Low oil pressure after shim

[Rawze]

* Clogged oil filter.

* bad oil pump.

* Spun bearing in head somewhere.

* Cracked/loose piston cooling nozzle.

* Bad o-ring in pick-up tube

* Oil pump mounting shimms not installed correctly.

* Bad or incorrectly installed bypass valve.

just some guesses. Have you pulled off the engine valve cover and cranked it up to see if oil is gushing around any cam bearings or valves anywhere?

* Maybe re-torque the piston cooling nozzles and ensure one of them is not loose/cracked?.

* inspect for a spun crank bearing in the bottom end since they have been replaced?.