tolrance's,gaps, parts and combinations of parts and ofcourse the machine shop are all a big factor to the failure of built motors. Stock motors have been pushed harder then built motors with no problems at all, Zo had a stock short block and 620rwhp and he was at the strip couple times a month runing mid 10s, Dean also has a stock short block with close to 300 runs on it and he runs mid to low 10s with no problems, just stating that the stock bottom end is strong and ppl beat on them, but then again they have the best tuner tune their cars.