Short answer
The decision does not rest on how badly the part is damaged but on four things at once: can the part be brought back to dimension, has the damage passed to the neighbouring part, is the downtime the repair creates longer than replacement, and will the same failure repeat. Where all four point the same way the decision is easy; where they diverge, dimension decides. A part that cannot be brought back to dimension is not repaired, because an out of dimension part in service damages not itself but its neighbour, and the bill arrives on the second round.
Four questions
- 1
Can the part be brought back to dimension?
Where surface damage has not gone beyond the body, grinding and plating can restore the dimension. Where there is a crack, where the dimension has fallen below the lower limit, or where the hardened layer has gone entirely, the part will not come back to dimension and the repair discussion ends there.
- 2
Has the damage passed to its neighbour?
A worn bushing affects the piston, a worn tool seat affects the retaining pin, a scored cylinder affects the seal. Renewing one part and leaving its neighbour in place damages the new one along the same path. While it is apart, measure the neighbouring part as well.
- 3
How much downtime does the repair cost?
A repair is usually cheaper than the part and more expensive than the downtime: the part goes to a workshop, waits its turn and comes back. Compare it with a new part on the shelf not by price but by how many days the machine stands still.
- 4
Will the same failure repeat?
If the cause that damaged the part has not been removed, a repair only postpones the calendar. Where lack of lubrication, a wrong angle or dirty oil continues, a repair made without removing the cause is paid for twice.
Parts that are usually replaced
- Sealing elements: not repaired, replaced as a set. Changing one seal and leaving another invites a fresh strip-down.
- Retaining pins, locks and washers: cheap, critical and quick to finish. A worn one is replaced, not built up with weld.
- Bushings: a bushing at its dimensional limit is replaced. The bushing is what keeps piston and tool in line; dimension is not negotiable there.
- Diaphragm: an ageing and fatigue part with no repair.
Where repair makes sense
The piston surface is the typical example: where the body is sound and only the surface is scored, grinding and re-plating return the part to dimension. The same applies to the cylinder bore, though there the dimension approaches its lower limit sooner.
On the housing and bracket side, welding is sometimes the right answer: a worn face on the bracket that carries the attachment can be recovered with build-up weld. What matters is that the heat-affected zone does not reach a working surface or a hardened region.
The tool is not on this list. Welding on the tool or its shank destroys the hardened structure and makes the part brittle. A worn tool is replaced.
No decision without a measurement
The commonest mistake on site is deciding by eye: "a bit worn, it will do". Wear limits are model specific and printed in the manual; neither repair nor replacement can be decided without putting a caliper reading next to the manual table. Read the limits from your manual, or ask HKM with your model code.
Do and don’t
Do
- Measure the neighbouring parts while it is apart; the second strip-down is the expensive one.
- Work the repair decision through downtime as well, not through part price alone.
- Find and remove the cause that damaged the part before sending it for repair.
- Change sealing elements as a set.
- Record the measurements; the wear rate only becomes visible through records.
Don't
- Do not weld on the tool or its shank; the hardened structure is destroyed.
- Do not run a part that has fallen below its lower dimensional limit on the grounds that "a little clearance is fine".
- Do not weld up a cracked part and carry on; the crack travels on under the weld.
- Do not have the same part repaired twice in a row; the second failure is telling you the cause persists.
- Do not decide by eye; without a caliper and the manual table you are guessing.
Two habits that make the decision easy
The first is record keeping. Where nobody writes down how many hours each part lasted, every failure arrives as a surprise and the decision is taken under pressure. Where the same part has finished twice around the same hour figure, it is no longer a failure but a maintenance item you can plan.
The second is stock. A business with its critical parts on the shelf decides calmly: it repairs where repair makes sense and fits a new part where it does not. A business without stock is pushed towards "making do with what we have" every time. We covered which parts belong on the shelf in the critical stock guide.