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BMW infamous for Conrod bearings?
BMW is infamous for engine bearing failure on their line of M3 (S65) and M5 (S85) engines. In 2019 BMW acknowledged this failure by publishing rod bearing clearance specifications. Engine builder experts have stated that BMW engines have been built with a bearing clearance under the recommendation from the performance industry. BMWs also have been shown to have an issue called “tolerance stacking” which can further reduce the bearing clearance to below acceptable dimensions causing premature conrod bearing failure (typically a spun bearing). When conrod bearing failure occurs, the vehicle typically needs a complete replacement engine. As a result, many S65/S85 BMW owners will change their rod bearings before they have an issue.
In this article we will concentrate on the S65 engine in the E9x (E90, E92, E93) M3 as they are the most common.
The Causes
In simple terms there are three factors that contribute to conrod bearing failures in the S65 engine: 1. A very narrow clearance specifications on conrod bearings from BMW 2. The specification of 10W60 oil which is extremely viscous at low temperatures 3. The factory programmed 1500rpm cold start procedure in the ECU that runs the engine at higher than idle speed at a time when the engine is most vulnerable to wear, and the factory specified oil has less chance of penetrating the narrow clearance between the conrod bearings and the crankshaft.
Lack of proper clearance limits oil lubrication between the crank journal and the rod bearings. This causes premature wearing of the rod bearings. Thicker oil exacerbates this. This rubbing also creates debris which can damage other engine parts. Because of this, we see BMW S65 and S85 engines wearing through their bearings early. In some cases when a tolerance stack was a factor, there have been engine failures on cars with very low mileage.
The usual method of detecting rod-bearing wear in most engines is to carry out an oil analysis which looks for the presence of rod bearing materials suspended in the oil.
However, it appears that during the production of the S65 and S85 engines BMW changed the metal materials in the conrod bearings to a harder mix of tin-aluminium. The commonly held belief is that BMW did this to make S65 conrod bearings last longer after many incidences of "early" conrod bearing wear (largely based on generally held expectations as there is no published spec for bearing life). The commonly used materials for conrod bearings are lead-copper, which as they wear, give a clear indicator of the degradation process when doing oil change analysis. The newer BMW tin-aluminium materials do not show up in analysis as easily (or at all). Another major drawback to the harder materials is that if oil starvation occurs and bearing-to-crankshaft contact occurs, the new bearings are more likely to damage the crankshaft resulting in a major re-build or replacement engine.
This leaves the only foolproof way of diagnosing rod bearing wear as physical examination of the bearings themselves. This process is so close to the cost of rod bearing replacement (as the bolts are single use and would need to be changed anyway) that a rod bearing change is recommended

A worn set of the Lead Copper Bearings

A worn set of the later Tin Aluminium Bearings

Our Solution
Bromspec Motor Works has carried out a significant number of conrod bearing replacements on E9x series M3 vehicles with the S65 engine. Whilst BMW and others offer replacement conrod bearings they duplicate those used in production of the vehicle and inevitably suffer the same issues eventually. We have chosen to use coated bearings from VAC together with rod bolts from ARP which are suitable for both street and race use.
VAC's S65 bearings suitable for both forced induction (typically supercharged) and N/A builds, their coated bearings are "friction resistant" making thermal breakdowns that can occur due to lack of lubrication a much less common occurrence. Helping to prevent premature engine failures on street cars, and spun bearings under stress at the track when things are most critical.
ARP conrod bolts are our choice fixing as they are race proven, stronger and are less complicated to use than the standard BMW torque to yield bolts (an involved 11 step procedure required).
The above combination is also the only one which has to our knowledge been post install tested at various stages by Harrop and found to have little or no wear.
We also specify fully Synthetic 5W30 oil as opposed to the original 10W60 to ensure the maximum chance for there to be oil in the bearings all the time.
As an option we can also have the 1500 rpm cold start tuned out with a flash tune

Two E92 M3 one of which recently had conrod bearings replaced by us.

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