Older cars usually feature solid valve lifters, which would often require frequent adjustments to ensure a tight clearance between it and your engine's intake and exhaust valves. Later on, hydraulic lifters were created to solve this problem by ensuring that there's zero valve clearance between the lifters, as well as your engine's pushrod and rocker arms. Hydraulic lifters consist of a steel cylinder that features a hollow piston within them, which allows the motor oil to flow through. The oil can thus use hydraulic pressure to regulate the movement of the hydraulic lifters, while also maintaining that zero valve clearance.