Because the crankcase is separated from the cylinder and scavenge space by the diaphragm plate on a two stroke crosshead engine, provision must be made for the piston rod to pass through the plate without oil from the crankcase being carried upwards, or used cylinder oil contaminated from products of combustion being carried downwards. It is also highly undesirable to allow the pressurized air in the scavenge space to leak into the crankcase.
The Piston rod passes through a stuffing box which is bolted into the diaphragm plate. The stuffing box casing which can be split vertically, as shown in the photo, contains a series of scraper rings and sealing rings which are each made up of three or four segments. On the outside of each set of segments is a garter spring which provides the tension to hold the ring segments against the piston rod. There is a clearance between each segment to allow for wear. The rings are either bronze or can comprise of replaceable cast iron lamella fitted into a steel backing ring.