What was the deciding factor for the 1.5mm hole? I can see if there is a restrictor in the venturi end to keep oil from gushing into the venturi that would make sense but why restrict it going all the way up to the catch tank?
The sheer amount of oily air that blasts in and out of the catch tank if you don't have some restriction in the line covering everything in oil is impressive. To block the vent completely would likely cause big pressure swings in the crankcase which might/would blow out an assortment of seals. Putting a couple of restrictions in the line still allows decent venting but breaks the vent down into chambers to try and knock more of the oil out of the vented gasses before they get to the atmospheric vent filter on the top of the tank. The 1.5mm drilling was to more or less match the restrictor that S&S have in the manifold fitting.
