Published July 16, 2026 at 07:05
Deep cylindrical bores with precision diameter requirements are one of the toughest challenges in workshop metrology. Standard internal micrometers have a neck depth of 50-150 mm — enough for most workpieces, but completely inadequate for:
- Hydraulic cylinders (typically 1-3 meters long with Ø200-500 mm inner diameter).
- Lathe headstock spindle bores (typically 500-1500 mm in length).
- Oil and gas pipes (typically 6-12 meters, but diameter verification at the intake opening is sufficient).
- Gearbox and transmission bores (typically 300-800 mm deep).
- Pump housing inspection (depth varies with pump size).
The Schwenk 280-510 mm instrument with 1500 mm measuring depth is designed specifically for this category.
The physics challenge: a LONG rod assembly demands rigid construction
A 1500 mm-long measuring probe has inherent rigidity problems:
Elastic deflection under its own weight: A 1500 mm × 25 mm-diameter steel rod supported at one end deflects by ~15 mm at the far end. This would shift the measuring position dramatically.
Thermal expansion: The steel rod’s length changes by 12 μm/m/°C. With a 5 °C difference between the handle and the far end, length drift reaches 90 μm — significantly larger than the measurement tolerance.
Torsion: During measurement, rotational pressure is applied at the handle. A long rod assembly can twist, which displaces the probe’s position during measurement.
Schwenk’s solution: the rod is a large-diameter HOLLOW cylinder (typically 35-45 mm outer diameter), a thermally insulated handle, and a torsion-stable housing construction. Full mechanical integrity across 1500 mm.
Two-point measuring principle
The instrument uses 2-point internal measurement — two opposing contacts that ride against the bore wall:
- Fixed reference contact: One side of the measuring tip is mounted on the fixed base. The reference for the measurement.
- Spring-loaded probe: The other side has a spring mechanism that presses against the opposite bore wall with constant force.
- Dial indicator: The spring probe’s position (relative to the fixed base) is mechanically transferred to a dial indicator at the operator’s handle via an internal linkage mechanism.
Standard workflow:
- Select a setting ring at the nominal diameter (e.g., Ø350 mm if the bore diameter should be ~350 mm).
- Place the instrument’s probe in the setting ring.
- Zero the dial indicator. The instrument is now calibrated to the nominal diameter.
- Insert the probe head into the actual bore.
- Read the dial indicator. A positive value = the bore is LARGER than nominal. Negative = SMALLER.
- Move the probe head deeper into the bore. Take readings at every 200-300 mm position.
- Map the diameter deviation along the entire depth of the bore.
What the deviation pattern reveals
Diameter variation along the bore depth reveals manufacturing or wear issues:
Constant diameter: The bore is CYLINDRICAL — perfect.
Taper — larger at the opening, smaller at the bottom: A common wear pattern on hydraulic cylinders. The piston passes the opening many times, wearing out the surface. The bottom (which the piston only touches at full stroke) is less worn.
Barrelling — larger in the middle, smaller at the ends: Often caused by localized HIGH-TEMPERATURE EXPOSURE (e.g., heat buildup during operation). The metal expands and deforms plastically in the middle.
Ovality at specific depths: The piston has not moved symmetrically — caused by incorrect piston ring installation or asymmetric piston loading.
Local “steps” in the diameter profile: Piston ring action has locally worn the cylinder wall. A standard wear pattern.
Economic relevance: hydraulic cylinder repair
Hydraulic cylinder wear is one of industry’s most EXPENSIVE maintenance categories:
- A 3-meter hydraulic cylinder for heavy machinery costs 50,000-300,000 DKK to replace.
- A replacement typically requires 8-24 hours of production downtime.
- REPAIR (if possible) costs 15% of replacement + 4-8 hours of downtime.
Critical decision: CAN the cylinder be repaired, or MUST it be replaced? The decision is based on the diameter deviation profile from the instrument:
- Max deviation < 0.2 mm: repair possible (honing + piston ring replacement).
- Max deviation 0.2-0.8 mm: repair possible but requires a new cylinder bushing.
- Max deviation > 0.8 mm: the cylinder must be replaced.
Without a deep-reach instrument like the Schwenk 280-510: the decision is based on guesswork or costly disassembly.
With the Schwenk instrument: the decision is based on ACTUAL diameter data. The instrument typically pays for itself after 1-2 repair decisions.
Interchangeable probe heads
The 280-510 mm range is NOT covered by a single probe. Standard practice is INTERCHANGEABLE probe heads:
- Probe 280-350 mm.
- Probe 350-430 mm.
- Probe 430-510 mm.
Each probe screws onto the far end of the rod. Swapping takes 30-60 seconds. All probes share the same 1500 mm rod-and-handle unit + the same dial indicator.
