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bazhart said:Yes a plain bearing (as fitted to earlier 911 variants up to M96 engines) has a pressurised oil feed which all plain bearings need but the ball bearing in the M96/97 IMS can be exposed to plenty of splash oil if the seal is removed.
Oil does seep in if the seal is still there - it can hardly be called an oil seal - OK for dust and thick grease perhaps - but someone seems to have forgotten that the engine temperatures inside would turn the grease thin like oil anyway and eventually seep out.
The shaft is hollow and eventually can fill up with very old smelly and probably carcinogenic oil through the leak in the inner seal.
The inclusion of the seal could be connected to manufacturing storage time as a grease filled sealed bearing has longer shelf life than an open unsealed one? Don't forget the new boss brought in to transform Porsche financially had a washing machine manufacturing background!
All they had to do was pull the outer seal off during assembly! leave the existing grease there and allow it to wash out during running in to be replaced with fresh oil.
The smaller bearings have too thin an outer track to prevent the interference fit from altering the pinch between different shafts, spindles and bearings when the tolerances stack up for a tight fit. The larger bearing has a much stronger thicker outer casing and of course does not require a tighter fit - so sustains the designed clearances better.
On diss- assembly some housings and outer bearing diameters in the smaller bearing show signs of "pick up" during assembly or running and pushing another new bearing in without measuring the housing and correcting any errors can lead to further problems down the line. Bearings have different fits (or tightnesses) in their design and manufacture and we chose a replacement bearing with more clearance to compensate. We may replace a larger bearing on a high mileage engine during reassembly and have not seen "pick-up" there.
Feeding oil from a pressurised feed will not deliver it into the bearing under pressure - just flow - and if it is high flow it would starve/reduce the engine oil delivery on hot tickover when the pressure is low anyway and well below the relief valve setting - so there is nothing you can do about it. Whereas as soon as an engine starts there is a massive amount of oil splashed into the bearing area - after all - if you want some proof - just consider that a high mileage car with the seal still in place and the bearing still OK has managed to lubricate sufficiently after all the grease has gone and just relying on the tiny amount of oil that is managing to splash into it past the slightly worn seals - not being replenished and therefore running hotter than the main oil supply! What more evidence do you need that splash oil without the seal restricting it - is adequate?
Baz