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

 

While you were replacing the oil pump, did you take a look at the rod and main bearings? Maybe take a Plasti-gauge reading?

 

Mod please remove this post.

 

Welcome to the site.

This is actually an engine only topic not a 73-79 truck topic.

FTE is huge and has engine forums for engine related topics.

Maybe a Moderator will move this to the 385 engine series forum for you?

Also, may I direct your attention to post #1 of this thread.

https://www.ford-trucks.com/forums/s...d.php?t=102632


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.