Case oil mods - what are they?
 

I’ve seen some references to case modifications for better oiling for race builds but I’m not clear what they are.

Assuming one has a good oil pump, a cross drilled crank and did the oil bypass modification, what other specific case modifications can be done to improve flow of oil where it’s needed?

I read that some builders have modified case by enlarging passage way to middle bearing? Is that a common modification that helps? When should that be done and what else can be done? Are there specific machine shops that know how to do this?

Yes the #4 main bearing oil delivery passage in the case is enlarged to allow more oil to this main bearing. Reason being is because it is intended to benefit the cross drilled crankshaft. The crank is cross drilled to feed #2 and #5 rod bearing journals because these two are in the center of the crank, thus fed oil last in factory configuration. They're fed last because the case delivers oil to the crank from the ends- #1 main and #8 main bearing locations

Of course you also need to open the oiling hole in your #4 main bearing to see the benefit of drilling the case passage larger.