The in-house Caliber 6401 is a brand-new, compact chronograph caliber that now powers this refreshed 38mm Royal Oak Chronograph. It’s got all the proper specs expected of a modern chronograph caliber, with a fully integrated architecture featuring a vertical clutch, a column wheel, and an instantaneous jumping date at midnight. The power reserve improves to 55 hours from 40 hours in this new caliber, along with a higher frequency of 28,800 VpH. The brand notes that the clutch mechanism in this movement is patented, with the new design offering a simpler layout that results in fewer parts, improved robustness, and a more refined tactility for the pushers. What this Caliber 6401 does not have, however, is the flyback functionality seen in the brand’s larger chronograph caliber. But it’s a compromise for a good reason, allowing the new Royal Oak Chronograph 38mm to remain about as thin as its predecessors, at 11.1mm.