There is a great deal more to pistons in internal combustion engines than most people realise. For example, because the top of the piston (crown) has to cope with temperatures of around 260°C, it expands more at the top than at the bottom. This being so, the top is made smaller than the middle and the bottom.
The top of the piston is round, but the bottom (skirt) is oval to prevent piston slap when cold. However, once the engine has warmed up, the skirt expands across the piston pin axis and becomes round.
Most pistons are made of alloys of aluminium and expand more than steel when heated. To control expansion, sometimes a steel strut is built into the piston to help control expansion. Still on the subject of expansion, we must not forget the piston pin. If it is too tight, it will prevent proper expansion of the piston and hinder oscillation of the connecting rod.
“NICE SMOOTH FIT”
Follow the manufacturer’s guide when fitting the pin to the piston as the so-called interference fit can vary considerably between makes and models perhaps, say, 0.020 to 0.040mm to 0.003 to 0.008mm on others. The pin to rod clearance on an ordinary engine should be a “nice smooth fit” in the small end. To achieve this, pour boiling water over the piston, then insert the pin. It will be nicely gripped when the piston cools.
Pistons are easily damaged, especially when removing or fitting the piston pin. Never re-use the circlips. Make certain that the new ones fit fully into their slots. Mark rods and pistons so that they can be refitted in matched pairs. Note the mark showing the front of the piston and rod for reassembly. Also mark rod and piston to enable fitting in matched pairs.
Follow recommendations in the manual for ring gaps.
Too small a gap may bring piston ring failure. Ring-gapping must be done on an unworn part of the bore. Be certain to enter the ring squarely.
A worn bore may have a wear ridge at the top of its travel and new rings could break if they hit this ridge. Bores tend to taper when worn and this contributes to ring wear as the rings move deeper into their grooves during downward motion of the piston and expand to fit the worn part of the bore as it rises, contributing to wear of both rings and lands.
You will see that the ring in its relaxed state is larger in diameter than the bore and, until gapped and compressed by the cylinder walls, is slightly out of round.
The springiness in the ring is known as static tension, which helps to seal the ring to the bore, but it is the gas pressure behind the ring which holds it against the bore wall.
To check the fit of the piston in the bore (the skirt being oval) clean both bore and piston and place piston upside down into the bore. Then, with a feeler gauge, measure the thrust faces and skirt bottom. Follow the manual for precise clearance recommendations.
Excessive piston clearance could allow rings to twist and break at TDC. - Star Motoring