Replacing a failed hydraulic cylinder is easy when you already know the dimensions. It becomes slow, expensive, and frustrating when a cylinder arrives from the supplier and does not fit the machine — the pin spacing is wrong, the port thread is a different standard, or the closed length pushes the rod end into the frame it was supposed to clear.
This guide covers the eight dimensions that decide whether a replacement cylinder is interchangeable, how to measure each one, and what to do when the old cylinder is too corroded, too damaged, or unmarked to read.
The eight dimensions that determine interchangeability
A hydraulic cylinder is interchangeable with an existing installation only when its critical dimensions match. Seven of the eight dimensions below can be measured from the outside; one — bore — usually requires opening the cylinder or reading a data plate.
| # | Dimension | What it is | Why it matters |
|---|---|---|---|
| 1 | Bore diameter | Internal diameter of the barrel | Sets push force |
| 2 | Stroke | Travel from fully retracted to fully extended | Sets how far the load moves |
| 3 | Closed length | Pin centre to pin centre, fully retracted | Sets the retracted envelope |
| 4 | Extended length | Pin centre to pin centre, fully extended | Sets the fully deployed envelope |
| 5 | Rod diameter | Diameter of the piston rod | Sets buckling and side-load capacity |
| 6 | Pin-to-pin distance | Centre distance of the mounting pins | Sets whether it bolts to the frame |
| 7 | Mounting type | Clevis, trunnion, flange, cross tube, rod eye | Sets mechanical compatibility |
| 8 | Port size and thread | SAE, NPT or BSPP port specification | Sets hydraulic connection |
If any one of these is wrong, the cylinder will not work correctly — and in several cases it will not physically fit. Collect all eight before ordering.
How to measure each dimension
1. Bore diameter
The bore is the internal diameter of the cylinder barrel, and it is the single number that sets how much force the cylinder can produce. Push force follows F = P × A, where A is the piston area derived from the bore.
If the cylinder has a readable data plate, the bore is stated on it. This is by far the easiest route.
If the cylinder must be opened, remove the cap end or the tie rods, then measure the internal diameter across at least three positions — two perpendicular readings plus one on the diagonal. Readings that vary by more than a fraction of a millimetre indicate ovality or scoring, which matters: a worn bore will not hold pressure with new seals.
If the cylinder is sealed and unmarked, you cannot measure bore directly. See the fallback method later in this guide.
Never size a replacement from rod diameter. Rod and bore are independent dimensions, and cylinders with different bores routinely use the same rod diameter on the same machine. Confusing the two is the most common cause of an under-powered replacement.
2. Stroke
Stroke is the distance the piston travels between the two ends of its range. Measure it with the rod fully retracted, then pull the rod out fully and measure the difference.
Extend slowly and watch what actually stops the motion. If the rod hits a mechanical stop on the machine frame, or you can hear the cushioning engage at the end of travel, that is the usable stroke — and it may be slightly less than the cylinder’s nominal stroke.
If the cylinder is telescopic, treat each stage separately; total stroke is the sum of the individual stage strokes.
3. Closed length
Closed length is the distance between the two mounting points with the cylinder fully retracted. This is the dimension that determines whether the cylinder fits inside the machine’s retracted envelope, and it is very often the one that is reported incorrectly.
Measure between the centres of the mounting pins, not the outside faces of the mounts. For clevis mounts this is the clevis pin centre to the rod eye centre. Include the mounting hardware in the reading if the replacement will use the same pins.
4. Extended length
Extended length is the same pin-to-pin distance with the cylinder fully extended. It should be close to closed length plus stroke, but confirm it by measurement rather than calculation — buffer and stop tubes, cushioning recesses and rod-end fittings can all make the two figures differ slightly.
Check this dimension against the machine. An extended length that is too long will drive the rod end into the frame at full travel.
5. Rod diameter
Measure the polished rod at a point clear of the gland and clear of any surface damage. Measure in two perpendicular directions.
Rod diameter governs two things: how much side load the rod can tolerate before bending, and how much push force it can carry without buckling. A thin rod on a long-stroke push application is the classic cause of bent rods. If the old rod is bent, replace it with a diameter one step larger rather than copying the original.
6. Pin-to-pin distance
Pin-to-pin is the centre-to-centre spacing of the mounting pins when the cylinder sits at its nominal zero position. For most pivot-mounted cylinders this is the same reading as closed length — which is why it is listed separately here, because machines with an adjustable mount can differ between the two.
Measure the pin diameter as well. It is a separate dimension from the spacing, and a 30 mm pin will not drop into a 25 mm bracket no matter how well the rest of the cylinder matches.
7. Mounting type
Identify how the cylinder attaches to the machine, and record it in standard terms rather than describing it loosely.
| Mounting type | How it attaches | Recorded as |
|---|---|---|
| Clevis (cap end) | Single pin through a clevis bracket | Mounting type + pin Ø |
| Trunnion | Integrated pivot pin near cap or head end | Mounting type + pin Ø |
| Flange | Bolted to a machined face | Flange dimensions + bolt circle |
| Cross tube | Pin through a welded tube | Pin Ø + pin centres |
| Rod eye (rod end) | Pin through the rod end | Eye bore Ø + thickness |
A cylinder that is otherwise identical will not bolt up if the mounting style differs. This is the dimension most often described vaguely — “the usual one with two pins” is not enough to order against.
