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Food for thought? I agree with most on this but do not agree with the theory that oil pressure can be arbitrarily increased by adjusting or reshimming the relief valve. Take this situation - a JD (lets say a 420) has been worked and taken care of for a good thirty years. However throught time the oil pressure has slowly shown less as when the engine is at normal operating temperature. Now in this particular engine the oil pump happens to be capable of pumping relativewly the same volume as when new. But the increse in bearing clearance has allowed enough space for all the oil that the pump can prouduce and then some to leak by. Consequently the pressure will go down when the bearings leak more than the capacity of the oil pump to produce. In this situation the relief valve stays closed and no oil is bypassed as the pressure in the system never reaches the bypass point. My theory is that when a motor is in the above condition, tightening or shimming up the relief valve will have no effect on oil presure. I could see one exception to the above theory. Lets say that through the years the spring in the relief valve has lost some of its tension or a lot of its tension because of say a bad run on the quality of metal in that particular spring. Here, which I feel would be a rare case, the pressure could be increased by instaling a new spring or putting more tension on the old tired one. If someone disagrees with the above theory, let me know please.
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