Engine - Pistons & Crank

The crankshaft is of the V8 cross-plane design and is made of forged steel (alloy C38 = 99% iron, 0.18% carbon, 0.17% silicon, 0.44% manganese, 0.20% copper, 0.03% sulphur) with a hardened surface layer and 6 balance weights.

It is supported in the crankcase by the five bearings shown above. The outer ones are lead-free three-material bearings (three metals with a plastic coating) and the main thrust bearing is at the centre and is a bi-material bearing (two metals with a plastic coating).  The "plastic" coating is a high strength polymer composite consisting of a polymer chain matrix filled with particles of solid lubricants (molybdenum disulfide and graphite).

The connecting rod is forged from sintered steel.

Sintering is a process by which powdered metals are formed under pressure and then carefully heated to fuse them together. This adds strength.  The connecting rods are then fracture-split at the big hole to allow for a better connection after the bearings are inserted and bolted down.

This is done by carefully introducing an imperfection at the desired fracture-split points, and then an abrupt lateral sheering force (i.e., a big hammer) is applied to fracture the rods at the big hole. The ragged surface has more area to support a better connection once the two halves are bolted together again around the crankshaft.

The small end has a formed hole that allows for a more even force distribution on the power stroke (see stress diagram below).

The piston heads are manufactured by the German company Mahle who have been manufacturing pistons since 1920.

They are made of lightweight cast aluminium with an iron particle reinforced synthetic resin coating to allow them to slide across the Alusil cylinders.

The design of the piston head is very carefully considered to work with the Valvetronic III (BMW's variable valve lift technology), the valves and the way they open, and direct high-pressure fuel injection into the cylinder. Computer models were run to design the shape so as to best promote the complete dispersion and combustion of the fuel.

All of the components we have just discussed are beefed up versions for the M series engine, as they have to deal with the considerably higher force (well in excess of 7000 lb) being directed onto the piston heads, down the connecting rods, and onto the crankshaft in support of the > 500 ft-lb of torque the M5 generates at the crank (the base N63 engine generates only 440 ft-lb, so everything can be weaker and hence lighter for better fuel economy).