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Inside a BMW front strut assembly the bump stop is the component that absorbs the final portion of suspension travel. When the coil spring reaches full compression, the stop deflects progressively and keeps the damper piston, the top mount and the body structure out of a hard, uncontrolled impact. The part is small compared with the struts it protects and is often overlooked during routine service, yet a worn stop announces itself through harsh bottoming and knocking sounds from the front axle long before damage spreads to neighbouring components. Buyers sourcing a BMW strut bump stop 31336771352 typically want three assurances from a supplier: exact dimensions that match the original part, a compound that holds its properties through years of compression cycles, and a factory able to repeat the same specification across batches.
Trade buyers approach this category from several directions. Some search by specification, for example a polyurethane bump stop with a stated bore and free length. Others search by supplier intent, such as a BMW bump stop manufacturer or wholesale source in China. Many go straight to the OE reference and type 31336771352 into a search engine or a parts catalogue. Still others start from the vehicle itself, looking for a front axle bump stop for a particular BMW model, or from a custom moulding requirement for their own brand. The sections below follow those angles in order, from numbering and function through dimensions, material, quality control and commercial terms.
BMW assigns structured eleven digit references across its parts catalogue, and 31336771352 follows the standard pattern. The leading 31 group identifies the front axle division, which places this stop within front suspension hardware. The following blocks narrow the assembly within that division, and the closing digits define the individual part and its revision. Because every digit block carries meaning, two bump stops that look almost identical on a shelf can belong to different axles or different model generations, so the full reference rather than visual resemblance should drive the purchase decision. Confirm 31336771352 against a catalogue entry or against the part removed from the vehicle before a bulk order is released.
Bump stops in the 31 series are inspected whenever front struts are serviced, and distributors covering the BMW aftermarket stock them alongside struts, top mounts and dust kits. Application data is confirmed with every order so that buyers avoid carrying obsolete references in inventory.
During normal driving the coil spring and damper carry the load and the bump stop waits untouched. As the suspension approaches full compression, over a pothole, a speed bump or a heavily loaded corner, the stop begins to deflect and resistance builds gradually. That progressive cushioning absorbs energy that would otherwise pass straight into the damper piston, the top mount and the body structure. A tired or missing stop shows up as harsh bottoming, knocking from one corner, and accelerated wear on shock absorbers and mounts that were never designed to absorb full stroke events alone.
BMW front axles carry the engine weight and take much of the braking load, so compression events at the front suspension are frequent and energetic. The stop is part of the chassis tuning: compound hardness and free length are selected so the transition into the stop is progressive, which is why substituting a stop with a different length or hardness changes how the front axle settles over sharp inputs. Keeping the specification close to the original preserves the ride character the platform was tuned for.
The table below lists the standard specification of this replacement part as supplied from the factory programme.
| Item | Specification |
|---|---|
| Product name | Strut Bump Stop |
| OE reference | 31336771352 |
| Bonpora number | 4110123 |
| Fitment | BMW (confirm by OE number) |
| Material | Polyurethane |
| Size | Φ20 x Φ66.5 x 80 mm |
| Country of origin | China |
| Warranty | 1 Year |
| Delivery time | 15-30 Days |
| Packaging | Neutral export packaging |
| Custom service | OEM and custom design support |
The three figures in the size line each control a different part of the fit. The 20 mm bore is the primary check because the stop slides onto the damper rod: a loose bore lets the stop wander and wear unevenly, while a bore that is too tight cannot be seated fully. The 66.5 mm outer diameter defines the contact area where the spring seat and dust cover close onto the stop. The 80 mm free length is compact, which indicates an application where the stop works through a short, controlled compression stroke, and it is the dimension most sensitive to substitution: a longer stop engages too early and firms the ride, while a shorter one engages too late and lets the chassis land hard. Measure the removed part at rest with a caliper and compare all three values before ordering. Photographs of the original part, including any step, flange or taper, allow the factory to confirm the match quickly.
