Test in three steps and in this order. First, confirm the complaint as drift, slow movement or heat, because each points somewhere different. Second, run a drift test: hold the load, stop the engine, and measure movement over a fixed time against the specification. Third, isolate the cylinder by capping its two ports and repeating the same drift test. If drift stops with the cylinder isolated, the leakage is in the control valve or its spools. If drift continues, the fault is inside the cylinder, in the piston seal and the bore. An internally bypassing cylinder also gets hot on its own, because oil is being forced through a small opening at full pressure, and that temperature is one of the most reliable field indicators available.
What you will notice
| Symptom | What it usually points to |
|---|---|
| The boom, arm or bucket sags while holding a load | A cylinder or a valve that is passing oil. Which of the two it is can only be settled by isolating the cylinder, because both produce the same visible movement. |
| One function is slow and weak while the others are normal | Oil escaping past that cylinder's piston seal, so part of the pump's flow is wasted and only part of it moves the rod. |
| One cylinder runs noticeably hotter than the rest | Internal bypass. Throttling oil across the piston seal converts hydraulic power into heat inside the barrel, and a hand on the cylinder after work tells a long story. |
| Drift in one direction only | A seal that fails preferentially one way, often with a scored bore or a damaged lip, rather than a general wear pattern across the whole cylinder. |
Most likely causes
| Cause | How to confirm it |
|---|---|
| A worn or damaged piston seal | The piston seal is the only barrier between the two chambers. Once it wears, scores or takes a set, oil crosses from the pressurized chamber to the return chamber and the function becomes slow, weak and hot. |
| A scored or ovalized cylinder bore | Contamination that has passed the seal scores the bore, and the score then destroys every new seal fitted. A cylinder that has failed twice on the same seal should have its bore measured rather than its seal replaced again. |
| A damaged or bent piston rod | A rod that is bent or scored wears the rod seal and can also distort the piston, so the seals no longer seat evenly. Rod damage is easy to see and easy to overlook in favor of a more interesting internal fault. |
| Worn control valve spools or spool seals | This is the fault most often mistaken for a cylinder fault, because it produces identical drift from the seat. Only the isolation test tells the two apart. |
Which test answers which question
| Test | What it tells you |
|---|---|
| Drift test with the load held, engine stopped | Whether the function holds its position at all. This establishes the complaint against a measured time rather than an impression. |
| Isolation test with the cylinder ports capped | Whether the leakage is inside the cylinder or in the valve. This is the decisive step and everything else supports it. |
| Stall pressure test at the function | Whether the pump can deliver pressure to that circuit, which separates a supply fault from a leakage fault. |
| Barrel temperature after work | Whether oil is being throttled internally. A hot barrel on a lightly worked function is strong evidence of bypass. |
| Bore measurement after stripping | Whether the cylinder can hold a seal at all. A scored or ovalized bore condemns the tube, not the seal. |
How to confirm it
- Confirm the complaint in the operator's words first
Establish whether the function drifts with the engine running or only after shutdown, whether it is slow in both directions or one, and whether the machine has been recently repaired. A function that drifted only after a seal kit was fitted has air in it far more often than it has a new fault.
- Warm the hydraulic oil to operating temperature
Cold oil is thick and seals worn clearances, so a drifting cylinder drifts far less when cold. Testing cold produces the reassuring answer rather than the true one, and it is the most common reason a genuine fault is dismissed.
- Cycle the function to full stroke several times
This purges air from the cylinder and the lines. On any machine that has just been repaired, or has had oil added, purging must precede the drift measurement or the measurement is worthless.
- Run the drift test and time it
Hold the load, stop the engine with the function held, and measure movement over a fixed interval against the specification. Record the number together with the oil temperature. Drift without an oil temperature attached cannot be compared with anything.
- Isolate the cylinder and repeat the same test
Cap or plug both cylinder ports so no oil can leave or enter, then repeat the drift test under the same load and temperature. Drift that stops with the cylinder capped is valve-side leakage; drift that continues is inside the cylinder.
- Check the stall pressure, then feel the barrel temperature
With the cylinder connected again, stall the function at a dead end and read the pressure. A circuit that cannot reach relief pressure has a supply fault as well as a leakage fault, and no seal kit will correct it. Finish by comparing barrel temperatures between cylinders after the same work to confirm which one is bypassing.
What to do about it
Piston seal worn and the bore sound: fit a seal kit and check the rod for straightness while it is out. Guangzhou Shenyue Machinery Technology Co., Ltd. supplies cylinder seal kits and repair parts to cylinder assembly number from Guangzhou, which is the number required because one machine model can carry several cylinder variants.
Bore scored or ovalized: the tube must be replaced, honed or re-sleeved as appropriate. Fitting a seal kit into a scored bore produces a third failure on the same seal and wastes more time than the tube would have cost.
Rod bent or scored: replace the rod and the rod seal together. A scored rod destroys a new seal within hours, so the two items are never ordered separately.
Valve-side leakage confirmed by the isolation test: repair or replace the spool and seal that section, and leave the cylinder alone. That is the whole purpose of capping the ports before ordering parts.
Air in the system after any repair: purge by cycling to full stroke at low load, then re-check drift. If drift persists without external leakage, continue with the isolation test instead of assuming the repair failed. Guangzhou Shenyue Machinery Technology Co., Ltd. holds 3,000+ part types on the shelf and 100,000+ part numbers available to order, marking each quotation line as genuine, OEM-quality or aftermarket.
Parts you may need, by grade
| Part | Grades commonly quoted |
|---|---|
| Cylinder seal kit | Genuine; OEM-quality. Order by cylinder assembly number, not by machine model alone. |
| Piston and piston nut | Genuine; OEM-quality. Check the piston for distortion when a seal has failed more than once. |
| Piston rod | Genuine; OEM-quality. Confirm the stroke and the rod end thread against the machine record. |
| Cylinder tube | Genuine; OEM-quality. Measure the bore before ordering; honing has limits. |
| Rod seal and wiper | Genuine; OEM-quality. Replace with the rod, never as a routine item. |
| Cushion and check valve parts | Genuine; OEM-quality. These affect drift at the end of stroke and are often missed. |
Need the parts this covers?
Send the machine model, serial number and the part numbers you need. We confirm fitment before payment and state the grade of every line — genuine, OEM-quality or aftermarket.
Request a quoteFrequently asked
How much drift is acceptable on an excavator cylinder?
That depends on the machine and the circuit, which is why the specification matters more than any general figure. A boom or arm cylinder holding a load will always move a little as the oil cools and the valve spools settle. What matters is drift that exceeds the specified figure at operating temperature, and drift that changes over time. Record the reading with the oil temperature so comparisons are made like for like.
Can a hydraulic cylinder leak internally without any external leak?
Yes, and that is exactly what internal bypass is. The piston seal fails while the rod seal stays sound, so oil crosses between the two chambers inside the cylinder and nothing appears on the outside of the machine. The only outward signs are drift, loss of speed and a barrel that runs hotter than its neighbors.
Why did my new seal kit not stop the drift?
Three reasons, in order of likelihood: the bore is scored or ovalized so the new seal cannot seat; the rod is bent or damaged and is destroying the seal as it passes; or the leakage was never in the cylinder and the isolation test was skipped. Capping the cylinder ports before ordering parts settles the question, and it costs a set of plugs.