Originally Posted by BMRLVR
I have no doubt that cost is always a factor, however, at a production rate of about 100 engines per day, there is time to have extra QC and be tighter on tolerances.
One thing I do have knowledge of is engine assembly, and I can safely tell you that I have seen many mass produced engines from automated assembly lines that have issues from either improper clearances, defective parts or incorrect assembly (gaskets or o-rings left out). I have no doubt that machines are able to make perfect engines provided that every bore, part, fastener, etc. are of perfect dimension and free of defects, however at extremely high volumes I don't think manufacturers are inspecting every part for quality or dimension, they are probably only inspecting every 100th or 1000th part. This less than 100% QC is the main reason for recalls, and manufacturers warranties, to deal with the inevitable problems that get past the machines.
I am still confident that an engine that is hand assembled by a craftsman who is verifying every measurement with a hands on nature is going to be built tighter compared to an engine that is built by a machine where maybe every 100th engine is pulled from the production and inspected. I may be wrong on this but this is my feeling. I also think that if BMW were hand assembling the S54 and S65 blocks that they wouldn't have had the main bearing issues that seem to be cropping up because they would have had the oil clearance adjusted to the proper specs instead of trusting in mass produced parts that may be off 0.0005" here and 0.00025" there causing issues down the road!
Mostly agree here. There can be both good and bad hand assembly and machine assembly. It actually comes down to both quality assurance and engineering - the actual design. Engineers are responsible for setting tolerances on each part. The balance is on cost vs. engineering function. Obviously some dimensions are way more critical than other, again here main bearing clearances and piston-bore clearance being tight whereas something like a bearing width or total camshaft length is not nearly as critical. Problems arising from these values not being proper to insure good and long engine function can be design, manufacturing or inspection. Certainly more critical dimensions receive more scrutiny in each domain. I'm not up on the details of the SPC (statistical process control) of engine block and internals machining. I am pretty confident though that on critical parts such as these, contributing to these kinds of high precision fit requirements, 100% in line inspection is occurring. Again machines can do this extremely fast and extremely inexpensively.
I've never argued that all machine built engines are superior. Only that given the choice for a high production volume engine, I would place my bets (and personal buying choice if I actually had one...) on the machines for the vast majority of the process. Even in the case of hand assembly, the vast majority of the part production and inspection is still likely done by machine (I guess that is obvious to many but still worth stating again). As discussed prior, we can eliminate the human from the line but for any reasonably high volume we can not eliminate the machines. Bringing it full circle faults due to machines are ultimately problems with the human team behind the machines themselves (again could be design, QC, inspection, maintenance, etc.).
It would indeed be fascinating to see a video like the one above of the 3er main assembly for engines, comparing something like the Honda Accord (very high volume) to something the the M156 (AMG Engine - mass produced but much smaller volumes).