If you're keeping the same gears, cover the pinion with a rag when drilling the air hole. Since we went to deeper cogs, we pressed new bearings on the new pinion with the stock shims, hoping the pinion depth would be right. New pinion races were tapped into the housing, and the pinion was installed. If the depth is right on when checked with the pinion depth gauge, install the crush collar, yoke, and seal to adjust bearing preload. This is a good reason to have a shop set up the gears if you don't have the tools or experience in this department.