Forum bmwbike.com

Are there any specific engine noises that indicate the early stages of crankshaft roller bearing big end or small end bearing failure

I don't know about the small end - that's a bushing. If it is not put together right, or has too much play, it may knock, but not fail.

The big end begins to make a knocking as the bearing and rod pin wear and their clearance opens up. If the clearance opens up enough, the piston can begin to hit the web of the crankshaft, which begins to make another knock that can really grow loud. All this will happen before the big end bearing "fails" by seizing or breaking. In other words, I think your ears give up before the engine does.

Hi Ian, Yes indeed, I have ridden several old BM's with terminal big end knock. The engine might sound fine at tick-over, but as soon as it's put under load like gentle accelleration, a distinctive knock will occur. In fact regard ANY engine knock with the greatest suspicion!
The best way to monitor and engine for internal condition is to carefully examine the engine oil at every change which, in the case of a BMW roller-bearing engine, should be at 1000 to 1200 mile intervals. The oil should be tarnished only. Traces of metal swarf from whatever source will collect in the rocker covers, therefore is easily spotted. Investigate any such metallic particles IMMEDIATELY and avoid running the engine because they WON'T go away!
Main bearings will rumble at idle. If there is any play, which of course there shouldn't be, it can be detected by removing the front engine cover and watching for movement at the crank nose as you operate the clutch lever, i.e. the clucth mechanism will push the revolving crank forward.
In both cases there are no quick fixes, the engine will have to be dismantled and thoroughly investigated.
Quiet = good, noisy = bad.
Gerry