Anyway...

Been a busy week and a long day.

When the Northstar was being designed, GM found that holes in the main bearing bulkheads to allow each bay to exchange air with its neighbors were practically a necessity. Because of the Northstar's lower crank case design, each main bearing bulkhead comes down almost to, if not touching the surface of the oil, essentially sealing off each bay from its neighbors, dramatically increasing pumping losses. In testing otherwise identical prototype engines, it was found that engines without windows gave up something like 30+ HP to engines with windows.

The conventional Northstar uses a die cast, open deck dry liner block. Because it's die cast, the holes can't be cast in place. GM machines them with a boring bar from the front of the engine. They are in approximately the location the cam would occupy, were the Northstar a pushrod engine. An expension plug is inserted in the back of the hole in the forward bulkhead (separates the 1-2 bay from the timing drive). The shaft for the intermediate timing sprocket covers the forward side of the hole, which is then used as a small plenum for oil from the front main bearing on its way to the cylinder heads and secondary timing chain tensioners.
Since the holes are machined with no subsequent operations, they have sharp edges.

The supercharged Northstar block is a sand cast closed deck dry liner design with thicker liners (smaller bore) than the die cast blocks. Because the block is sand cast the windows can be cast in place, and since they can be cast in place, they can be cast with radiused edges. In development testing, the rounded edges were found to be worth about 5 horsepower.

Since it would only take a couple of hours and pretty much no money, I went ahead and did this to my block.

This is what the windows looked like stock--sharp enough to cut yourself on:

This is what I could do with a die grinder:

A machinist frined suggested that I take a long strip of sandpaper and see-saww it back and forth through the window. I tried that and it worked much better than the die grinder. This is the finished product:

It may not look like much in the picture, but that's a bad angle. If you could feel the difference between the two, you'd be amazed.

There are other sets of bay-to-bay breathing windows on either side of the main bearing bores. They are 1/2" wide and 1" tall and straddle the block split. They would be great candidates for porting with a relatively simple CNC mill program, but since I didn't have that and did have a die grinder, I used the die grinder.

Before:

After:

Alan Johnson has told me that his engines experience oil drain back problems when they spend a lot of time "up at high C", as he puts it, and tipped at odd angles because a sand car can't keep its front wheels on the ground. The oil drain back passages in the side of the block are cast in place for most of their length, but are finished using a mill. This leaves sharp corners and such in the flow path. I used the same technique to smooth them as I used for the main b2b windows:

The oil drain back holes also go through the lower crank case, oil manifold, and are cast into the sides of the oil pan. They ultimately empty out below the surface of the oil in the pan. The holes in the lower crank case are cast most of the way and finished by a mill as are the holes in the block. However, the diameter is not consistent between the milled and cast portion of the holes, and the location is frequently off. There is room in the lower crank case to enlarge these holes to a consistent diameter via milling or drilling on a press.

The oil manifold is the next step. For those of you unfamiliar with the Northstar's oil system, the oil galleries that feed the mains are cast into the underside of the lower crank case as troughs. They are turned into passages by the oil manifold, which is similar to the valve body in an automatic transmission... it is a flat aluminum plate with holes in it at the right locations, and a linear seal inlaid into it to seal the troughs in the bottom of the lower crank case. From these troughs, the oil goes around the shanks of the main bolts on the left side of the engine and into the main bearings. The troughs would be another interesting CNC porting project.
The oil mainifold could in theory be drilled out to a larger size in the oil drain back holes, however this comes very close to the groove in which the aforementioned seal resides.
In a max effort engine, I would have a new piece of aluminum laser cut with the appropriate holes, and then have a groove for o-ring stock milled in to it, then have the plate milled or ground flat.

I have not looked at the drain back passages in the oil pan yet.

Also in a max effort engine, I might see if I could install pressure regulators in the cylinder heads in the passage where each head gets oil from the block. This would help reduce one of the causes of oil drain back problems... excessive top end oiling.

This is what the workbench looked like when I was done:

Need to balance the crank and one other big task before I have everything in line to reassemble.

I'm curious as to why you are doing it that way instead of drilling. I would think drilling would be easier to balance. I would think grinding the OD would end changing the shape and result in worse balance. I might be wrong though. But if I were to touch the OD, I think I would turn it rather than grind it just to be a bit smoother.

Ummm, in simple terms that means you want to remove more weight from the outside so you have less rotating mass, or something to that effect? Lowering rotating mass=higher revs? If I'm correct is my thinking, I now understand why you are doing it that way....