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I have a 77 cs 460 motor I have replaced oil pump ywice oil sending unit and the heads pistons looked fine after 15 min warm up loses pressure and oil pump knocks
Regarding the actual topic of Oil pressure (not in title of thread)...Your cam & crank bearings are worn out.
As the engine increases in temp the automatic action is for metal to expand.
This expansion (of the bearings) causes oil pressure to drop because there is more space between the bearings and the journals.