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Maintaining an M – Bearings: It’s a Hydrodynamic Principal

Posted in Maintaining an M Car, Maintenance

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The crankshaft is never in direct contact with the bearing, there is always a film of oil in between. Therefore it’s not about finding a better quality bearing. Besides, there is no “better quality bearing”, the black calico ones are the OEM bearing coated with Calico’s CT1 dry film lubricant, claiming better friction characteristics but decreasing bearing clearance. We do not recommend a smaller clearance.

Engine builders and keyboard warriors argue that it’s the bearing clearance that is the cause, some say too small and others say too large. All naturally aspirated M cars are prone to bearing failure. We are of the opinion that the clearance is too small. At the same time, we also feel that a leading cause is the long service interval as well as the fact that owners drive their cars hard without knowing the condition or level of the oil in the sump. We recommend an oil change every 10000 KMs or annually, with the correct grade of oil. We recommend doing a wear check analysis when doing a service. If there is metal contamination in the oil, it could be too late but it is a good indicator of the condition of the bearings. Therefore we also recommend swapping out the big end bearing shells as a preventative measure. We have seen cars starting to run bearings at 50000 KMs and others at 140000 KMs, many factors at play. Early cars did have tolerance issues on clearance. There were cases of cars picking up bearing knocks at 20000 KMs.

If you have just bought a used M car, a bearing swap is a must. As a rule of thumb, we suggest a bearing swap every 80000 to 100000 KMs on all naturally aspirated M cars. On race cars, we do an oil change after every 2nd, 3rd or 4th event, depending as well as physically check the bearings annually.

In most cases when the S85 and S65 engines run bearings a very expensive new crankshaft is required. About R52000+vat for the M3. When we rebuild these engines, we micro-polish the journals, a bit more clearance but still within tolerance. More and more of these cars are running bearings and breaking engines now that they are out of warranty.

The trend is always the same with each generation. 15 to 20 years ago we used to do a lot of bearing checks, preventative replacement and engine repair due to bearing failure on the E36 M3. These cars are in collections and do very low mileage these days, so bearing jobs are much less common on the older cars.

We have seen many cars where the owner did not notice the bearing knock, leading to catastrophic failure where a con-rod and piston smashes through the cylinder block, totally destroying the engine.