I just rebuilt my 84 Scrambler engine, an inline 6 258.  New everything.  It runs but I can't get the oil pressure off 0 without revving the engine, and then it only gets to about 20psi.  I have slaved in a mechanical gauge and it give the same indication as the electrical gauge.  I origingally installed a new oil pump, and have replaced it again.  I didn't have oil pressure problems before the teardown.  I didn't fool with the pressure relief valve until after the problem occured. Since then I have taken it apart cleaned it up and reinstalled, but that didn't help.  I am lost .  Does a drawing exist that shows how the oil is ported through the engine, so I could maybe see what I might have missed in the rebuild.  

It's hard to say. I've seen lots of older 258s with seemingly low oil pressure that ran forever. It's pretty common. But a new engine... doesn't sound good. If both the mechanical and electric gauges are reading the same you may want to look into a performance high-volume oil pump rather than a stock replacement.  

did your build include a reground crank?
were new cam bearings also installed?  

sounds like the oil pump drive bearing or gasket problems, but I would say gears. did you put the mechanical oil pressure in the same hole in the block as the electrical sending unit?  

it had good presure before....recheck your bearing clearances with plasti-gage..  

Since my last posting I have replaced the pump (again), plugged the pressure relief valve, check the gap on one of the crank bearings, and replace the oil filter.  All to no avail.

Mudb8
did your build include a reground crank?
Yes, but I don't know which journal.

were new cam bearings also installed?
New cam new bearings

it had good presure before....recheck your bearing clearances with plasti-gage..  
All seemed real close on install.  Pulled the #3 journal when I replaced the oil pump and I couldn't get .0015 between the bearing and the crank.

Stumpbumper
sounds like the oil pump drive bearing or gasket problems, but I would say gears. did you put the mechanical oil pressure in the same hole in the block as the electrical sending unit?
Yes I hooked up the mechanical gauge in the same hole.

I'm wondering now if maybe I left something off on the install of the cam.  I had a machine shop install the cam bearings, and they bored the cylinders.  They also provided the crank bearings and new pistons.  When I got the block back all the freeze plugs were missing, and an oil plug on the back of the engine close to the cam shaft rear plug was missing.  Is there a plug on the front end inside the timing chain cover maybe that I left off?  I found the rear plug when I manually ran the oil pump before installing the engine. It was behind the flywheel and poured oil all over my garage floor.  A similar plug under the timing cover would not be so obvious.  

pull the distributor and turn the oil pump drive with a electric drill to see if you generate presure that way.  

Well after trying several suggestions, I finally got around to pulling the timing chain cover and there is a plug that goes in the oil channel just behind the cam shaft gear.  Yep, it was missing.  Replaced it, put things back together, and ran the pump with a drill.  60psi. YES!! Put everything else back together and at idle I have a steady 60psi on the dash gauge.  A little more cleaning up and I'm ready to roll.  Thanks to all for you suggestions.  

right on, I couldn't remember if there was one there!  

Hate to re-hash the same topic but I am frustrated by a rescent re-build.  Same engine as lascrambler but in a 1985 CJ7.  Prior to re-build the oil pressure bounced from 20 to 40 and spit oil out the exhaust and smoked like a chimney.  I replaced the head and the pressure went to 60 but still spit oil and smoked.  Found 2 cracked pistons in 4 and 6 but the block was fine.  The engine was not pushed to failure.  I then replaced the pistons, rings, timing chain, timing gears, oil pump, main and rod bearings, had the rods machined and fitted with the new pistons and replaced the lifters.  I had a restoration company do the engine rebuild and everything gauged out fine with plasti-gauge and the like.  Got the engine back in and no oil pressure unless it is reved and then only to 20-25psi.  I have replaced the oil pump again to no avail.  What next?!  I bought the jeep 9 months ago and am itching to go.  But weather and mechanical has kept me out of it.  I will listen to any suggestions that are reasonable.  Thanks!!