Internal bore surface roughness is a decisive quality characteristic for hydraulic cylinders, engine liners, bearings, bushings and precision tubing, directly affecting sealing performance, lubricant retention, friction and service life. This testing solution explains how internal bore roughness parameters such as Ra, Rz, Rk and Sa are measured and evaluated in accordance with ISO 21920, ISO 4287 and ISO 25178, covering sample preparation, instrument selection, in-situ and laboratory methods, and result interpretation for bores ranging from a few millimeters in diameter to deep honed barrels.
Technical Overview
Internal bore surface roughness controls the functional behavior of mating assemblies. In hydraulic and pneumatic cylinders it governs seal friction, leakage and wear; in engine liners it determines oil film retention and oil consumption; in bearings and valve bores it influences torque, noise and fatigue life. Because the measurement surface is located inside a cylinder, the geometry itself creates the principal metrological challenge: restricted access, small diameters, large depth-to-diameter ratios, and the need to position a sensor at the bottom of the bore without degrading trace accuracy.
This solution combines several complementary techniques to cover the full range of industrial bore geometries:
- Contact stylus profilometry with dedicated bore probes, evaluated under the ISO 21920 profile surface texture standards, which supersede and consolidate the former ISO 4287, ISO 4288 and ISO 13565 series.
- Portable in-situ bore roughness gauges with articulated or pivoting probe arms for on-machine and field measurement of assembled components.
- Areal (three-dimensional) optical methods such as focus variation, confocal microscopy and white-light interferometry in accordance with the ISO 25178 series.
- Replica (impression) techniques for internal features that cannot be accessed directly, and reference specimen comparison for rapid shop-floor screening.
- Harmonized evaluation against national equivalents such as GB/T 3505 and JIS B 0601 where regional specifications require them.
Testing Samples
Hydraulic and Pneumatic Cylinder Bores
Honed or roller-burnished cylinder tubes with typical diameters from 20 mm to 500 mm and lengths up to several meters. Typical specification limits are Ra 0.1 µm to 0.4 µm, with tight requirements on waviness and circumferential uniformity. Sampling is performed at defined angular positions and axial stations to detect taper, barrel shape and local defects such as chatter marks or honing irregularities.
Engine Cylinder Liners and Block Bores
Plateau-honed bores where the load-bearing plateau structure and the deep honing valleys must be evaluated jointly. In addition to Ra and Rz, the stratified functional parameters Rk, Rpk, Rvk, Mr1 and Mr2 (formerly ISO 13565, now ISO 21920-2) are assessed, together with honing cross-hatch angle where specified by the drawing.
Sleeve Bearings, Bushings and Valve Bores
Thin-wall and small-diameter internal surfaces with diameters from approximately 3 mm upward. These parts frequently require skidless measurement of the axial generatrix to capture scoring, chatter and feed marks left by reaming, boring or burnishing.
Deep-Drilled Holes, Precision Tubing and Fuel System Components
Gun-drilled and trepanned holes, injector bodies, common-rail components and pump bores with high depth-to-diameter ratios. These samples are measured using extended-reach probes, areal optical methods, cross-sectioning or replication, depending on accessibility and whether destructive preparation is permitted.
Testing Items
The test program is defined by the drawing callout and the functional intent of the surface. Commonly measured parameters include:
- Amplitude parameters (profile): Ra (arithmetical mean deviation), Rq (root mean square deviation), Rz (maximum height of the profile), Rt and Rmax.
- Spacing and hybrid parameters: RSm (mean spacing of profile elements) and Rdq (root mean square slope), which describe the texture direction and the sharpness of the surface asperities.
- Stratified functional parameters: Rk (core roughness depth), Rpk (reduced peak height), Rvk (reduced valley depth) and material ratios Mr1 and Mr2, derived from the Abbott-Firestone material ratio curve, essential for plateau-honed cylinder liners.
- Material ratio curve: Rmr(c) at a specified section height for sealing and bearing compatibility assessment.
- Waviness and primary profile: Wt and Pt for the detection of machine-tool-induced medium-wavelength errors inside the bore.
- Areal parameters: Sa, Sq, Sz, Ssk and Sku in accordance with ISO 25178-2, reported when three-dimensional optical measurement is applied.
Testing Methods
Sample Preparation and Measurement Planning
Samples are cleaned by degreasing and ultrasonic cleaning to remove cutting fluid, oil and swarf, and inspected for burrs at the bore mouth that could obstruct probe entry. Unless otherwise specified, the measurement trace is taken along the axial generatrix of the bore, with measurement positions distributed at several angular stations (typically 0°, 120° and 240°) and several depths (near mouth, mid-length and bottom). The cutoff wavelength and evaluation length are selected according to the expected roughness range and the rules of ISO 4288 and ISO 21920.
