end surfaces 61 and 63 as the pistons advance to their respective tc locations in the cylinder bore fuel is injected through the nozzles 100 into the charge air and the mixture of charge air and fuel is compressed between the pistons 60 and 62 when the mixture reaches an ignition temperature the fuel ignites combustion results driving the pistons apart toward their respective bc locations fig2 is a graph that shows the oil pressures at the wristpins of a representative opposed piton engine during a full cycle of the engine from a crank angle ca of 0 u00b0 at tc to bc 180 u00b0 ca and back to tc 360 u00b0 ca for several main gallery oil pressures as can be seen in fig2 for a given gallery pressure the oil pressure at the wristpins is at its highest at tc and at its lowest at bc if main gallery pressure is below an engine specific threshold the pressure of the column of oil in the connecting rod drilling supplying the wristpin will go negative and the wristpin will not have pressurized oil available until such time in the cycle that the column pressure is positive again as per fig2 for a given engine geometry and crankshaft speed a main gallery pressure below five bar results in the oil pressure at the wristpin being negative at 180 u00b0 ca bc at a main gallery oil pressure.