Trade-off: the probes built for different diameter ranges have different probe mechanism geometries. Standard practice is to BUY the full set of probes to cover the entire 280-510 mm range.
Setting rings for zeroing
Calibration rings (setting rings) with a known diameter are NECESSARY to zero the instrument. Standard sizes for this instrument’s range:
- Ø300 mm setting ring: 4,000-8,000 DKK.
- Ø400 mm setting ring: 8,000-15,000 DKK.
- Ø500 mm setting ring: 15,000-30,000 DKK.
The calibration ring’s diameter must lie WITHIN the instrument’s measuring range and match the NOMINAL value of the cylinder to be measured.
Alternative: the instrument can be sent to a calibration lab for verification without needing to own setting rings. Periodicity: annually. Cost: 3,000-8,000 DKK per verification.
Measuring depth 1500 mm — the critical specification
Measuring depth 1500 mm = the maximum length the probe head can be inserted into the bore from the opening. This is not the same as the rod length:
- Total rod length: ~1550-1600 mm (including the probe head).
- Effective measuring depth: 1500 mm (probe head + inspection margin).
Practical implications:
- 1-meter cylinder: measurement of the ENTIRE bore from opening to bottom.
- 1.5-meter cylinder: measurement almost to the bottom (max ~1.4 meters).
- 2-meter cylinder: measurement of only the upper HALF (1.5 meters from the opening).
- >2-meter cylinder: INSUFFICIENT. Requires longer-rod variants.
For 2-3 meter cylinders, special variants of the Schwenk instrument exist with 2000-3000 mm rods. These are even larger and more challenging to handle.
Weight and handling
The instrument weighs approximately 15-25 kg depending on the probe head. Handling:
- Typically requires 2 operators for safe handling (especially when carefully inserting the probe head into the bore).
- The handle must be supported at an ergonomic height to avoid strain.
- The instrument must NOT be dropped — the probe head is sensitive, and damage requires complete replacement.
Applications
Hydraulic cylinder maintenance: The standard tool for wear diagnosis in hydraulically driven machinery.
CNC lathe service: Headstock spindle bore verification.
Large engine maintenance: Large diesel engine cylinder inspection.
Oil and gas industry: Wellhead diameter verification, pipe inspection.
Wind power service: Tower flange diameter, gearbox bore verification.
Marine/shipping service: Hydraulic cylinder maintenance on vessels. Propeller shaft bushing verification.
Paper and cardboard production: Cylinder roll diameter, calender roll inspection.
Power plant service: Turbine service, generator bearing verification.
Marine hydraulics: Steering cylinder verification on heavy vehicles and marine equipment.
Alternative measuring methods for deep bores
3-point Holtest (covered previously): Better precision (±5-10 μm vs ±50 μm for this Schwenk), but max depth ~500 mm. For shorter bores.
Borescope + camera-based measurement: Provides visual inspection + digital measurement. 100,000-500,000 DKK. Good for visual defect identification but less suited to precision diameter measurement.
Internal laser scanner: 200,000-1,000,000 DKK. Provides a full 3D scan of the bore interior. Overkill for simple diameter verification.
Custom-made measuring tools: For much larger bores (>500 mm) or extra deep ones (>1500 mm), purpose-built tools are required. Price varies.
This Schwenk 280-510×1500 mm: 42,200 DKK. The standard solution for this specific large-diameter + deep-reach combination.
Specifications
- Model: Schwenk internal measuring instrument, size range 280-510 mm diameter.
- Measuring depth: 1500 mm.
- Diameter range: 280-510 mm (requires interchangeable probe heads).
- Dial indicator type: analog with 0.01-0.001 mm graduation (depending on the built-in dial indicator).
- Precision: ±0.05 mm typical.
- Repeatability: 0.01-0.02 mm.
- Measuring principle: 2-point internal with fixed + spring-loaded probe.
- Thermally insulated handle.
- Rod material: hollow-cylinder steel for rigidity + low weight.
- Requires setting rings for zeroing (sold separately).
- Weight: approximately 15-25 kg.
What you get for your money
Schwenk internal measuring instrument with analog dial indicator, measuring range 280-510 mm (via interchangeable probe heads), extreme measuring depth of 1500 mm for deep cylindrical bores, 2-point internal measuring principle with spring-loaded probe, thermally insulated handle to prevent body-heat drift, strong hollow-cylinder rod construction. 42,200 DKK. NOTE: Setting rings and additional probe heads are sold separately.
The standard tool for hydraulic cylinder maintenance, CNC lathe service, large engine maintenance, the oil and gas industry, wind power service, marine service, and other applications where diameter verification DEEP INSIDE cylindrical bores 1-1.5 meters deep is critical. The price premium over a standard internal measuring instrument is dramatically justified when the alternative is complete disassembly of equipment to access the bore opening from both sides.
Read more: Schwenk internal measuring instrument 280-510 mm in the shop →
