As illustrated here, the reason for warming are ASSUMED to be "seized" type interpretation, when there's far more nuance and practical reason to warm first. Undoubtedly, for engines that need to be considered for power and lifing at their peak, its a prudent strategy to warm and carry out valid system checks prior to use in helping to correct anomalies trackside rather than producing an uncertain result when out of the pit box.
Many envision "seized" to be piston clearance, when the majority of engines have the piston ultimately expand more than the bore and risking seizing from too much heat, not cold.
Many production engines use low expansion metallurgy focus in production of pistons to achieve closer cold status clearance with bore, but not raising risk of hot seizure.
More performance orientated piston metallurgy heads towards durability under extreme combustion demand, while accepting "loose" tolerance to bore when cold. There's room to grow that piston, in other words, and still avoid hot derived interference fit under maximum demand within usage cycle. This more typically used in competition engines.
-----THIS COULD BE IMPORTANT IN THIS TOPIC----- We'll come back to it, and the "Ambient" role in this.
IF ---- the designers were now to make a PU with geometric 18:1 design, that at the exact 1600cc (base on the machined bore and combustion surfaces) then it can be seen that the rules would be infringed -----from that design geometry when comparing the volume inside the cylinder divided into the volume left inside the combustion chamber at TDC. indisputable we can all see.
BUT ----- and its a big one, if mercedes have seen in the inspection wording and method that calculations COULD be different , that may be inside the rules. To judge this would need exact method of inspection and the precise wording to extend our view and assessment of potential.
It's possible they've made use of the piston being smaller than the bore (see statement above) to "argue" that the swept volume is that of the piston, and not the bore diameter. Which would produce a lower recognised geometric compression ratio ----- at ambient temperature------ which brought the geometric calculations to inside the 16:1 ratio specified within the rules. An interpretation of the words published by the FIA in other words.
The bores are designed specifically not to have great expansion, that to increase accuracy and power potential. The piston usually MUST accommodate that design ideal to function at full deployment, by being smaller and by the margin that is driven by favouring durability over expansion characteristics in the alloying mix /design used.
Whether the check on ratio ever happens at track is another question. If it were FIA inspection of components and detailed measurements going into build is an important part to understand. If they looked at the the design and actually measured prior to build, then to enact the phrase "at all times adhere to the rules" would then provision the exclusion of a competitor IF an engine where to be sealed and stripped after racing, would be interesting to know.
Are they regularly testing trackside or not is important. We don't know enough here to be definitive.
