Going back to Allen's question (two postings back), it has not been a smooth ride for eight cylinder engines in the locomotive size range. EMD had a number of problems with their eight, not so much in locomotives as in power generating service where the engines have to run at synchronous speed (generally 720 RPM or 900 RPM depending on the type of generator) continually. EMD uses very large camshaft counterweights rather than balance shafts to provide the necessary balance solution for their engine, and many of the earlier eight cylinder crankcases had problems with cracks developing in the end sheets around the counterweights. This peaked in the early 645 production when most of the generating applications went to 900 RPM (567's had generally been run at 720). There were a series of modifications issued to increase the bolting and doweling of the camshaft bearing supports, and finally they adopted a heavy crankcase design with much thicker end sheets.
I can't speak much for GE's experience with their eight cylinders, there were not enough of them out there that I encountered. However there is one very significant difference between the EMD and GE design practices and the ALCO 251 arrangement. In EMD and GE engines the cylinders are directly across the engine from each other and apply the connecting rod load at the center of the crankshaft journal, in the ALCO 251 the cylinders are offset bank to bank, so that the rods from opposing cylinders seat on the journal next to each other. This imposes a rocking force couple to the loading on the crank throw in addition to the usual reversal of force you get on the crank journal of a four stroke cycle engine due to the intake stroke. It adds one more complex element to the design process.
I apologize again to those who want a "pure" forum, for talking about non locomotive applications. But as I have pointed out before, if you only look at the locomotive side of the business you only get part of the story.