Porn

Well greased piston precisely positioned with the head held close to the cylinder bore, throw the switch, piston slides back and forth within the cylinder chamber. The objective is to release the load into the cylinder chamber.
Do a dry run. Ensure that the constant friction exerted on the piston causes expansion and proper clearance provided by the cylinder head for free piston movement in the cylinder chamber.
Excessive clearance, on the other hand, based on the over use of the cylinder chamber, may cause compression and slippage of the piston. Unused chambers may cause high friction and increase in piston noise.  
Cylinder chambers are of various dimensions and some of them call for excessive force for piston penetration.  The crankshaft has to be carefully positioned to help align the piston as it moves into the cylinder bore.
The piston head is the top surface (closest to the cylinder head) of the piston which is subjected to tremendous forces and heat during normal operation.  Some chambers have profiles cut into them to reduce piston mass and to provide clearance for the rotating crankshaft counterweights.
Piston designs are based on benefits and compromises for optimum overall performance. In addition to inherent pressure, a piston ring seals the combustion chamber through applied pressure. Applied pressure is pressure applied from combustion gases to the piston ring, causing it to expand. Constant friction has to be maintained to keep the piston in the expansion mode. Compression may lead the cylinder chamber to shut of operations.
Another piston ring design consideration is cylinder wall contact pressure. This pressure is usually dependent on the elasticity of the chamber, its wear and tear and usage pattern. The taper angle of the piston provides contact that routes excess oil on the cylinder wall to the oil ring for return to the oil reservoir.
Once the piston builds up enough pressure the opening of the piston head releases its load in spurts, either in the lower chamber or the chamber above. The release of the load signals the end of the operation. Sometimes two or three chambers are lined up for assembly line piston operations.