Cut gap

The ring expands when heated. The ring temperature is higher than the cylinder temperature. The rings receive the calories from the pistons whereas the cylinders are cooled by the outside air or the water cooling circuit.
The rings give off some of the calories accumulated through contact with the cylinder walls and their temperature is somewhere between the temperature of the cylinder wall and that of the piston head. The dilatation of the cylinders and their bore is inhibited in most cases by the dilatation of the complex form of the partitioned block and there is no way of guaranteeing the bore dimensions will increase to the extent indicated in the basic dilatation calculation.
Finally, the dilatation coefficient of the ring cast iron exceeds that of the cylinder cast iron. The developed length of the ring must be reasonably short when the engine is at operating temperature to avoid any risk of contact between the two ring ends. This means that the developed length of the ring must be shorter than the developed circumference of the cylinder bore, in particular in the most critical cases where the temperature difference between the ring and cylinder is at its highest (start up, bad lubrication, high revs...).

The dilatation coefficient of standard ring cast iron (FGL, FGS) is:
- from 20 to 200°C: 1.07 millimetres per developed metre for 100°C variation
- from 200 to 500°C: 1.30 mm millimetres per developed metre for 100°C variation

For applications other than thermal applications, the gap will be defined according to the operating environment, operating temperature, nature of the fluid or gas to be sealed, etc. 

Setting ring / CV : straigh joint / CO : hand angle joint