Reinforced ARP bolts for OEM rods BMW S65B40

The BMW engine concerned

The BMW S65 is the petrol V8 of 4.0 litres with 32 valves of the M3 (E90, E92, E93). The S65B40 produces 420 hp and was built from 2007 to 2013.

The original connecting rod bolt of the S65 carries the reference 11 24 7 834 522 (M9 x 1): BMW prescribes new bolts, washed and oiled, tightened in three sequences of 6 Nm, 20 Nm then 130 degrees, with loosening between them, which makes it a single-use part.

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 rod bolts with ARP CA625+ bolts, tightened by measuring stretch, the method ARP recommends for this part, or failing that by torque. The CA625+ version uses Custom Age 625+, a super-alloy whose typical tensile strength ranges from 260,000 to 280,000 psi according to ARP. It combines fatigue life and toughness, is unaffected by atmospheric corrosion or oxidation, and remains immune to hydrogen embrittlement and stress corrosion. By comparison, classes 10.9 and 12.9 of original fasteners have a minimum of 150,800 to 176,900 psi. The CA625+ bolts hold up to 11,250 rpm (calculated estimate).

Technical specifications

Tightening torque and stretch

ARP recommends supplementing torque tightening with stretch measurement, which gives a more accurate preload. Torque and stretch figures assume ARP lubricant.

The values above come from the ARP 201-6001 instruction sheet applicable at the date of publication; it is authoritative in the event of any difference. Read the ARP 201-6001 instruction sheet

Assembly is done without thread locker, as the ARP instruction sheet calls for none: ARP Ultra-Torque grease is applied to the threads and under the head of each bolt.

Once the fasteners are fitted, the ARP instructions call for the big ends to be checked and resized by a machine shop.

Measuring bolt stretch

Stretch measurement supplements torque tightening and gives the most accurate preload. It is done with the bolt in place: the gauge, fitted with a dial indicator, bears on both ends of the bolt and shows the stretch achieved, to be brought to the value in the table above. This avoids the variations caused by friction, which matters on a highly stressed engine.

Technical guide

Torque and stretch figures are only valid with ARP assembly grease, applied to the threads and under the head.

The ARP brand

Founded in the early 1970s by Gary Holzapfel, who came from the aerospace sector, ARP (Automotive Racing Products) was born to put an end to fastener-related engine failures in racing. Its thousands of part numbers, designed and manufactured in California for the cylinder head, the main bearings and the connecting rods, come in several materials depending on the application and are recognised for the consistency of their manufacture.

Vehicle compatibility

OEM reference (rod bolt): 11 24 7 834 522 (M9 x 1)

Why is it the most cost-effective solution?

Tightened by torque then angle, the original rod bolt reaches its yield point and stays stretched for good. Reinforced and therefore stronger, the ARP CA625+ bolts withstand much higher engine speeds and inertia loads: the risk of a fastener-related failure falls, and with it the risk of an engine repair out of all proportion to the price of the kit.

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?

A forged rod, hot formed under high pressure from a steel such as 4340, gains density and homogeneity compared with a stock rod. Where the original rod, often made of sintered metal, is aimed first at cost, a 4340 forged rod shows about +19% yield strength, +8% tensile strength and +19 to +37% fatigue strength. It is the cornerstone of a solid high-power build. For your engine: