There is a need for much more information in order to diagnose problems such as these. Was the oil pressure in flight ok during the winter and spring but now it is too low? What sort of oil temperature do you see? What is the oil temp at idle? What is the oil temp at startup? How long does it take for the idle oil temp to stabilize at 35 after the first startup? (This no longer
counts since the mechanic has changed the relief valve.) When did you last change the oil and with what grade? (Straight 50 weight such as Shell 100 would be correct for this engine.) Think of your oil system in this way: The oil pump delivers oil in a volume that is proportional to engine speed. The oil in the oil galleries flows past the bearings in proportion to the pressure and inversely proportional to viscosity. The oil pressure relief valve is a coarse regulator which begins to limit pressure at 35 lbs ( higher in yours). It is common practice to shim the relief valve spring on the assumption that it has weakened over time and is the cause of your low oil pressure. This is seldom the case and your relief valve should be returned to its' original configuration at some convenient time. The function of the valve is to prevent damage to the engine when the oil is too thick. I had a car engine which would rupture the oil filter on each startup because the valve was frozen. When you feel like running your engine we need to know how the pressure behaves during flight. What is the pressure when you take off? The oil will be thick and the pump will be able to achieve the pressure set by the relief valve until the oil warms up unless something else is wrong. When you level at cruise does the oil pressure follow rpm? Do you have higher pressure at 2500 than at 2200 rpm? If so then you have a loose engine or thin oil. If not then you have a faulty gauge or relief valve. What is the oil pressure at idle on landing? If the engine is loose I expect to find something like 5 to 10 psi. higher than that suggests that the engine is ok internally. Should you find that the engine is really loose then you are very close to splitting the case and spending a lot of money. The last thing that you may choose to do is remove the cylinders and measure the clearance of the rod bearings. The measurement is performed with plastigauge which your mechanic is well familiar with. If you find that the rod bearings are loose then you may suspect that the mains are also loose but there is no way to measure them without splitting the case. Before removing the cylinders your mechanic should check the torque of the crankcase through bolts. If they are loose then the case may be loose enough to allow a main bearing to spin in the case which would not be visible until the case is split but would be a big problem. You don't want to think about it but if you should end up rebuilding your engine and need a crankshaft, there is an STC allowing the use of an O-200 crankshaft in the C-85. You get to change the pistons and rods too but you get a 6% larger engine and the O-200 shaft is $1000 cheaper than the C-85 shaft. My last overhaul is like yours kind of vague. It was done some time ago in a log book now lost so I knew when I bought the plane that the engine might need overhaul at any time. It turned out that three of the cylinders were pretty junky at 400 hours but the lower end of my engine has shown no troubles. Good luck Bob Condon
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