8. Port size and thread
Measure the port thread at both the cap end and the rod end. Hydraulic ports come in three widely used standards, and they are not interchangeable:
- SAE (straight thread, common on mobile equipment in North America)
- NPT (tapered pipe thread, common on imported equipment)
- BSPP (British Standard Parallel Pipe, common on European and Japanese equipment)
A G 3/4 BSPP port and a 3/4-16 NPT port look similar in diameter but will not seal against each other’s fittings. Note the thread standard, not just the nominal diameter, and check whether the two ports are the same size — some cylinders have a larger cap-end port than rod-end port.
How to measure when the cylinder is sealed or unmarked
Older cylinders, cylinders from failed machines, and cylinders that have been repainted or painted over often carry no readable identification. Four fallback methods work in practice.
Method 1 — Measure everything external. Closed length, extended length, stroke, rod diameter, pin-to-pin, pin diameter and mounting type are all measurable without opening the cylinder. That covers seven of the eight dimensions.
Method 2 — Look for casting or stamping marks. Manufacturers nearly always mark the barrel, cap or rod end with a model code, a serial number, or a logo. On painted cylinders, the mark is often still legible under raking light or after light wire brushing.
Method 3 — Identify the mounting geometry. The mounting style, pin diameter and pin spacing will often uniquely identify a cylinder from a specific machine model. Machinery builders frequently used one cylinder type across an entire product line, so identifying the machine can identify the cylinder.
Method 4 — Open the cylinder as a last resort. If bore must be measured, the cylinder has to be opened. This is only worth doing if the machine is already out of service for other work, because disassembly and reassembly carry their own risk of damage.
For cylinders where bore cannot be confirmed, supply everything you have measured and state clearly which dimension is unconfirmed. A supplier who knows what is unknown will propose a practical solution; a supplier given six certain dimensions and one guess will order the wrong part.
Measurement mistakes that cause failed orders
| Mistake | Result |
|---|---|
| Using rod diameter as the bore | Cylinder fits but delivers too little force |
| Reporting closed length face-to-face instead of pin centre-to-pin centre | Cylinder is too long or too short in the envelope |
| Giving the nominal port size without the thread standard | Ports do not connect |
| Copying the old cylinder’s rod diameter when the old rod was bent | New rod bends again under the same load |
| Measuring stroke from the old cylinder without watching for the mechanical stop | New cylinder over-travels into the frame |
| Describing mounting in words instead of using a standard term | Supplier cannot match the frame |
Before sending an enquiry, check that the bore is either measured or explicitly flagged as unconfirmed, and that the thread standard is stated. Those two omissions account for most mismatches.
Build a replacement specification that can be ordered
Compile the measurements into a specification a supplier can quote without asking follow-up questions: bore, rod diameter, stroke, closed length, extended length, pin diameter, pin-to-pin distance, mounting type, port thread standard, and the working pressure the machine operates at.
Add two more items that are often omitted and are frequently needed: the rod end style (clevis, eye, threaded, spherical bearing) and the seal medium — the hydraulic oil grade and any temperature range the cylinder sees in service. Seal compound must match both the fluid and the temperature, and a cylinder specified without this can arrive with seals that will not survive the duty.
If a cushioning feature is fitted on the original, or if the machine has position feedback from a rod-mounted sensor, note that too; both affect the build and the price.
Standard dimensions you will see most often
Standard bore sizes follow the ISO 3320 series. Recognising these makes it easier to sanity-check a measurement and to ask for a quotation in the usual units.
- Standard bore series (mm): 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250, 320
- Common port threads: G 1/4, G 3/8, G 1/2, G 3/4 BSPP; 3/8, 1/2, 3/4 NPT; SAE straight thread equivalents
Common mounting designations for tie-rod cylinders include MP1 (cap-end clevis), MP2 (detachable clevis), MT1 and MT2 (trunnion types), MF1 and MF2 (rectangular flanges), and MS2 (side lugs). Recording the standard code removes ambiguity.
The fastest path to a correct replacement
Identify what the cylinder is and how it is mounted, measure the eight dimensions, and state clearly anything you could not confirm. That single specification sheet — bore, rod, stroke, closed and extended length, pin size and spacing, mounting type, and port thread standard — is what turns a supplier conversation into a firm quote and a cylinder that fits on arrival.
If you have the measurements for an existing installation and want a quotation against them, send them over with the machine type and working pressure. We will tell you which specification fits, and which dimensions need confirming before production.
Frequently asked questions
Can I identify a hydraulic cylinder by its dimensions alone?
Sometimes. Mounting type, pin diameter, pin spacing and closed length will often identify a cylinder used across a single machine model or product line. For a reliable match, supply bore, rod diameter and stroke as well — these three define the load capacity and cannot be inferred from anything else.
What is the difference between closed length and pin-to-pin distance?
On most pivot-mounted cylinders they are the same measurement taken the same way. They differ when the mounting is adjustable, or when closed length is specified to the outside faces of the mounts rather than to the pin centres. Always state which reference you measured from.
Do I need to know the pressure rating to replace a cylinder?
The rating determines what the cylinder can withstand, but for a like-for-like replacement the machine’s existing working pressure is the number to quote. Supplying it allows the supplier to confirm that the replacement is rated for the duty rather than merely dimensionally identical.
Can I measure bore without dismantling the cylinder?
Not directly. Bore is an internal dimension. Read it from a data plate if one is legible, or have the cylinder opened. Everything else on this list is measurable externally.
Should I order a larger bore to get more force?
Only after checking the rest of the circuit. A larger bore draws more oil for the same stroke, which changes the speed at a given pump flow and can overload the pump. Sizing for force is covered in detail in our hydraulic cylinder bore size calculation guide.