Steel Shim
The Listed Service Life and What Stands Behind It
A steel shim, also called a valve shim or shock shim, is installed inside the shock absorber piston assembly, positioned in the piston valve system or base valve system, and works together with valve seats, springs and orifice passages. It is made from Sandvik 20C, SK4 or SK5, C100S, 65Mn, or stainless grades. During the compression and rebound strokes the valve shim opens or closes as the oil flows, regulating the flow rate through the orifice passages and generating the required damping force. Development and production follow the IATF 16949 quality management system, and the warranty covers one year or 100,000 km, whichever comes first, for material and manufacturing defects.
Bonpora lists a service life of typically not less than 100,000 km or 5 years under normal operating conditions. That figure describes a stack working inside a healthy damper with clean oil and an intact seal. When a damper is serviced, the realistic question is not whether every shim has reached a nominal distance, but whether this particular stack still behaves as it did when it was approved. The rest of this guide sets out the signals and the decisions.
What Actually Limits Shim Life
Three mechanisms shorten shim life, and they leave different traces. Knowing which one is present decides whether a rebuild will last.
Fatigue at the Deflection Zone
Each stroke bends the shim group and then releases it. Over millions of cycles the repeated deflection can start a crack at the point where the shim leaves its seat or its clamp, and a cracked shim changes the opening behaviour of the group rather than failing outright. Fatigue is the mechanism most sensitive to the deflection amplitude, which is why a damper that works near the end of its travel wears its shims faster than one used on smooth roads.
Permanent Deformation and Creep
A shim that has been pressed against its seat for a long period can take a slight set, so the group no longer returns to a flat rest position. The result is a low speed force that has drifted from the approved value, often without any visible damage. Very thin shims at the lower end of the 0.1 mm to 1.0 mm thickness range are the most sensitive to this behaviour.
Contamination and Oil Condition
Shims do not wear in isolation. Water, metal particles and degraded oil all reach the valve area if the rod seal or the dust cover has failed, and particles trapped under a shim seat tilt the group and change its rest position. When contamination is the cause, replacing the shims without addressing the seal simply repeats the failure.
How an Aged Stack Shows on the Vehicle
Shim ageing rarely announces itself in a single dramatic way. It shows as a slow drift in behaviour, which is why a written baseline is worth having.
| Reported behaviour | What it usually indicates |
|---|---|
| Body floats or oscillates more over small bumps | Low speed force reduced, often preload or shim set |
| Harshness over sharp edges | High speed force increased, group stiffer than intended |
| One corner of the vehicle behaves differently | Uneven wear between two dampers, or an assembly difference |
| Damping fades after a long run | Oil condition or gas pressure, not necessarily the shims |
| Noise from the damper at low amplitude | Play in the retaining device or a deformed shim |
| Sudden change after a rebuild | Assembly error rather than wear |
Because several causes produce similar symptoms, the practical rule is to measure before deciding. A force curve comparison between two dampers of the same axle is the quickest way to see whether the difference is real, and it separates a worn stack from a difference in oil or gas charge.
Rebuild or Replace the Shim Set
When a damper is opened for service, the shims are the cheapest part in the assembly and the most influential on the result. The decision table below reflects that trade-off.
| Condition found | Decision | Reason |
|---|---|---|
| Shims flat, clean, no visible crease | Reuse after cleaning and measuring | No mechanism has shortened the life |
| Slight set but the curve is within target | Reuse with a re-measured stack height | The rest position must be re-established, not assumed |
| Crease, bent edge or bore damage on one shim | Replace the affected shim | A local deformation changes the opening sequence |
| Crack at the seat contact line | Replace the group | A cracked shim can release material into the valve |
| Wear marks or fretting under the seat | Replace the group and inspect the seat | The seat may also need attention |
| Contamination present | Replace the group, the seal and the oil | Otherwise the new stack is contaminated immediately |
Bench Verification Before Refitting
Where a force measurement rig is available, verify the assembly before it goes back on the vehicle. Compare the compression and rebound curves against the target, and then compare the two dampers of the same axle as a pair.
Pair Matching
A pair that differs noticeably will be felt as an asymmetric ride, and it is far cheaper to correct on the bench than after fitting. Note the measured curve with the rebuild record so a later comparison is possible. Where the axle pair uses a matched stack, build both dampers in the same session from the same shim batch, which removes the batch variable from the comparison.
When No Rig Is Available
Without a rig, use a reference damper of known behaviour and a hand comparison of the resistance through the full stroke, and treat the result as a screening check rather than a measurement. Keep one damper per common reference set aside as the reference unit, refresh it when a new stack is approved, and never fit a damper that feels noticeably different from the reference without an explanation.
