Regular-production BMW 6-cylinder engines employ bucket-type hydraulic lifters, actuating the valves directly with minimum noise and no periodic adjustment. For an engine with the S54’s rpm potential, BMW M engineers needed less reciprocating mass.
To achieve this, they created a different actuating mechanism, using finger-type rocker arms. Pivoting on their own shafts (one on the intake side, one on the exhaust), these small arms reach out to provide the actuating surface between camshaft and valve. As the entire arm does not move the distance of valve lift, its effective reciprocating mass is less than its actual mass – and it weighs less than the “bucket tappets” in the first place. When all is said and done, the effective mass is 30% less; in turn, this allows lighter valve springs, which further reduce inertia. The system also has less friction.
As there is no hydraulic maintenance of valve clearance, it does have to be inspected periodically. Lead engine engineer Helmut Himmel asserts that it is unlikely that clearance will ever require adjustment, but if so it is done with shims (tiny metal discs of various thickness) without removing the camshafts.
Whereas the “regular” 6-cylinder engines have a simplex (single) primary chain driving the exhaust camshaft and a smaller secondary chain driving the intake camshaft from there, the S54 has a full duplex (double) chain driving both camshafts directly. As usual with BMW engines, the chain is hydraulically tensioned and needs no periodic adjustment or replacement.