We've mentioned how this boss makes a measurable difference in the rear main bearing clearance in past buildups, and Nelson wholeheartedly agrees. "If you don't believe it, measure it yourself. It makes a difference, and the main bearing clearances are too critical not to take this into consideration. If your machinist doesn't do this, find another machinist." He chose .0028 inch for a final clearance dimension; no more, no less.