I have a 401 with about 500 miles on a rebuild that had great oil pressure, at idle it was around 60 cold and 20 hot, then one day I cranked it and nothing. Here's the entire story
Engine - '77 401 complete rebuild, I built the engine. .030" over, crank ground .010"/.010" heads resurfaced, deck blocked, new bearings, rotating assembly balanced, new timing cover, new pump, pump housing cover lapped on glass to be perfectly smooth and flat. I checked the pump gear clearances both on the end and side and everything was in spec according to my Haynes manual.
First I checked the mechanical gauge, wound up replacing the line and gauge, no help.
I pulled the oil pan, oil was clean, no shiny specs, nothing in the pick-up, re-installed the pan, pulled the dizzy and tried to prime with a drill, drill turned noticeably easier than when I built the engine, oil pressure went up to around 12 psi on the drill, during the build, the drill got 40+ psi.
I pulled the oil pump cover and oil pump gears. No noticeable wear in the oil pump housing or on the gears, you can see where the gears rubbed the end-plate, but barely and no marks deep enough to feel or catch a finger nail on.
Where do I go from here, pull the pan again and re-inspect the bearings? I have a 304 that had good oil pressure when it was pulled, I could pull the timing cover from it and try it on the 401, but I don't see the point as the pump still looks new. Any suggestions?