C63 AMG 6.2 V8 (M156)

The hand-built naturally-aspirated 6.2 V8 (M156) is one of the great modern AMG engines — savage, charismatic and, mechanically, immensely strong in the bottom end. But it carries two well-documented, expensive faults that completely define the buying decision. First, the early cylinder head bolts (made of a material that corrodes/stretches under thermal cycling) can fail and cause coolant leaks and loss of seal; Mercedes fitted revised bolts from engine number 60658, so pre-~2011 cars must be confirmed either to have the updated bolts or to have had the fix done. Second — and the subject of a US class action — the intake camshafts and lifters were improperly heat-treated and wear prematurely, typically by ~100,000 miles, producing a persistent ticking/rattle (worsened by the engine's normal cold-start oil 'bleed-down' tap); left unchecked the cam lobe eats through the lifter. On top of this it is a genuinely expensive car to run (fuel, tyres, brakes, AMG servicing). A car with the head-bolt fix, documented cam/lifter health and full AMG history is superb and a future classic; an unchecked one is a five-figure trap. Buy on inspection, not on emotion.