span, each head covers a narrow range — typically 6–8mm per head — keeping measurement uncertainty low across the entire set. You’re not working near the extremes of a long-range instrument; each head is in its sweet spot.
- Three Mitutoyo precision setting rings included (24.999mm, 34.997mm, 45.002mm). These ground reference rings are what make this set genuinely self-contained. Zeroing a Holtest correctly against a traceable ring gauge is the only reliable way to establish accuracy — without them, you’re relying on a gauge block stack or borrowing a ring from another set. Having them matched to the instrument set removes that problem entirely.
- Ideal for: Checking reamed bores and drilled-and-bored holes to H6/H7/H8 fit standards, verifying hydraulic cylinder and engine bore diameters, inspecting housing bores for rolling-element bearings, confirming bore size after honing on a cylindrical grinder, and production QC checks on machining centre output where a plug gauge pass/fail isn’t sufficient for recording actual deviation.
Condition
Used (Good Condition / Full Working Set)
All five Holtest heads are in good used condition. The measuring contacts show normal service wear consistent with workshop use, but there is no visible damage to the carbide tips, no bent or stiff contact stems, and the thimble action on each head is smooth with no binding or backlash beyond normal instrument feel. The thimble graduations and barrel scales are clearly legible on all five instruments — no significant scratching obscuring the graduation lines. The three Mitutoyo setting rings (NO.177-139, NO.177-140, and NO.177-178) show a light surface patina on their outer flanks typical of handled toolroom gauges, but the precision bore surfaces are in good order with no visible scoring, pitting, or deformation that would compromise their use as zeroing references. The spanner wrench for the contact locking mechanism is present in the case. Some surface marks and minor cosmetic wear are present on the instrument bodies — this is honest working-tool condition, not display condition, and has no bearing on measurement function. As with any used precision bore-measuring instrument, buyers should verify each head against its corresponding setting ring before first use in a critical application, and re-zero as part of normal pre-use procedure. If traceability is required for a quality system, calibration against a certified master ring is recommended.
Included
- 5 x Mitutoyo Holtest Three-Point Internal Micrometer Heads (Series 368, 20–50mm range set)
- 3 x Mitutoyo Precision Setting Rings — NO.177-139 (24.999mm), NO.177-140 (34.997mm), NO.177-178 (45.002mm approx.)
- 1 x Spanner wrench for contact adjustment
- 1 x Original Mitutoyo fitted timber storage case
Not Included
- Calibration certificate (none supplied — verify zero against setting rings before critical use)
Brand Information
Mitutoyo Corporation, founded in Japan in 1934, is the benchmark name in precision metrology at the toolroom and production inspection level globally. Their instruments are manufactured to consistently high standards of accuracy and repeatability, and sit firmly at the top of the industrial metrology tier alongside Mahr, Tesa, and Starrett — well above generic import alternatives in terms of long-term stability, surface finish quality, and calibration traceability. Made in Japan is not a throwaway line with Mitutoyo; it reflects a manufacturing culture where the tolerance on the tool making the part is held tighter than the tolerance on the part itself. Mitutoyo instruments are found in ISO-certified QC labs, production machine shops, and serious home toolrooms alike — and critically, replacement parts, setting rings, and calibration services remain widely available for their product lines.
Specifications
| Feature | Specification |
|---|---|
| Brand | Mitutoyo |
| Series / Model | Series 368 — Set No. 368-903 |
| Type | Holtest Three-Point Internal Micrometer Set |
| Measuring Range (Set) | 20–50mm (five individual heads in stepped sub-ranges) |
| Graduation / Resolution | 0.001mm (1 micron) |
| Accuracy | ±0.003mm (refer to Mitutoyo Series 368 specification sheet for per-head accuracy; verify against setting rings) |
| Contact Type | Three-point self-centring, carbide-tipped anvils |
| Setting Rings Included | 24.999mm (NO.177-139), 34.997mm (NO.177-140), 45.002mm approx. (NO.177-178) |
| Construction / Material | Hardened steel body, carbide measuring contacts, knurled thimble |
| Country of Origin | Japan |
| Case | Original Mitutoyo fitted timber case |
| Condition | Used — Good Condition |
| Included Items | 5 x Holtest heads, 3 x setting rings, 1 x spanner wrench, timber case |
| Not Included | Calibration certificate |
Keywords
Mitutoyo 368-903, Mitutoyo Holtest set, Mitutoyo three-point bore micrometer, internal micrometer 20-50mm, bore mic set, Holtest 368 series, three-point internal mic, toolroom bore gauge, precision bore measurement, bore micrometer with setting rings, internal micrometer Australia, machining centre bore inspection
