The skirt

The simplest form of this is illustrated in figure, in which the skirt forms a downward tubular extension below the ring belt this arrangement is called a solid skirt. It is the strongest form of skirt and is always used for engines where the loads on the piston are particularly great.

To permit the use of small clearance with aluminium alloy pistons when cold, special types of skirt construction are used which involve some degree of flexibility of the skirt. For example, the skirt may be almost completely separated from the ring belt except at the gudgeon-pin bosses, and split down one side. This allows it to fit in the cylinder with very little clearance when cold, expansion due to rise in temperature is taken up by reducing the width of the split. The split usually extends the full length of the skirt and is at a slight angle, to avoid leaving a ridge on the cylinder as wear occurs.

When assembling split-skirt pistons on to the connecting rod it is important that the split side of the skirt is on that side which bears against the cylinder during the compression stroke, so that the the unsplit and stronger – side takes the greater side thrust during the power strokes.

The high piston load of a modern engine has meant that the split skirt has been superseded by low-expansion, solid-skirt pistons such as the thermal slot or link strut types.

Thermal slot.

Cutting a lateral thermal slot at the bottom of the ring groove on the thrust and non-thrust sides of the piston directs the heat from the hot (240С) piston crown to the gudgeon-pin boss reions. In some designs these slots are extended axially downwards into the skirt and terminate with a circular hole. Thermal expansion of the crown and bosses, combined with the effect of the gas load, pulls in thrust faces and allows the piston to be fitted with a small clearance. To compensate for this action the piston is oval when cold.

Link strut pistons have special alloy steel (invar) plates cast into the skirt. This material has an extremely low coefficient of thermal expansion, so the resistance given by these plates limits the expansion of the skirt.{jcomments on}