In the hydraulically actuated, air spring return exhaust valve design used on large two-stroke engines, the valve spindle is closed by compressed air (the "air spring") rather than a mechanical coil spring, and the opening motion is generated by hydraulic pressure acting on a piston or piston block at the top of the valve housing. Because both hydraulic oil and compressed air are involved, the valve has no rigid mechanical link to the rocker/cam; instead the hydraulic oil above the air spring is what drives the valve open and repositions it.

The term "virtual tappet" refers to the effective, controllable clearance or cushion that exists between the hydraulic actuator piston and the valve spindle extension. In a conventional mechanical tappet system the clearance must be adjusted manually. In this hydraulic system there is no physical tappet screw; instead the design creates an equivalent controlled clearance by the oil film and by the dimensional relationship between the actuator piston and the lower end of the valve spindle extension. The virtual tappet is set by machining the spindle extension to a defined length and by ensuring the piston block is positioned so that, when the valve is closed, there is a small pre-determined axial clearance (typically of the order of a few tenths of a millimetre). This setting is carried out by measuring between the piston and the spindle extension, or by using spacer/adjusting shims, and confirming the cold clearance against the manufacturer's figure. The air spring also provides a controlled cushioning effect so that the "tappet" is effectively compliant.

Furrowing and cutting of the valve seats: The seats become damaged because of burning of deposit, fuel-related corrosion and erosion. When combustion deposits or particles of uncarbonised fuel and hard sodium/vanadium compounds become trapped between the valve seat and valve insert, they act as an abrasive. The hard, brittle ash particles also soften and stick at high temperature. The high seating velocity and the excavating action of gas flow can then literally plough "furrows" round the seat and produce localized "cutting" in the valve-facing surfaces. Thermal loading and the differential expansion between spindle and seat ring further worsen it. Poor atomization and excess combustion advance promote burning on the seat land. Keeping the seats clean by proper valve rotation, correct fuel quality and adequate cooling reduces this damage.

Valve drop is the complete loss of the valve drive/retention, where the hydraulic oil pressure fails (e.g. loss of pump pressure, oil viscosity reduction, valve spindle fracturing at the neck or the spindle extension breaking) and the air spring supply fails simultaneously, so the valve head goes into the cylinder uncontrolled. The valve can then hit the piston crown at top dead centre, bending the connecting rod, breaking the crown, and leading to extensive damage to the running gear (piston, liner, crosshead and connecting rod). The mechanism usually involves failure of the hydraulic system security interlocks combined with a fractured spindle.