Nearly every CLK63 listing draws head-bolt questions, and the community consensus is that failure is rare, that Mercedes and specialists do not recommend preemptive replacement absent symptoms (coolant loss, oil contamination), and that the job is only worth doing if the valve covers are already off. Proactive replacement quotes cluster around $3,700 for bolts plus cam-adjuster plates.
M156 head bolt anxiety on the CLK63
Mercedes-Benz CLK55/CLK63 AMG (C209) · mechanical