OOPS!! mystyped,sorry, should read like this:

The plugs with cold thermal range F (Fredda), have a body with thinner walls and shorter body, so they dissipate heat better and faster to the engine head.

The plugs with hot thermal range C (Calda), have a body with thicker walls and longer body, so they dissipate heat slower to the engine head.

The number of threads is the same on both types of plugs.

Although never actually cc'd a head with each type, in theory the F type should give less compression, but difference should be ultra minimal

AFM