Accurate measurements are essential when repairing a hydraulic cylinder, ordering seals, replacing a damaged unit, or specifying a custom cylinder for existing equipment.
Two cylinders can look nearly identical and still be incompatible. A different stroke, mounting center, pin-hole diameter, or port thread may prevent a replacement from fitting or working correctly. Bore and stroke are only part of the information you need.
This guide covers the main dimensions used to identify different hydraulic cylinder types and designs, including the bore, rod, stroke, mounting geometry, pins, ports, threads, and internal components. The examples use both imperial and metric units.
Measure the bore, rod, and stroke
The three basic cylinder dimensions are:
Bore diameter
Piston rod diameter
Stroke length
Clean the cylinder before starting. Useful tools include a digital caliper, tape measure, micrometer, thread-pitch gauge, straightedge, and an inside bore gauge if one is available. Record each result on a measurement sheet.
If a cylinder on U.S. equipment appears to use imperial components, record the original readings in inches before adding metric conversions.
Bore diameter
The bore is the inside diameter of the cylinder barrel, not the tube's outside diameter.
On a disassembled cylinder, measure the barrel with an inside micrometer or bore gauge. Take readings at several points and in at least two directions. A worn barrel may no longer be perfectly round.
Example:
Measured bore = 2.500 in
Metric conversion:
2.500 × 25.4 = 63.50 mm
Do not calculate the bore from the tube's outside diameter. Barrel wall thickness varies by design.
If the cylinder cannot be fully disassembled, first search for the manufacturer's model number and drawing. When those records are unavailable, you may be able to measure an accessible internal surface with a telescoping gauge, inspection mirror, or another indirect method. Treat that result as an estimate until a drawing or internal component confirms it.
Rod diameter
Measure the polished piston rod on an unworn section with a micrometer or digital caliper. Take several readings around its circumference.
Rust, scratches, damaged plating, and heavily worn areas can produce misleading readings. Avoid those areas where possible.
Do not record the rod only as "about 1.5 inches." Write down the actual measurement and the probable nominal size:
Measured rod diameter: 1.499 in
Likely nominal rod diameter: 1.500 in
Stroke length
Stroke is the distance the piston rod moves between the fully retracted and fully extended positions.
Measure between the same two reference points in both positions:
Stroke = Extended reference length − Retracted reference length
For example:
Retracted pin-center distance: 34.25 in
Extended pin-center distance: 46.25 in
The stroke is:
46.25 − 34.25 = 12.00 in
Overall extended length is not the same as stroke.
For field identification, try to measure stroke and overall mounting dimensions within approximately 1/32 to 1/16 in. Measure diameters more precisely with a caliper or micrometer. These figures are field-measurement targets, not universal manufacturing tolerances. Confirm final fit requirements against the original drawing.
Record the mounting, pins, and end styles
Hydraulic cylinder mounting geometry matters as much as bore and stroke. Cylinders with the same bore and stroke may use completely different mounting arrangements.
Common mounting styles include:
Clevis mount
Single-lug or eye mount
Double-lug mount
Trunnion mount
Flange mount
Foot or side mount
Cross-tube mount
Threaded rod end
Spherical bearing mount
Parker cylinder documentation also lists mounting style, rod-end connection, bore, stroke, and mounting dimensions as separate specification variables.
Pins and clevises
For a pin-mounted cylinder, record the following dimensions:
| Dimension | What to measure |
|---|---|
| Pin diameter | Actual outside diameter of the pin |
| Pin-hole ID | Inside diameter of the mounting hole |
| Lug thickness | Width of each mounting ear |
| Clevis opening | Inside distance between the clevis ears |
| Mount width | Total width of the mounting feature |
| Center-to-center length | Center of the base pin to center of the rod-end pin |
| Bearing width | Width of the spherical bearing or bushing |
Measure a worn pin hole vertically and horizontally. Wear can make the hole oval, and using only its largest diameter may lead to an incorrect replacement. Measure an unworn pin or bushing as well when one is available.
Where a caliper can reach, aim to record mounting features to about 0.005 in (0.13 mm). This is a recording target, not a required pin-fit tolerance. The correct clearance or interference depends on the bushing, bearing, load, and original engineering specification.
