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I have a 2002 5.3l fresh rebuild. The only thing I didn't do is cam bearings (could not get tool in time) and barbell (didn't know about it till after the fact. Have 20# oil pressure at startup and falls off fast. Makes noises that don't sound good to me. Only has about 1000 miles on rebuild. Had time and money issues so had to do it cheap and fast. I know that's not the best way. I have put 3 oil changes in it in the 1000 ish miles to keep oil pressure in the "safe zone" These are all of the parts that I purchased. I have since purchased the barbell. Any help would be much appreciated.
Did you replace the O ring for the pickup tube where it plugs into the oil pump? I had a motor with zero oil pressure issues start doing similar things. Chased it for a while with no luck, thinking that O ring looked fine. When I was finally out of ideas, I broke down, pulled the pan and replaced it as a last ditch effort. Problem solved.
"Have 20# oil pressure at startup and falls off fast."
Pressure fall to 0? As mentioned, replace oil pick up tube O ring? Did you pull apart new pump, clean up casting and verify relief slides freely?
If you want to find where the oil is bleeding out (not building pressure) you need to... pull pan. Block off oil pick up and oil ports to filter on oil pan rail. Connect fitting to LF oil galley port (16M x1.5) and inject pressurized oil in (should not need more than 30psi) to see where oil leaking on the pump side. Could be pick up tube O ring, oil pump relief valve stuck open, cracked pickup tube. Connect fitting to LR oil galley port and see where bleeding oil. Could be cam bearing walked out, bearing clearances excessive, etc. This test will clearly identify where you need to focus next.
Thank you all for responding. Got alot going on right now. Sorry didn't get back sooner. I did not check clearances I know it's not the right way. Engine had stock bearings with 240,000 miles with no bearing problems so I just slapped new stock sized one in. Don't have the tools to check with. pickup tube o ring replaced twice so far.
oil pressure does not go to 0# but gets down to about what I think is 8# on the dash.
I have not pulled the pump apart as of now. I haven't even had time to look at it unfortunately.
Hopefully going to get to work on it this weekend if anything goes like I need it to.
Will pull the pump and check that because this is the second time I was told to look at that since it happened.
Hopefully I will be able to swap a stocker back in this weekend so I can take my time getting this one right.
I will try to keep everybody apprised on what's going on as soon as I have time. Again, sorry for taking so long.
Ever heard the expression that an assumption is the mother of all screw ups!
How do you know if the crank and rods required the use of stock size bearings if you didn't measure everything first? I don't believe that's your issue though but I did hear some chirping which could be an indication of cam bearings not getting the oil it needs. Hopefully you don't have any spun cam bearings that will lead to a wiped out cam and lifters. Have you dropped the oil and cut open the filter looking for any signs of bearing material?
In my line of work I tell my guys to never assume. It only makes an *** out of U and ME
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Oil pressure issues 
Sometimes the truth hurts. 