Reinforced ARP bolts for OEM rods BMW S50B30 S50B32

The BMW engines concerned

The BMW S50 is the 24-valve inline six-cylinder petrol engine of the M3 (E36) and the Z3 M. The S50B30 (3.0 litres, 286 hp, European version) was built from 1992 to 1995, then replaced by the S50B32 (3.2 litres, 321 hp), produced from 1995 to 2001.

The original connecting rod bolt of the European S50 is a steel bolt with an M10 thread and 45 mm under-head length (reference 11 24 1 405 890), tightened to torque and then through an angle: this method brings it to its yield point and makes it single-use, BMW requiring its replacement each time the rods are removed.

Reinforced ARP bolts for connecting rods

At every revolution, the rod bolt takes the pull of the rod and piston assembly, which tries to force the cap away through inertia. As original equipment, it is sized for stock power and is replaced at every strip-down.

This kit replaces the original bolts of the six rods with ARP2000 bolts, tightened by measuring stretch, the method ARP recommends for this part, or failing that by torque. ARP2000 is an alloy steel developed by ARP and heat treated to 220,000 psi tensile strength; its threads are rolled after heat treatment. Standard engine fasteners commonly fall into classes 10.9 and 12.9, i.e. 150,800 to 176,900 psi minimum strength: ARP2000 sits 24 to 46% above that. In ARP2000, these bolts are rated for 11,750 rpm at most (calculated estimate).

Technical specifications

Tightening torque and stretch

The method ARP recommends combines torque with a stretch check, for an accurate preload. The torque and stretch figures in the table apply with ARP lubricant.

Figures taken from the ARP 201-6102 instruction sheet, revision in force on the day of publication. Should any difference appear, the official instruction sheet takes precedence. Read the ARP 201-6102 instruction sheet

No thread locker is called for by the ARP instruction sheet for this kit: the bolts are fitted with ARP Ultra-Torque grease, on the threads and under the head.

According to the ARP instructions, the big ends must be checked and resized with the fasteners in place; plan a visit to a machine shop.

Measuring bolt stretch

Tightening to stretch means setting the preload on how much the bolt actually stretches, rather than on a torque affected by friction. With the bolt fitted, the dial gauge is placed on both its ends and the bolt is tightened until the stretch given in the table is read. It is the safest method on a highly tuned engine.

Technical guide

Tightening is done with ARP assembly grease, on the threads and under the head: the torque and stretch figures in the table were established with it.

The ARP brand

ARP (Automotive Racing Products) came into being in the early 1970s, when Gary Holzapfel, who came from the aerospace industry, decided to tackle the engine failures caused in motorsport by inadequate fasteners. Today the brand produces thousands of part numbers in California for the cylinder head, the main bearings and the connecting rods, in materials suited to each application, with manufacturing known for its consistency.

Vehicle compatibility

OEM reference (rod bolt): 11 24 1 405 890 (M10 x 45 mm)

Why is it the most cost-effective solution?

The original rod bolt, taken to its yield point by torque then angle tightening, keeps a permanent stretch. The ARP2000 bolts, reinforced and stronger, withstand higher engine speeds and inertia loads, which reduces the risk of a fastener-related failure and saves a far heavier engine repair.

Matching connecting rod bearings

Removing the connecting rods is the right time to replace their bearings: here are the ones that fit this engine.

Why not step up to forged connecting rods?

Hot forged under considerable pressure, generally from 4340 steel, a forged rod has a denser and more uniform structure than a stock rod. Against the original rod, frequently made of sintered metal and optimised for production cost, it brings about +19% yield strength, +8% tensile strength and +19 to +37% fatigue strength: enough to build a reliable high-power engine on. For your engine: