Mains priority oiling modification

I have been lead to believe that it's possible to redirect oil flow in my block (304 holden) to convert it to mains priority oiling. I've seen videos of how to go about this, and supposedly it's a done thing.

What other aspects should be considered before I commit to making this modification?

Yes it is possible although this isn't a modification I've personally made. Basically this modification reduces the oil flow to the top end of the engine and the lifter bores. Once consideration is that you won't be able to use hydraulic lifters with a mains priority oiling system.

I'd consider this only if you're currently having problems with your bearing life, or alternatively you're building a serious competition engine. It's less of a concern for a mild street engine.

can i thread hi jack slightly, its common practice in RB engines and 3SGTE engine to run a restrictor to the head.

my question is where is best to read oil pressure after this modification? in the block (before restrictor) or in the head (post restrictor)?

My old old engine builder did this on a 4A-GE in order to resolve a breathing problem.

It didn't resolve the breathing problem, which wasn't a breathing problem anyway, and seriously shortened the valve train life.

Just be aware to the possible side effects.

And engine builders doing things that are "common practice" without really understanding WHY!

usually i would agree but from the 3rd revision on with the 3sgte toyota included a restrictor in the head gasket to restrict flow to the head.

so it can be done safely to the right engines.