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Home › Knowledge Base › Field Case: A CAT 330D Boom Cylinder That Ate Three Seal Kits — the Fourth Fix Lasted
Case in one line: A 330D boom cylinder leaked three times in a year — three seal kits, three failures. On the fourth inquiry we asked for photos of the cylinder head: a dent-and-score on the bore was scratching every new seal as it slid in. The fix: head-polishing notes + a proper seal kit. One year on — zero leaks.

How it went

DayWhat happened
Day 1Fourth inquiry for the same seal kit, "three replaced, three leaks." Stock confirmed by number — and we asked for photos of the bore and rod
Day 1Photos showed a scored dent inside the bore — the root cause. Every new seal was being cut at installation
The planSeal kit shipped as requested, plus head-prep notes (stone/fine emery around the bore, deburr, clean before fitting)
Day 5Kit arrived; customer polished the bore and fitted
One-year follow-upZero weeping. Customer's words: "The first three were parts-swapping — this one was a repair."

Four root causes of repeat seal failure (check before fitting)

Root causeHow to checkFix
Dented/scored head or boreShine a light into the borePolish the damage out before fitting
Rod scoring / plating flakingRun a finger along the rodLight: polish; heavy: re-chrome
Abnormal pressure spikesRelief-valve pressure readingReset to spec before replacing parts
Aged shelf-stock sealsCheck the supply channel and dateBuy fresh, properly sourced kits

Appendix: 4 ways to get the exact part right the first time

MethodWhat you provideResult
1. Part number (fastest)The part number / VOE number / CAT number — nothing else neededStock and price confirmed within minutes; 100% precise
2. Model + serial number + yeare.g. EC210B + machine serial + year 2015The same model can have several versions — the serial number pins it down
3. Photo of nameplate / old partA clear photo of the machine nameplate, or the printed/stamped number on the old partWe verify fitment from the photo — our responsibility, not yours
4. Fault description + replacement historyBriefly describe the symptom and whether/when this part was replaced beforeWe help judge what to replace — and whether a root cause is still hiding

Send any one of the four to our online chat to lock in the right part. The more you share, the faster and more accurate the quote.

Parts used in this case

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Frequently asked questions

Can we polish the head ourselves?

Light scoring: yes — stone or fine emery in a circular motion, never crosswise; deep damage goes to a cylinder shop for boring.

How are seal kits matched?

Bore size + part number is surest; no bore data? Photograph the nameplate or measure the old kit and we'll match it.

Repair the cylinder or buy an assembly?

Bore and rod sound? A seal kit is by far the best value. Scored bore or bent rod? That's when an assembly makes sense.

Got a part number or a photo?

Send it to our online chat — in-stock parts quoted the same day.

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