We have several turbo engines that make 1700 H.P. all the time with a Dart aluminum block and have never had main problems,Dart and Brodix have the strongest mains in the business.Our Sb2.2 circle track engines make 844 H.P. and run on the average 600 miles between rebuilds and the mains are never out of round,I suspect the guy is telling you this to sell his motor.Bill