Quote:
Originally Posted by regular guy
Some will say (and have already said) that the 702/703 bearings changed dimensions because of the materials change. I've got to be honest, that sounds very compelling on the surface but there's one thing about it that really bugs me. Nobody has explained why the harder material would require extra clearance when 1) the bearing is never supposed to touch the journal, and 2) it would seem that it has less friction than the older lead/copper design. So to me, the dimensional changes of the new bearing weren't based on a simple materials change, but were much more deliberate.
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A softer lead faced bearing can run a tighter clearance because they are less sensitive to tolerance, misalignment and other issues as should there be any metal to metal contact the softer outer lead layer is sacrificed not the crank journal surface (as would be if using a harder bearing surface). True, in principle, the bearing is not supposed to touch the journal but it is well known (even to BMW) that in real life it can.