I just recently rebuild my engine and had heard many issues of not getting oil pressure on a rebuilt engine. I primed the oil pump off the engine in a bath of oil and cranked the pump gear in either direction until I could see oil coming out of either gallery when the direction was reversed. I then carefully installed the pump without tipping it, into the bottom of the motor. I then filled it with oil and started to crank the engine a few times on the starter. No oil pressure was evident. I then hooked up an air line to the oil tank breather and set the pressure gauge to about 25 PSI. I left it running for about twenty mins, I could hear air leaking from the cap. I then cranked the motor a fews times on the starter and suddenly I got oil pressure.