Polyurethane is the standard compound for this replacement because it holds together two properties that conventional rubber struggles to combine: high energy absorption and resistance to permanent set. A stop that takes a permanent squeeze after repeated hard compressions loses free length and engages later in the stroke, which quietly defeats its purpose. Polyurethane also tolerates ozone, road salt, oil mist and wide temperature swings better than rubber compounds, which matters for vehicles operated in coastal regions, winter salt belts and hot climates. The dedicated polyurethane bump stop range lists further references moulded from the same compound family.
| Property | Polyurethane | Conventional rubber |
|---|---|---|
| Resistance to permanent set | High, keeps free length | Lower, can take a set |
| Ozone and storage ageing resistance | Strong | Moderate |
| Consistency between batches | Controlled by formulation | Varies with compound |
| Typical service life in this role | Longer | Shorter |
| Hardness tuning range | Wide | Limited |
The aim is not to make the suspension firmer. A correctly specified polyurethane stop feels much the same as a healthy original part in normal driving and differs only at the end of the travel, where it cushions the final movement instead of letting the chassis land on metal.
Anything that multiplies compression events shortens the life of a bump stop. Vehicles that regularly carry passengers and cargo, taxi and ride hailing fleets, models used for towing, and cars driven on rough or unpaved roads compress the front suspension far more often than average use. Road condition matters as much as load: repeated hard crossings of potholes and speed cushions load the stop harder than gentle inputs at the same speed. Inspection intervals should be shortened for these duty cycles, and stops are best replaced whenever front struts are rebuilt or springs are renewed, because the additional labour is minimal once the assembly is apart. Since the stop shares its working space with the damper, worn stops and tired shock absorber parts are frequently found during the same inspection, and replacing both together restores the original damping envelope in one visit.
The earliest signs are a knock or thud from one front corner over sharp bumps, followed by a firmer, harsher ride as the stop loses height or begins to crumble. Because the same symptoms can also point to a worn damper or a broken spring seat, confirm the condition of the stop with a physical inspection rather than replacing parts on guesswork.
The parts are moulded from polyurethane formulations selected for the load range of the application, then finished and checked before packing. Dimensional checks confirm bore, outer diameter and free length against the specification. Hardness checks confirm the compound sits within its target band. Sample parts from each batch are compression tested to verify recovery after repeated full stroke events, and visual inspection removes parts with flow marks, splits or flash that could seed a tear in service.
Standard references such as 31336771352 are supplied from existing tooling, which keeps unit cost and lead time low. Where an OE reference is unavailable or the original part is discontinued, the factory supports OEM and custom work through its custom parts service: the original part or a sample is measured, a drawing is confirmed, and a new mould is produced. Custom projects can adjust compound hardness, dimensions, colour and marking to the requirement of the buyer, subject to minimum order quantities agreed per project. Distributors building their own branded ranges for the BMW and wider European aftermarket use this route regularly.
Orders are packed in neutral export packaging, which allows distributors to place the parts under their own brand without repacking. Standard delivery time is 15 to 30 days depending on order volume and the production schedule. The parts carry a 1 year warranty against material and manufacturing defects, and batch records are retained so that any field claim can be traced back to its production batch. Mixed containers and consolidated shipments with other suspension components are routine and help buyers fill a container economically.
The reference covers one stop. Front struts are normally serviced in pairs so that both corners of an axle behave the same way, and bulk orders are usually placed in even quantities. Carton quantities can be aligned with warehouse preferences before shipment so the count matches the receiving side.
A correctly specified stop is not felt in normal driving because it only works in the last portion of the compression stroke. It engages progressively and cushions the final movement. Differences in ride character usually trace back to dampers, springs or tyres rather than to the stop itself.
Match the full reference against a parts catalogue entry for the vehicle, or compare the removed part with the three dimensions listed in the table above. When in doubt, send the OE number together with photographs to the factory for cross checking before a bulk order is released.
The OE reference, the required quantity, the destination port and any marking or packaging preference. A photograph of the original part helps when the reference is uncertain. The factory replies with availability, unit price at the stated quantity and the earliest production slot.
Buyers looking for a BMW strut bump stop under reference 31336771352, or planning a wider programme of polyurethane and rubber suspension components across brands, can contact the factory with the reference, quantity and destination. The sales team confirms availability, quotation and production time, and can arrange samples for fitment verification before a bulk order is placed.
