Why did I spin a main bearing?

I have had main bearing problems in a 360 and a 340 block. They would both kill the center 3 bearings like some kind of balance problem. I rebuilt the 360 four times!! I was about to scrap the block as it was the only part that wasn't changed the previous four times. Long story short----The main oil feeds that intersects the lifter galley hadn't been drilled the right diameter all the way through---So the bearings would receive just enough oil to live--Until this engine exceeded 5000 rpm. I drilled them out to 9/32" and found that the bottom 4 inches wasn't drilled from the factory--which is why it lived for so long--never having enough cam or valve spring to rpm over 5000rpm. I found the same thing on the 340 block--actually worse.

Both engines are now living happily--The 360 with J heads making 430hp and the 340/418 W2 heads making over 500hp.

I drill them no matter the application. J.Rob