Flange dimensions
For a flange-mounted cylinder, record:
Flange outside width or diameter
Flange thickness
Number of bolt holes
Bolt-hole diameter
Bolt-circle diameter or horizontal and vertical hole spacing
Pilot or locating-register diameter
Distance from the flange face to the rod-end reference point
Rod-end threads
For a threaded rod connection, measure:
Major thread diameter
Threads per inch or metric pitch
Threaded length
Male or female thread
Shoulder diameter
Distance from the shoulder to the rod end
Use a thread-pitch gauge instead of estimating the pitch by eye.
For example, 1.000-14 UNF-2A is not interchangeable with a metric thread simply because the diameters appear similar.
Identify ports, threads, and sealing methods
Do not identify a hydraulic port by apparent diameter alone.
Common cylinder port connections include:
NPT
BSPP
BSPT
SAE straight thread or UNF
Metric straight thread
ASME B1.20.1 is the primary U.S. standard for NPT and related general-purpose inch pipe threads.
Use this field identification sequence:
Measure the outside diameter of a male thread or the opening of a female thread.
Determine the TPI or metric pitch with a thread-pitch gauge.
Check whether the thread is straight or tapered.
Inspect the sealing method.
The following sizes show why diameter alone is not enough:
| Nominal size | NPT | BSP |
|---|---|---|
| 1/8 in | 27 TPI | 28 TPI |
| 1/4 in | 18 TPI | 19 TPI |
| 3/8 in | 18 TPI | 19 TPI |
| 1/2 in | 14 TPI | 14 TPI |
| 3/4 in | 14 TPI | 14 TPI |
NPT has a 60° thread form, while BSP uses a 55° form. BSPP is parallel and normally seals against a washer or O-ring instead of sealing through thread interference. BSPT is tapered. Parker identifies BSPP under ISO 228-1 and BSPT under ISO 7-1.
Both 1/2 NPT and 1/2 BSP may have 14 threads per inch, so TPI alone cannot distinguish between them.
SAE straight-thread ports commonly use UN or UNF threads with an O-ring seal. The hydraulic seal does not depend on thread interference.
When documenting a port, photograph the thread next to a ruler. Include the sealing surface, O-ring seat, adapter, and any stamped pressure rating or fitting identification.
Measure a cylinder while it is installed
Some cylinders must be measured on an excavator, agricultural machine, press, trailer, or other equipment because removal is not immediately practical.
Safety takes priority over dimensional accuracy. Hydraulic pressure can remain stored after the machine has been switched off. OSHA hazardous-energy requirements state that stored or residual energy must be relieved, disconnected, restrained, or otherwise rendered safe before servicing. The procedure may require bleeding pressure from the hydraulic system.
Follow the equipment manufacturer's lockout, tagout, and pressure-relief instructions. Isolate the energy sources, mechanically support any load that could move, relieve stored pressure, and verify isolation before approaching a component that may move unexpectedly.
Never loosen a hydraulic fitting as an improvised pressure check.
Useful field tools include:
Digital caliper
Tape measure
Steel rule
Telescoping gauge
Depth gauge
Thread-pitch gauge
Inspection mirror
Flashlight
Magnetic markers
Straightedge
When the full stroke is not accessible
If the machine prevents full extension, measure the rod at two controlled machine positions. Record:
Exposed rod length at position A
Exposed rod length at position B
The corresponding attachment geometry
These readings establish known travel between the two positions. They do not necessarily reveal the cylinder's full design stroke.
Before estimating the remaining travel, look for unused polished rod length, mechanical stops, the original machine specification, a cylinder model number, or a dimensional drawing.
Magnetic markers or removable tape can help track rod movement. In a confined area, photograph each measurement while the caliper or tape is still in place. Take one overview image and several close-ups so each reading can later be matched to the correct feature.
Measure internal components for repair or replacement
A disassembled cylinder provides access to hydraulic cylinder parts and the dimensions used for seal, bearing, and custom-part identification. Record:
Piston outside diameter
Piston width
Gland diameter
Wear-ring dimensions
Seal-groove diameter
Groove width
Groove depth
Rod-guide or bearing clearance
Piston thread
Rod-end thread
Cushion components
Barrel wall thickness
Measure seal grooves with a depth gauge and caliper. Do not select a replacement seal only from the worn seal's outside dimensions.
Before reverse engineering a cylinder, look for:
Manufacturer name
Model number
Serial number
Date code
Stamped part number
Equipment parts manual
Original engineering drawing
The original specification is usually more reliable than a measurement taken from a heavily worn component.
