Oil sump pick-up Oil temperature control valve Oil filter Oil pressure  relief valve  by-pass to  sump Oil cooler Oil pressure gauge connection Hydraulic lifters Governor pad To propeller Camshaft bearing Crankshaft bearing Starter bushing Idler shaft bushing Accessory drive bushing Figure 6-15.  Oil circulation through the engine.  Oil reaches a hollow camshaft (in an inline or opposed  engine), or a cam plate or cam drum (in a radial engine),  through a connection with the end bearing or the main oil  manifold; it then flows out to the various camshaft, cam drum,  or cam plate bearings and the cams. have a greater capacity than the pressure pump. This is needed  because the volume of the oil has generally increased due  to foaming (mixing with air). On dry sump engines, this oil  leaves the engine, passes through the oil cooler, and returns  to the supply tank. A thermostat attached to the oil cooler controls oil  temperature by allowing part of the oil to flow through the  cooler and part to flow directly into the oil supply tank. This  arrangement allows hot engine oil with a temperature still  below 65 °C (150 °F) to mix with the cold uncirculated oil  in the tank. This raises the complete engine oil supply to  operating temperature in a shorter period of time. The engine cylinder surfaces receive oil sprayed from the  crankshaft and also from the crankpin bearings. Since oil  seeps slowly through the small crankpin clearances before it  is sprayed on the cylinder walls, considerable time is required  for enough oil to reach the cylinder walls, especially on a  cold day when the oil flow is more sluggish. This is one of  the chief reasons for using modern multiviscosity oils that  flow well at low temperatures. When the circulating oil has performed its function of  lubricating and cooling the moving parts of the engine, it  drains into the sumps in the lowest parts of the engine. Oil  collected in these sumps is picked up by gear or gerotor-type  scavenger pumps as quickly as it accumulates. These pumps  A simple form of a wet-sump system is shown in  Figure 6-16 .  The system consists of a sump or pan in which the oil supply  is contained. The oil supply is limited by the sump (oil pan)  capacity. The level (quantity) of oil is indicated or measured  by a vertical rod that protrudes into the oil from an elevated  Wet-Sump Lubrication System Operation 6-13