Contact Stylus Profilometry with Bore Probes
A skidless stylus instrument conforming to ISO 3274 is used, fitted with a bore-access probe or a pivoting measuring arm. The diamond stylus tip radius (typically 2 µm to 5 µm) and the static measuring force are matched to the surface hardness to avoid tip wear or scratching. The profile is filtered with the default Gaussian filter, the evaluation length normally comprises five sampling lengths, and parameters are computed in accordance with ISO 21920-2. Instruments are calibrated and verified per ISO 12179 using artifacts traceable to ISO 5436-1 reference standards, ensuring full metrological traceability to national standards.
Portable In-Situ Bore Roughness Gauges
For parts that cannot be transported to a laboratory, or for machining-center verification before workpiece removal, portable drivers with radial or articulated probe arms measure bores in place. This method supports process control of honing and boring operations and enables measurement of large cylinder tubes and housings without cutting.
Optical and Areal 3D Methods
Focus variation, laser scanning confocal microscopy and white-light interferometry provide three-dimensional areal datasets compliant with ISO 25178-6. These methods are preferred for small bores, fuel injector spray holes, and cross-hatch characterization, because they capture texture direction and valley structure that a single profile may miss. Their limitation is line-of-sight access, so they apply mainly to open or shallow bores and to sectioned samples.
Replica (Impression) Technique and Cross-Sectioning
Where no instrument can physically reach the surface, a low-shrinkage replica compound is applied to the internal surface, cured, removed and measured in the laboratory; replication fidelity is validated against a reference surface of comparable roughness. For failure analysis and metallurgical correlation, samples may be sectioned along the bore axis to expose the full generatrix for both profilometric and microscopic examination.
Data Evaluation and Reporting
Acceptance decisions follow the ISO 4288 / ISO 21920 rules: under the default 16% rule, the surface is considered acceptable if not more than 16% of all measured values of the parameter exceed the specified limit, while the max-rule variant requires every individual value to comply. Test reports state the measured parameters, cutoff and evaluation length, filter type, instrument and method, measurement positions, ambient conditions and an estimate of measurement uncertainty, providing an unambiguous basis for drawing conformity assessment.
Applications
- Hydraulic and pneumatic cylinders, actuators and telescopic tubes for construction, agricultural and industrial machinery.
- Automotive and marine powertrain components: cylinder liners, engine block bores, connecting rod small ends and camshaft bores.
- Aerospace landing-gear actuators, servo valves and fuel metering components.
- Oil and gas downhole tools, drilling equipment and flow control internals.
- Pumps, compressors, injectors and common-rail fuel system parts.
- Sleeve bearings, bushings, gear bores and general precision machined components.
FAQ
What is the minimum bore diameter that can be measured?
Dedicated bore probes with small stylus arms typically measure bores from approximately 3 mm in diameter. Smaller features, such as injector spray holes, are measured by areal optical methods on sectioned parts or by replication, depending on part value and permitted preparation.
Can the bore be tested without cutting or destroying the part?
Yes. Portable bore roughness gauges with pivoting probe arms measure open bores in place, and replica films capture the texture of inaccessible internal regions for subsequent laboratory measurement, allowing finished or expensive parts to be tested non-destructively.
Which parameters should be specified for a sliding bore versus a sealing bore?
Sliding and lubricated surfaces, such as cylinder liners, are usually specified with the stratified parameters Rk, Rpk and Rvk to control plateau and oil-retaining valleys. Simple static or dynamic sealing surfaces are commonly specified with Ra and Rz, supplemented by Rmr(c) where lubricant film behavior is critical.
Which standards apply to the measurement?
Profile measurement and parameter evaluation are governed by the ISO 21920 series, which consolidates the former ISO 4287, ISO 4288 and ISO 13565 standards; instrument characteristics follow ISO 3274 and calibration follows ISO 12179. Areal measurement follows the ISO 25178 series. GB/T 3505 and JIS B 0601 provide equivalent national parameter definitions where regional specifications invoke them.
How many measurement positions are required inside a bore?
Unless the drawing or the inspection plan dictates otherwise, a minimum of three axial traces at evenly spaced angular positions is recommended, each comprising the standard evaluation length of five sampling lengths. Deep or critical bores are sampled at multiple axial stations to detect local defects and form-related roughness variation.
How is traceability and measurement uncertainty ensured?
All instruments are calibrated against certified reference artifacts conforming to ISO 5436-1, and periodic intermediate checks are performed with the laboratory's own roughness standards. Measurement uncertainty budgets account for stylus geometry, filter effects, calibration uncertainty and bore-alignment contributions, and are reported with the results, ensuring that acceptance decisions are metrologically sound.