Convert between imperial and metric dimensions
Use the exact conversion:
1 in = 25.4 mm
| Inch | Millimeter |
|---|---|
| 1.00 in | 25.4 mm |
| 1.50 in | 38.1 mm |
| 2.00 in | 50.8 mm |
| 2.50 in | 63.5 mm |
| 3.00 in | 76.2 mm |
| 4.00 in | 101.6 mm |
| 6.00 in | 152.4 mm |
| 12.00 in | 304.8 mm |
Avoid rounding too early. A 1.500-in rod converts to 38.10 mm, but that does not mean a nominal 38-mm rod is interchangeable with it.
Calculate theoretical cylinder force
The theoretical extension force is:
Force = Pressure × Piston area
where:
Piston area = π × Bore² ÷ 4
For a cylinder with a 4-in bore operating at 2,500 psi:
Piston area = π × 4² ÷ 4 = 12.57 in²
Theoretical extension force = 2,500 × 12.57 = 31,425 lbf
Friction, pressure losses, and operating conditions reduce the force available in service.
To calculate retraction force, subtract the rod's cross-sectional area from the piston area.
Information needed to order a replacement cylinder
A replacement-cylinder request should include:
Bore diameter
Rod diameter
Stroke
Retracted pin-center distance
Extended pin-center distance
Base mounting style
Rod-end mounting style
Pin diameters
Mount widths
Port type and size
Port locations and orientation
Rod-end thread
Operating pressure
Equipment model
Working environment
Several clear photographs
Required quantity
Requested delivery date
Send these details through the inquiry form to reduce repeated dimensional checks. The measurements help determine whether a standard cylinder will fit or a custom industrial hydraulic cylinder is required.
Get help checking cylinder measurements
Use a printable measurement sheet before ordering a replacement. Record the bore, rod, stroke, mounting centers, pin sizes, ports, and rod-end dimensions in one place.
You can also contact our team and submit the completed measurements with photographs of the full cylinder, both mounting ends, and the hydraulic ports. We can check imperial and metric consistency, identify likely NPT, UN or UNF, and metric connections, and flag dimensions that need tighter tolerance verification. The same information provides the details needed to prepare a replacement or custom-cylinder quotation.
A complete measurement package allows the replacement cylinder to be evaluated more quickly.
FAQ
How can I measure the bore without fully disassembling the cylinder?
Start with the cylinder model number or manufacturer's drawing. If no specification is available, measure an accessible internal surface with a bore gauge or telescoping gauge where possible. Do not estimate the bore from the barrel's outside diameter because wall thickness varies. If you cannot reach an internal surface, leave the bore unconfirmed until the cylinder or piston can be measured.
How should I measure stroke if the cylinder cannot fully extend or retract?
Measure between the same reference points at two known positions and calculate the confirmed movement between them. If neither position is the mechanical end of travel, the result is partial travel rather than full stroke. Confirm the design stroke with the machine drawing, cylinder identification, or additional geometry.
How can I distinguish NPT, BSP, and metric hydraulic threads?
Measure the thread diameter and pitch, determine whether it is tapered or parallel, and inspect the sealing method. Compare the results with a thread chart. NPT has a 60° profile and BSP has a 55° profile. Metric threads are normally identified by major diameter and pitch, such as M18 × 1.5.
Which mounting dimensions are needed for a replacement cylinder?
At minimum, record the retracted center-to-center length, stroke, pin and pin-hole diameters, mounting widths, lug or clevis dimensions, and mounting style. For a flange cylinder, include the complete bolt-hole pattern and pilot dimensions.
How do I identify a rod-end thread?
Measure the major diameter, pitch or TPI, threaded length, thread gender, and shoulder dimensions. Use a pitch gauge and compare the readings with UN, UNF, or metric thread specifications. Diameter alone is not enough to identify the thread.
How should I convert cylinder measurements between imperial and metric units?
Multiply inches by 25.4 to convert them to millimeters. Keep enough decimal places during the conversion, then identify the likely nominal standard. A converted dimension that is close to a metric size does not make the components interchangeable.
What safety steps should I take before measuring an installed cylinder?
Follow the machine-specific energy-control procedure. Shut down and isolate the power source, lock out hazardous energy, support anything that could move, relieve stored hydraulic pressure, and verify that the equipment is de-energized before measuring around the cylinder. Do not rely only on the machine being switched off.