Jim Cameron wrote:  Got a mystery, and looking for any advice:  My oil pressure is persistently in the 69 - 70 psi range in cruise, over 80 on takeoff with full power.  I've removed, disassembled, cleaned and examined the oil pressure regulator assembly on the back of the oil pump.  At the moment I have the adjusting screw backed out as far as it will go (usefully), and still am having these high oil pressures.  I even scavenged a spring from another engine to compare the spring tension, which seemed about the same from one to the other.  I'm really scratching my head.  Any ideas how to get this down?  Also, would running with these oil pressures actually present a problem, or cause long-term damage?  TCM recommends 30 - 60 in cruise, but state a 100 psi upper limit.

For this time of the year your pressures are pretty high and I would be uncomfortable.  The real issue will be in cold weather where the pressure will be higher and the possibility of exploding the oil filter exists.  Before I tore anything apart or replaced anything, I would verify the accuracy of the aircraft pressure gage with a mechanical gage from an auto parts store.  If all gages say your pressure is really high, then what you have already done is what I would do.  Make sure the relief valve works....try putting air pressure on it and see if it moves off the seat.  Make sure nothing is blocking the bypass port.  What makes a very good pump (high pressure) is all of the clearances around the two positive displacement gears. The smaller the clearances are, the higher the pressure will be for a given orifice.  The engine bearing clearances, etc combined is the pump outlet orifice.  There are a lot of variables that affect pressure, but the system is pretty simple.  If the gages are correct, then the relief system is the number one suspect.  You know the oil system design is OK, so after that it comes down to the engine orifice.  I can't remember where the oil pressure is taken off of the engine. If it is away from the oil pump, then forget the next comment.  Make sure there isn't anything blocking the oil passage to the engine.  For example a gasket or something.

If you are running straight weight oil, multi-vis will reduce the oil pressure.

Craig Berland