Viscous drag probably doesn't matter for such short passages, what is important is the pressure lost in accelerating the steam from rest into the ports, then round a 90 (nearly) degree sharp bend, then accelerating again into the drilled holes which have smaller area than the fully open port. How important any of this is depends on the velocity of the steam, which again depends on the piston speed of the engine, which tends to be low at least for model locos. Opening out the holes into rectangular passages would be better, but might not be noticeable, if the pressure drop is already low, reducing it doesn't help much. It can all be worked out, but a bit heavy going for this forum. I keep threatening to do an article for ME, but more interesting things keep on cropping up,like watching paint dry

If you convert say 3 off round holes to one off slot with rounded ends, the area reduces by 24%. Wheher this matters depends on whether the passageway area is significant compared to the total area including piston, cylinder head and a bit of the bore. Steam is going faster in the passageways, so heat transfer will be higher. If using superheated steam, this effect will be much reduced.

A key issue is how the passage area compares to the diameter of the steam and exhaust ports. Although passages that are too small lead to 'wire drawing' passages that are too big increase the amount of 'dead' space added to the clearance above the piston, so there's a sweet spot between too narrow and too large.

Neil.