low oil pressure when hot

I'm having low oil pressure when oil is hot. No problem with cold engine. Presure light doesn't come on, but when I measure pressure on the right side og the engine, pressure is 0 bar on 3000 rpm. Bike is 1981 Z1000J transfered to ELR. I added Lockhart oil cooler. I don't know if there was presure before restoring, because I bought non-runner.
O-ring in oil pan and on the pump are mounted.
thanx

What has been done internally to the motor during the restoration?

GPz 1100 head, new OEM pistons 72,5mm, rebuild head, ... Bottom was only disassembled and checked. I didn't change oil pump. No smokes on the exhaust.

Your pressure gauge may not be sensitive enough.
These engines only run around 3 psi when hot ( high volume / low pressure system ) and 1 bar is 14psi so the needle on your gauge may hardly be moving at 3000 rpm.
When stone cold ( depending on the oil viscosity you are running ) you will probably be getting somewhere around 10 psi which then drops off quickly after a minute or so of running.

This is my gauge and it is on 0 when hot and on 9 when cold.

Your pic is a bit blurry but that gauge should be reading in PSI not Bar's if it's the same as mine.
9 psi is ok for cold but zero is not for hot.
However ! some of these aftermarket gauges aren't really that accurate though at low pressures if not a well know make and some are just more for show .
If it's a recent addition along with the rebuild I would have the pressure checked with a good quality gauge first before digging around inside the motor again.
The std oil pumps rarely give any trouble and the usual culprit is debris blocking up the gauze on the pick-up which is unlikely as you have had the bottom end apart and checked.

I went through my oil pump on my last build. #600 paper on a piece of glass switched gears and thinner gasket. I blew two Dorimi 30 PSI gauges right away. I contacted the seller and he sent me a new one. That was my 2 ND one. I would peg the gauge on rev. when cold. Finally ordered a 60 PSI gauge meant for a Sportster but no reading. It had a small jet in the opening. It was loose so I screwed it out. Would have fallen out in time. Got a reading and still do. OK. Same problem as you except the needle is just above 0 at idle. I'm convinced it's the oil it's self. Been using Castrol GTX 20/50 all my life on everything. After reading about the new oils how they're not the same, I'm thinking I should change which if hard for me. Same with my 69 F 250. Just put in STP and oil pressure is up 10 PSI.
Like Jefferson said, fire them all and start over. I'm paraphrasing.
Steve

Thank you both.. I guess you are right. I'll try with other gauge. I'll have to wait for a while because it is -12 C here and before I worm up engine I'll become ICE cube.

Unscrew the gauge and hold your thumb over the hole,do this before starting bike! :laugh:
That will tell you about as much as a gauge will on these bikes,Id also pull the sending unit off and check/clean it.

This is what I did to my new gauges.


This is what happened as soon as I revved her up.
Steve