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5.0L 24v - M70B50
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5.0L 24v - M70B50
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Set of 12 forged 4340 steel connecting rods for BMW V12 M70B50 and M70B54.
The M70 is BMW's first modern-era V12, launched in 1987. With an aluminium block and heads, a single overhead camshaft and two valves per cylinder, it uses a split engine-management layout, each bank of six cylinders having its own ECU. The M70B50 displaces 5.0 litres (4988 cc, 84 mm bore, 75 mm stroke) and produces 300 hp; the range is extended by a 5.4-litre evolution, designated M70B54 by the aftermarket, with more torque.
This twelve-cylinder powers the BMW 7 Series 750i and 750iL (E32 then E38) and the 8 Series 850i and 850Ci (E31), as well as the ALPINA B12 5.0. Renowned for its smoothness, the engine benefits from a reinforced rotating assembly as soon as you aim for high-mileage reliability or tuning: switching to forged connecting rods secures the bottom end and allows a controlled power increase.
Machined from high-strength 4340 steel, these reinforced connecting rods protect the bottom end as soon as loads rise. They become essential when fitting forged pistons, seeking greater reliability, raising the rev limit or gaining power, where the original rod reaches its limits.
Built to withstand the repeated stresses of tuned engines, naturally aspirated and forced-induction alike, they suit every motorsport discipline (rally, drift, drag, track and track days, hillclimb, time attack) and are equally at home in a reliability-focused rebuild or in Stage 2, Stage 3 and beyond builds.
| Reference | Brand | Profile | Bolts | Pin diameter OEM 22 mm | Small-end width | Big-end diameter OEM 48 mm | Big-end width | Center-to-center OEM 135 mm | Weight per rod |
|---|---|---|---|---|---|---|---|---|---|
| R-BMW-017-I | ZRP | I | ARP 2000-3/8"-38mm | 22 mm | 48 mm | 21.9 mm | 135 mm | 499 g | |
| R-BMW-017-I-L19 | ZRP | I | ARP L19-3/8"-38mm | 22 mm | 48 mm | 21.9 mm | 135 mm | 499 g |
| Rod | Bolts | Tightening torque | Recommended stretch |
|---|---|---|---|
| ZRP (R-BMW-017-I) | ARP 2000 | 61 N·m | 0.140 – 0.152 mm |
| ZRP (R-BMW-017-I-L19) | ARP L19 | 67.8 N·m | 0.152 – 0.165 mm |
Manufacturers recommend the stretch-gauge method as the reference: the instructions supplied with the kit always take precedence. The quoted torque figures correspond to assembly with ARP lubricant.
| 4340 is a low-alloy nickel-chromium-molybdenum steel used in aerospace and motorsport. Forged then heat-treated by quenching and tempering, it combines high tensile strength, excellent fatigue life and good ductility. Compared with an original sintered-steel rod, it gains around 19% in yield strength, 8% in tensile strength and 19 to 37% in fatigue life: real margin where sintered steel breaks under heavy load. It is the reference material for any high-performance connecting rod. |
 | I-Beam profile. Thin central section and reduced mass: the I-beam rod limits inertia and allows high engine speeds. It is the profile of choice for modern engines, rally builds and high-revving units, drift and time attack in particular. It is the profile chosen for this M40 / M43 kit. |
The rod bolt is one of the most heavily stressed parts of the engine. The right approach is to choose the material according to the real use of the build, then strictly follow the assembly torque and stretch.
 | ARP 2000 (tensile strength around 220,000 psi, i.e. ~1,517 MPa) is the high-performance standard. It covers builds up to around 700 hp and engine speeds in the region of 7,500 rpm. It is the most common choice for track, drift or rally use, with an excellent balance of strength and reliability and no particular storage requirement. |
 | ARP L19 (tensile strength around 260,000 psi, i.e. ~1,793 MPa) targets extreme use: beyond 800 hp, engine speeds above 8,500 rpm, drag racing and dragsters. Note that L19 is sensitive to corrosion and hydrogen embrittlement: it must be stored and fitted oiled, away from moisture. |
In addition to torque tightening, measuring rod-bolt stretch is the most reliable check for optimal preload. The check is done with the bolt fitted: place the gauge (dial indicator) on the two ends of the bolt and read its actual stretch, which must match the recommended value (see the torque & stretch table above). This method removes the friction variations inherent in torque tightening and secures the assembly on heavily stressed engines. |
 | ZRP : a Greek brand of high-performance forged connecting rods, made by Drakos Engineering in Athens. Each rod is forged from 4340 steel, precision-machined and balanced to stay reliable at high rpm. Available here in I-Beam profile and supplied with ARP 2000 or ARP L19 bolts, they are designed and inspected in Greece and used in rally, on track and in drift. |
 | 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 |
| Brand | Model (chassis) | Engine code | Power | Years |
|---|---|---|---|---|
| BMW | 7 Series 750i / 750iL (E32) | M70B50 | 300 hp | 1987–1994 |
| BMW | 8 Series 850i / 850Ci (E31) | M70B50 | 300 hp | 1990–1994 |
| ALPINA | B12 5.0 (E32, E31) | M70B50 | 350 hp | 1988–1994 |
OEM reference: 11 24 1 729 487.
When an original bottom end fails (a broken, bent or scored rod), going back to factory specification often means replacing both the rods and the pistons, frequently damaged together. The bill climbs quickly, just to return to the original state.
For a comparable or even lower budget, switching to 4340 forged connecting rods paired with forged pistons brings far greater reliability and safety margin, with real room to grow. It is the route chosen for more than fifteen years by European engine builders.
For the same budget: more reliability, more potential, more longevity.
