When the power level gets up to where they start to experience this problem, the block actually splits apart in a lengthwise fashion, between the cylinders where the block has the thinnest cross section. A girdle doesn't stiffen the block in that direction. Rob Harrison had even tried to run all thread through the block between each cylinder in a side to side pattern to try to add strength to keep his blocks from splitting but it didn't work. Once he started running a billet block that solved the problem, along with other design changes made to the block to help combat this problem.
