Aftermarket Cylinder Seal Kits for Select Hitachi ZX270/ZX280 Applications
Finding the correct aftermarket cylinder seal kit involves more than searching for an excavator model. Machines in the same product family may use different cylinders according to their generation, configuration, production range, or previous repairs.
This guide introduces four Ruijueparts aftermarket replacement seal kits—4639936, 4639938, 4639939, and 4649745—and explains their intended boom, arm, and bucket cylinder applications.
All brand and model names in this article are used solely to identify the intended equipment compatibility. The products discussed are aftermarket replacement parts supplied by Ruijueparts and are not represented as genuine or original equipment manufacturer parts.
Important: Model information is a reference rather than a fitment guarantee. Buyers should verify the machine serial number and cylinder assembly before ordering.
What Do These Four Seal Kits Have in Common?
They share several characteristics:
- They are used when rebuilding a hydraulic cylinder.
- They contain replacement sealing components rather than a complete cylinder.
- They are identified by reference part numbers.
- They cover cylinders within the excavator’s front working equipment.
- Their suitability must be checked against the installed cylinder.
- They are independently supplied aftermarket products.
The four kits are not automatically interchangeable. A seal kit intended for a boom cylinder should not be installed in an arm or bucket cylinder simply because all three cylinders are fitted to the same excavator.
Part Number and Intended Application
| Reference number | Intended cylinder position | Application reference | Verification priority |
|---|---|---|---|
| 4639936 | Boom cylinder | Select ZX270/ZX280 applications | Cylinder position and assembly number |
| 4639938 | Arm/stick cylinder | Select ZX270/ZX280 applications | Confirm arm cylinder identification |
| 4639939 | Bucket cylinder | Select ZX270/ZX280 applications | Confirm bucket cylinder identification |
| 4649745 | Boom cylinder | Select ZX270-3/ZX280LC-3 applications | Confirm machine generation |
4639936 Aftermarket Boom Cylinder Seal Kit
The 4639936 aftermarket boom cylinder seal kit is intended for selected boom cylinder applications associated with the ZX270/ZX280 range.
Boom cylinders raise and lower the excavator’s front equipment. Common reasons for inspecting a boom cylinder include visible oil around the rod or gland, reduced lifting performance, and attachment drift.
These symptoms do not always prove that the seals alone are responsible. Internal cylinder wear, a damaged piston rod, control valve leakage, or hydraulic system problems can produce similar behavior. The cylinder should therefore be inspected before a replacement kit is selected.
4639938 Aftermarket Arm Cylinder Seal Kit
The 4639938 aftermarket arm cylinder seal kit is intended for selected arm cylinder applications.
The arm cylinder—also called the stick cylinder in some markets—controls movement between the boom and arm. It operates repeatedly during digging, grading, and loading, making clean hydraulic oil and an undamaged piston rod important to seal life.
Customers should confirm the physical cylinder position because terminology may differ between parts catalogs and regions. “Arm,” "stick," and “dipper” can sometimes describe the same excavator section, but the applicable replacement kit must still be matched to the actual cylinder.
4639939 Aftermarket Bucket Cylinder Seal Kit
The 4639939 aftermarket bucket cylinder seal kit is currently listed for selected bucket cylinder applications.
The bucket cylinder controls bucket curling and dumping. Its location exposes it to dust, soil, moisture, and abrasive material. A damaged wiper, contaminated hydraulic oil, or scratched piston rod can shorten the service life of newly installed seals.
Before ordering, buyers should verify that the repair is for the bucket cylinder. The bucket and arm cylinders can look similar in photographs, but their dimensions, seal profiles, and kit contents may differ.
4649745 Aftermarket Boom Cylinder Seal Kit
The 4649745 aftermarket boom cylinder seal kit is also intended for a boom cylinder, with current product information associating it with selected ZX270-3 and ZX280LC-3 applications.
The “-3” designation is important. A ZX270 and a ZX270-3 should not be treated as identical applications simply because their model names are similar.
The fact that 4639936 and 4649745 are both described as boom cylinder seal kits does not establish that they are substitutes. Differences may exist between machine generations and installed cylinder assemblies. Interchangeability should only be stated when supported by your verified cross-reference records.
How to Verify the Required Seal Kit
Check the complete machine model
Record all letters, numbers, and suffixes. ZX270 and ZX270-3 should be treated as different model references during verification.
Identify the cylinder position
Confirm whether the leaking component is the boom, arm/stick, or bucket cylinder.
Record the machine serial number
Machines carrying the same model designation may have production changes. A serial number gives the supplier more information than the model alone.
Find the cylinder assembly number
Photograph any plate, stamping, or identification number on the installed cylinder. The cylinder number can provide stronger evidence than a general machine-model search.
Inspect the cylinder condition
A replacement seal kit may not correct leakage caused by a bent, scratched, pitted, or corroded piston rod. The gland, bore, grooves, bearings, and wear rings should also be inspected.
Compare the removed components
When identification information is unavailable, provide clear photographs and measurements of the removed seals. Measurements should support—not replace—part-number verification.
Contact Ruijueparts for a fitment check and quotation for 4639936, 4639938, 4639939, or 4649745 aftermarket cylinder seal kits.