Planning Intervals Across a Fleet
Shim condition follows damper condition, so the planning unit is really the damper rather than the shim. Band the vehicles by duty and set the damper check interval from experience with that duty.
| Duty band | Typical profile | Suggested damper check |
|---|---|---|
| Urban delivery | Frequent small inputs, many stops, heavy steering activity | Every 40,000 km or 12 months |
| General mixed | Mixed urban and highway, normal load | Every 60,000 km or 18 months |
| Heavy or towing | Near-limit load, rough surfaces | Every 40,000 km or 12 months, plus after impact damage |
| Salt belt or coastal | De-icing salt or sea air exposure | Every 40,000 km or 12 months, with a seal and cover check after each winter |
The check itself can be short. Look for oil weeping from the rod seal, damaged or hardened dust covers, a loose top mount and a visibly bent rod, then compare the effort through the stroke with the reference damper. A damper that passes those checks is a poor candidate for opening, while one that fails any of them should be removed and assessed on the bench where the stack can be measured.
Shim Stock and Rebuild Kits
A workshop that rebuilds dampers needs a small stock of shims organised so that a stack can be rebuilt without waiting. Group the stock by thickness step and diameter, keep a build sheet with each kit, and hold the sizes used by the most common references in greater depth than the occasional ones. Storage conditions matter as well: keep shims flat, dry and in rigid trays, because a bent shim found in a loose bag is unusable and the cost of the loss is far higher than the cost of proper packing.
For a fleet that outsources rebuild work, agree the shim specification with the rebuilder rather than leaving it open. If the rebuilder selects shims by availability, the resulting damping may be valid but different from the approved value, and the difference is then attributed to the damper rather than to the stack. Writing the stack sheet into the service agreement removes that ambiguity.
Warranty Evidence and Claim Discipline
The warranty covers one year or 100,000 km, whichever comes first, for material and manufacturing defects. Wear and fatigue found at a scheduled rebuild are not the same as a material defect, so a claim needs evidence that separates the two. Keep the rebuild record with the shim specification, the measured stack height and float, the oil used and the measured force curve, and keep the batch marking from the shim packing.
When a shim group fails earlier than the expected interval, the record answers the first question a supplier will ask, which is whether the stack was assembled to the approved values. A claim supported by measurements and a build sheet is straightforward to settle, while one supported by a description of feel is not.
Keeping Records That Make the Next Decision Easier
The final step in a good rebuild is the easiest to skip. Record the vehicle, the damper reference, the odometer, the stack specification, the measured values and the force curve result in one place, and keep it with the vehicle history rather than in the workshop diary. Two rebuild records for the same reference make the interval decision straightforward, because they show whether the stack reached the workshop at the planned distance or well before it. Over a year, that record set is what turns a general interval into a schedule fitted to your own operation.
Frequently Asked Questions About Shim Service Life
Does every shim need replacing at a damper rebuild?
No. Shims that are flat, clean and free of creases or cracks can be reused after cleaning and measuring, provided the stack height is re-established. A shim with a crease, a bent edge, a damaged bore or a crack at the seat contact line must be replaced.
What is the expected service life of a valve shim?
Bonpora lists typically not less than 100,000 km or 5 years under normal operating conditions. The figure assumes a healthy damper with clean oil and an intact seal, so a damper that has run with a failed seal is likely to reach the limit earlier.
Can a rebuilt damper be verified without a force measurement rig?
Only as a screening check. Use a reference damper of known behaviour and compare resistance through the full stroke, then record the result. Where possible, have the finished pair measured on a rig before fitting, because a pair that differs will be felt as an asymmetric ride.
How should shims be stored in a service workshop?
Flat, dry and in rigid trays or between boards, sorted by thickness and diameter with the size marked on the outside. Loose bag packing allows edge damage, and a bent shim cannot be straightened back to a usable condition.
Plan the Rebuild With the Right Data
Start from the steel shim range to check the thicknesses and diameters held in production, and read the steel shim specification outline for the base values to compare with your measurements. The failure analysis guide separates fatigue and deformation from a tuning problem, and the stack assembly guide covers the assembly checks that keep a rebuild repeatable. For workshop stock planning, the wholesale sourcing guide explains how the size range and price breaks work, and the manufacturing process guide shows why flatness and burr control decide how a stack behaves after a rebuild. The dampers that carry the stack are listed under shock absorber parts, and rebuild quantities can be discussed through the contact page.