A Kobelco excavator carries three working cylinders on the front end and often more on attachments, and every one is rebuilt at least once in a machine's life. Cylinders are attractive to rebuild because the alternative — a new assembly — is expensive and slow to arrive. They are also the assembly most often rebuilt badly, because the failure is read from the outside and the decision is made before the bore and rod have been measured. Guangzhou Shenyue Machinery Technology Co., Ltd. has supplied these assemblies since 2007 and this guide sets out what decides the outcome.
Start from the symptom, not from the leak
A weeping rod seal is a symptom, not a diagnosis. Three different failures produce fluid on the same rod. A worn rod seal leaks only when the cylinder is under pressure. A scored rod damages a new seal within hours of fitting, so the leak returns and the workshop is blamed. A damaged tube bore lets oil bypass the piston internally, which produces drift and heat rather than an external leak at all. Ordering seals against the first of those and fitting them to the third wastes a seal kit and a day of labour, and the machine comes back.
What to measure before anything is ordered
Four readings decide whether a cylinder is rebuildable and which parts it needs. Bore diameter at three points along the stroke shows whether the tube is oval or tapered. Rod diameter and straightness show whether the rod can be re-chromed or must be replaced; a bent rod will destroy a new gland no matter how carefully it is fitted. Rod surface condition under a light shows whether scoring has reached beyond the chrome. Finally, the gland and piston clearances show whether the wear rings and bushings are doing their job or the metal is already carrying the load. A workshop that records these four numbers can quote a rebuild with confidence; one that does not is guessing.
When the tube can be saved and when it cannot
A tube with light scoring, no more than a few thousandths deep and not reaching the seal travel line, can usually be honed and returned to service with a matching piston seal. A tube with deep gouges, ovality beyond specification or corrosion pitting along the stroke must be replaced — honing it only enlarges the bore and accelerates bypass. The same logic applies to the rod: light marking polishes out, but damage that has removed the chrome layer will continue to corrode under a new seal and leak again. Chrome plating and grinding a rod back to specification is a normal repair in most markets, and it is usually far cheaper than a new rod.
| Wear point | Reading that matters | Decision |
|---|---|---|
| Rod seal | Leaks only under pressure | Seal kit — check the rod first |
| Rod surface | Scoring depth vs chrome layer | Polish, re-chrome or replace |
| Tube bore | Ovality and scoring along stroke | Hone or replace the tube |
| Piston and wear rings | Clearance and ring condition | Rebuild group with the seal kit |
| Gland bushings | Side play on the rod | Replace with the seal kit |
The parts that turn a reseal into a rebuild
A seal kit alone restores the sealing function and nothing else. The parts that decide how long the repair lasts are the wear rings on the piston, the bushings in the gland that keep the rod centred, and the rod itself. Where those are left in place because they "still look serviceable", the new seals are working against the same clearances that destroyed the old ones, and the repair is measured in months rather than hours. On our list the cylinder-related lines such as cylinder and undercarriage components are quoted as a group for exactly this reason: seals, wear rings, bushings and the rod decision taken together.
Drift is an internal leak, not a seal problem
A machine whose boom settles overnight, or whose cylinder creeps under load, is bypassing internally. That is piston seal, wear ring and bore condition — not the rod seal. On a machine where the drift is accompanied by heat in the cylinder, the bypass is significant enough to be costing hydraulic power on every cycle, which is a fuel and productivity loss as well as a repair. Drift should be measured in millimetres over a fixed time with the machine at working temperature, because a cold machine can drift within tolerance and still fail when hot.
How we supply
Guangzhou Shenyue Machinery Technology Co., Ltd. has specialised in Kobelco parts since 2007, with 4 warehouses over 1000 m² in Guangzhou, 3,000+ part types on the shelf and 100,000+ part numbers available to order through original supply channels. Shenyue is an independent supplier and is not an authorized dealer of Kobelco Construction Machinery Co., Ltd.; for Kobelco, Hino, Isuzu, Yanmar, Cummins, Kawasaki, Nabtesco, KYB and KPM we supply genuine parts ordered through original supply channels alongside quality replacement alternatives, at prices close to agent level, and the five brands we hold formal authorization for — Mitsubishi, Garrett, TONGMYUNG, Marusan and NIKURA — are named on every quotation. Browse the hydraulic components range and the SK200 model page; the Kobelco published machine history is a useful cross-reference when a machine's generation must be confirmed.
Frequently Asked Questions
Do I need a new cylinder or can mine be rebuilt?
Most Kobelco cylinders are rebuildable unless the tube bore is deeply scored or corroded along the stroke. Measure bore ovality, rod straightness and rod surface condition first; if the rod can be re-chromed and the tube honed, a rebuild with a full group — seals, wear rings and gland bushings — restores normal service life.
Why does my rebuilt cylinder leak again after a few weeks?
Almost always because the rod surface was not corrected. A scored or corroded rod damages a new seal quickly, so the leak returns and the seal is blamed. Check the rod under a light and re-chrome or replace it before fitting a new seal kit.
What causes boom drift when there is no visible leak?
Internal bypass past the piston seal, wear rings or a damaged bore. This shows as drift and heat rather than an external leak. Measure the drift hot, then quote the piston group rather than the rod seal.
Can I get the rod and the seal kit in one order?
Yes, and it is the recommended way. Send the cylinder nameplate, the measured bore and rod readings and photographs of both, and Guangzhou Shenyue Machinery Technology Co., Ltd. quotes the complete rebuild group with the grade of each line declared.