I have to agree with Bruce on some of his statements. As someone who works with engines for a living I can tell you that during a rebuild or after a block or crankshaft comes back from machining you measure, measure, measure and then measure again.
By simply taking parts off of the shelf and putting them together without measuring, many times can result in failure since the tolerances are not within stated dimensions. I don't know if you guys remember the infamous "GM Piston Slap" fiasco a few years back. That issue started when they stopped the hand fitting of pistons in their blocks and relied on the tolerances of the pistons and bores being within stated dimensions.
I do agree that modern automated machining can make extremely tight tolerances and is very good, but, if you get out your feeler gauges, vernier, micrometer, or bore gauges you can actually show the areas where the measurements are barely within tolerance. A good engine has all of its measurements evenly within tolerance and not all over the place with some on the high side and some on the low side of spec.
I can guarantee you that there is some human involvement when building any good high performance engine. Even if each and every engine is not hand assembled, some of the parts or even a complete engine will be pulled from the assembly line every so often and measured to verify that all of the dimensions are indeed within spec and as a form of QC.
One final thing, even the S65 which BMW does not claim is hand assembled has three sizes for main bearings. If you look at parts list the bearings have colours assigned to them. This is due to the main bearing bore variance and the need to keep the oil clearance proper. Now I don't know if they hand measure and then select the bearings or if they have a machine measure but either way this shows that even high tech machining equipment can have some variance that has to be made up for with correctly sized bearing shells.
As for pistons, the S65 only shows one piston size. Since Mahle makes the S65's pistons and they are known as one of the best piston manufacturers in the world, they probably verify dimensions before shipping them, and BMW may very well check all of the bores of the blocks after machining and only build engines out of the ones that meet their specs.
The moral of this post is that having a skilled craftsman measuring and building you an engine is definitely worth it, as good as machines are, many times we have had to send back brand new parts that were supposed to be within spec but were indeed not even close. If you work with this stuff all the time you realize that machines, while as amazing as they are, do not always build the perfect parts. Finally, Even with automated assembly there is always a programmer or machine operator that has to be added into the equation too!
P.S.: Swamp, their is indeed evidence that hand fitting of pistons does offer benefit and it is not merely anecdotal, it is a fact and has been proven. While it may not always be measurable in terms of power, it will definitely be measurable in longevity. Excessive piston to bore clearance will result in piston slap and piston to bore clearance that is too tight will result in scuffing and excessive wear. Over time piston clearance that is too tight or too loose will make the bores wear out of round which can eventually lead to oil burning and/or loss of power. Basically if you want an engine to run quiet, make power and last, you need to ensure all of your measurements are nicely within spec. ( I know you know this but I thought I would add my $0.02)
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Last edited by BMRLVR; 08-24-2012 at 10:11 AM.