This paper develops Aa computer program for the unsteady flow analysis of diesel engine lubrication systems. The possibility of cavitations developing in the big-end bearing groove due to the reciprocating motion of the connecting rod is investigated at the designed running speeds. The flow passage geometry especially the head loss coefficients of the bearing grooves s affects the minimum values of the head and the point when the development of the cavitations is initiated As the loss coefficient at the bigend increases and that of the small-end decreases, then the cavitations can be initiated. An orifice can be used to suppress the development of the cavitations and the diameter ratio and the location needs to be carefully selected to avoid unnecessary pressure loss. The channel size in the connecting rod is also an important parameter for optimization of the overall lubrication system; as the pipe diameter decreases, the minimum head increases and suppresses the cavitations from developing The minimum head always appears at the trailing groove of the big-end bearing.