i built and/or fitted about 20 cg engines over the years,n/a and turbo,and blew a lot of them
but i never had any oil pressure problems, there is probably something amiss, rather than a pump issue
another member had that same problem after a rebuild a few years ago
there are 2 woodruff keys on that end of the crank, the pump, one may be missing ?
or a pressure relief valve problem maybe ?