Piston ring gap

[Ripcrow]

Can someone explain why there needs to be a gap in rings .i always believed that they were a tight fit to prevent blowby/oil burning.also any thermal expansion should be equal between cylinder piston and rings otherwise as the motor warmed up compression would leak :hDe:

 

[///]

You kinda answered your own question... thermal expansion.
As the ring heats up it expands and the gap reduces.
It also allows the ring to expand to compensate for cylinder and ring wear, in which case the gap gets larger but the ring is still providing an acceptable seal.

Thermal expansion of the different metals(aluminium piston/cast iron rings) occurs at different rates.