the casting is not purfect block to block. They may run them all day long....but that information came from Mike Smith...a man that has built race motors FOR Nissan and of course others. They have to last a while bro...and when you have thin cyln. walls what happens? HEAT! And things that get too hot are Bad as im sure you know. For instance...my block that i am using is good to 90mm....the block i originally had statr machine work on was only good to 88. I ended up with the 90mm block by good fortune...but they are not all the same.