Most OEM and aftermarket race cylinder blocks are configured for priority main oiling to protect the main bearings at high RPM. This oiling strategy gives priority to directly oiling the main bearings first. Pressurized oil within the main oil gallery follows a direct path to each main bearing first as seen in this photo courtesy of Dart Machinery. Note the direct oiling passage from the main gallery (B) to the upper main bearing housing bore. Vertical passages from the upper main bearing then oil the camshaft and valve train as shown and separate galleries (A) oil the lifters. In earlier OEM applications and many older race blocks the main gallery fed the cam bearings first and then the mains, often resulting in insufficient oil volume to support the crank in the mains.
Restricting upper engine oiling in earlier blocks provided some relief, permitting carefully built engines to perform well, but main bearing oil starvation was always lurking and designers sought a permanent solution. Stock oiling systems are fully adequate for stock applications and many high performance engines, but race engines required more. Hence priority main oiling, a simple solution that has largely put an end to main bearing failures due to oil starvation at elevated engine speeds.