Forged connecting rods BMW - 5.0L 40V - S85B50 for forged pistons

The BMW S85B50 engine

The BMW S85B50 is the M division’s naturally aspirated 5.0-litre V10, an engine born of the brand’s Formula 1 experience and without equal in its range. It displaces 4,999 cc with a 92 mm bore and a very short 75.2 mm stroke, racing-engine geometry that explains its appetite for revs. Forty-valve heads, ten cylinders and an almost instant response give 507 hp at 7,750 rpm and 520 Nm, with a red line at 8,250 rpm. It powers the M5 (E60/E61) and the M6 (E63/E64).

Built from 2004 to 2010, this V10 is as fascinating as it is demanding. The very high engine speeds it lives at work the bottom end constantly, and the standard connecting rods are a recognised weak point on high-mileage examples or engines taken beyond factory figures. Raising the limiter, running back-to-back track sessions or increasing power moves the limit to the bottom end. A set of forged 4340 steel rods, combined with forged pistons, removes that risk for good.

Forged connecting rods

Forged and then machined from heat-treated 4340 steel, these rods replace production parts whose margin shrinks as soon as the engine leaves its factory envelope. Forging aligns the metal grain with the load path, which clearly raises fatigue life, the decisive factor on a V10 accumulating cycles beyond 7,750 rpm.

Shot-peened, individually inspected and weight-matched to ±1 g across the ten rods in the set, they preserve the balance of an engine whose smoothness is part of its character. They cover the serious uses of a tuned M5 E60 or M6: circuit racing and track days, hillclimb, time attack, drift, rally and drag, and they support Stage 1, Stage 2 and Stage 3 builds up to full race engines. The ARP bolts supplied lock the assembly at very high rpm.

Technical specifications of the S85B50 forged connecting rods

Torque and bolt stretch

At the engine speeds the S85B50 V10 allows, the inertia load trying to tear the cap away grows with the square of engine speed: bolt preload is the only thing standing in its way. Tightening below the specified figure therefore means letting the cap breathe at every top dead centre; what follows is well known, a marked parting face, a hammered shell that eventually spins in its bore, and a rod lost even before the crankshaft. Tightening beyond it strengthens nothing: the bolt leaves its elastic domain, stretches permanently and loses preload with the first heat cycle, while the bearing bore distorts and the big end goes oval. All ten rods must therefore be tightened to the values in the table below, defined by the fastener manufacturer for ARP 2000 and ARP L19 in 3/8" and not transferable from one grade to the other. The stretch gauge is the reference tool: it measures what actually matters, bolt elongation, whereas a torque wrench only sees a figure that depends on friction. Assemble with ARP lubricant, the kit instructions taking priority.

Technical guide

4340 forged steel: what is it?

Connecting rod profiles

I-Beam profile. Its slimmer central section cuts reciprocating mass: less inertia, less load on the bearings and a freer climb through the rev range, which counts doubly on a short-stroke V10 turning to 8,250 rpm. Material stays concentrated where it works, so stiffness remains suited to track and competition use. It is the only profile offered for this engine, and the one most in keeping with its racing-engine philosophy.

How to choose your rod bolts?

The rod bolt is the most heavily loaded part of the assembly: on every revolution it takes the inertial force trying to pull the cap away from the rod body, and that force grows with the square of engine speed, so very quickly on a V10 of this kind. Two ARP grades are offered; the grade has to match real use, and the specified torque and stretch figures must then be followed exactly.

ARP 2000: the high-performance standard, with tensile strength around 220,000 psi (about 1,517 MPa). In 3/8" it covers builds up to 200 hp per cylinder and 8,500 rpm, which takes in the standard S85B50 as well as most optimised versions, including regular track use. It is found in circuit racing and track days, drift, rally, drag and hillclimb, chosen for its strength and versatility, with no special storage requirements.

L19: the grade for engines pushed to the extreme, with tensile strength around 260,000 psi (about 1,793 MPa). In 3/8" it allows up to 250 hp per cylinder and 10,000 rpm, the reserve to choose for an S85B50 with a raised limiter, entered in competition or destined for long sessions at full revs. The trade-off: the alloy is sensitive to corrosion and hydrogen embrittlement, and must be stored and fitted oiled, away from moisture.

Measuring bolt stretch

Buy the ARP stretch gauge →

The brands offered

ZRP, the Greek manufacturer of high-performance forged connecting rods run by Alex Drakos in Athens. Every rod is forged from 4340 steel, precision machined, inspected and then weight-matched across the full set of ten, an essential step to preserve the smoothness of a V10. For the S85B50 the range is limited to the I-Beam profile, with ARP 2000 or ARP L19 bolts in 3/8" depending on the power and rpm targeted. Design, machining and inspection all take place in Greece, and the rods are proven in rally, drift and circuit racing.

Glossary

1) Small end 2) Small-end diameter 3) Rod beam 4) Rod bolt 5) Big end 6) Rod nut / bolt 7) Rod cap 8) Big-end diameter 9) Center-to-center

Vehicle compatibility for the BMW S85B50 engine

These forged connecting rods are compatible with the factory BMW S85B50 engine and its builds. Vehicles concerned:

OEM reference: 11 24 7 838 623 (rod set).

Why it’s the most cost-effective solution?

On an S85B50, a rod that lets go at high revs almost always damages the crankshaft, the block and sometimes a cylinder head. This V10 is no longer produced and its specific parts are expensive and scarce: returning the engine to factory specification means buying rods and pistons again, with specialist labour and a long time off the road, only to end up back at the original limits.

For a comparable budget, sometimes less, a set of forged 4340 steel rods combined with forged pistons brings a far greater margin of strength, lasting fatigue life at high rpm and the freedom to develop the build without opening the bottom end again. On a V10 that has become sought after, it is also the best way of preserving the value of the car.

For the same budget: more reliability, more usable rpm, more longevity.