Forged connecting rods BMW M5 E34 - 3.8L 24V - S38B38 for forged pistons

The BMW S38B38 engine

The BMW S38B38 is the final evolution of the S38 inline-six, itself a descendant of the legendary M88 from the M1. Bored out to 3,795 cc (94.6 mm bore x 90 mm stroke), it crowned the BMW M5 E34 from 1992, producing around 340 hp at 6,900 rpm and 400 Nm of torque. Its 24-valve head, six individual throttle bodies and Bosch Motronic management make it one of the most accomplished naturally aspirated engines of its era.

Built until 1995, the S38B38 is still prized for its sound and its high-revving character. But its factory crankshaft and rods were sized for stock output: as soon as you raise the redline, build the engine or plan regular track use, the rotating assembly becomes the weak link to reinforce. That is exactly what these forged connecting rods are for.

Forged connecting rods

These rods are forged and then machined from heat-treated 4340 steel, a material whose fatigue life bears no comparison with a mass-production part. Pressing aligns the metal grain along the load paths, which protects the rod exactly where it works: the shank in compression and the small end in tension, on every revolution, on a 3.8-litre engine pulling to almost 7,000 rpm.

Every set of six is shot-peened, inspected piece by piece and weight-matched, a condition for not spoiling the legendary smoothness of the straight-six. These rods follow an M5 E34 wherever it is used: circuit racing and track days, hillclimb, drift, rally, drag and time attack, as well as Stage 1 reliability builds, Stage 2 and Stage 3, up to full race engines. The ARP bolts supplied with the set keep the cap clamped whatever the load.

Technical specifications of the S38B38 forged connecting rods

Torque and bolt stretch

On an inline six like the S38B38 the point is not merely to reach the right tension: it is to reach it twelve times in a row. An assembly is judged by its scatter, and nothing replaces a written record, bolt by bolt, of free length and then clamped length. The resulting difference must fall inside the range given in the table above, a range issued by the bolt manufacturer and not adjustable: it differs depending on whether the rod takes ARP 2000 or ARP L19 bolts. Torque, dependent on thread friction and head bearing, does not allow that fine control; a stretch gauge does, which is what makes it the reference method. Threads cleaned and then coated with an even film of ARP assembly lubricant, load built in stages while alternating between the two cap bolts. The instruction sheet supplied with the kit remains the source that prevails.

Technical guide

4340 forged steel: what is it?

Connecting rod profiles

H-Beam profile. Two thick flanges joined by a central web: that is the definition of an H-Beam rod, and the guarantee of very high stiffness in bending as well as compression. Slightly heavier than an I-Beam, it relies on strength rather than lightness, which suits a 3.8-litre with substantial capacity per cylinder producing 400 Nm. It is the only profile offered for the S38B38, and the one that best absorbs a torque-oriented build or added forced induction.

How to choose your rod bolts?

Nothing in an engine is more heavily loaded than the rod bolt: on every revolution it takes the force trying to tear the cap away, a force that grows with engine speed and with cylinder pressure. Two ARP grades are offered here; the right approach is to choose the grade according to real use, then fit to the specified torque and stretch figures, with no approximation.

ARP 2000: the benchmark high-performance grade, with tensile strength around 220,000 psi (about 1,517 MPa). In 3/8" it holds up to 200 hp per cylinder and 8,500 rpm, a very wide margin against the S38B38’s original 340 hp, even after optimisation. With no special storage precautions, it covers almost every use: circuit racing and track days, hillclimb, drift, rally and drag.

ARP L19: the grade for engines taken to the end of their potential, with tensile strength around 260,000 psi (about 1,793 MPa). In 3/8" it accepts up to 250 hp per cylinder and 10,000 rpm, which makes it the logical choice for a boosted, stroked or race-entered S38B38. The alloy is, however, vulnerable to corrosion and hydrogen embrittlement: storage and fitting must be done oiled, away from moisture.

Measuring bolt stretch

Buy the ARP stretch gauge →

The brands offered

ZRP, a Greek workshop based in Athens and run by Alex Drakos, specialising in high-performance forged connecting rods. Forging, machining and final inspection all take place in Greece, and every set of six is weight-matched before dispatch. For the S38B38 the catalogue focuses on the H-Beam profile, available with ARP 2000 or ARP L19 bolts depending on the level of the build. ZRP rods run in rally, drift and circuit racing, which says more than any sales pitch.

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 S38B38 engine

These forged connecting rods are compatible with the factory BMW S38B38 engine and its builds. Vehicles concerned:

OEM reference: 11 24 1 401 358 (rod set).

Why it’s the most cost-effective solution?

When the bottom end of an S38B38 lets go, a spun or broken rod at high rpm, the bill climbs fast: rare parts, specialist labour, downtime.

For a budget comparable to tired second-hand rods, you fit a brand-new forged set here, calibrated and made reliable once and for all. No need to revisit it with every upgrade.

For the same budget: more reliability, a comfortable power margin and the peace of mind of a rotating assembly built to last. It is the most rational way to preserve an engine that has become rare.