When an excavator boom reaches the end of its stroke, the massive weight of the arm and bucket generates incredible momentum. If the piston hits the cylinder head at full speed, it will shear the head bolts or crack the cast iron. To prevent this, boom cylinders have a "cushion" at the rod-end and cap-end. As the piston nears the end of the bore, a tapered spear on the piston enters a recess in the cylinder head, trapping the oil and forcing it to exit through a small, adjustable orifice called a needle valve.
Over time, shock loads and debris can score the tapered seat of the needle valve. If the valve seat is damaged, the cushion is destroyed. The operator will complain of a "hammering" sound when they boom up or boom down to the full extent of the stroke.
The fatal mistake mechanics make is cranking the needle valve shut to try and restore the cushion. If you force a damaged needle valve closed, you completely block the oil flow. The hydraulic pressure builds up against the trapped piston with nowhere to go, and the cylinder will literally explode the head off the end of the barrel. If adjusting the needle valve doesn't smooth out the stop, the cylinder must be removed and the cushion spear and needle valve seat must be machined and lapped. Never force the needle valve; it is a metering device, not a plug.
