Reinforced ARP bolts for OEM rods BMW M52B28 S50B30US S52B32US

The BMW engines concerned

The BMW M52B28 is the 2.8-litre inline six-cylinder petrol engine (193 hp) of the 328i (E36, E46), 528i (E39), 728i (E38) and Z3 2.8. The kit also covers the S50B30US (3.0 litres) and S52B32US (3.2 litres) six-cylinder engines of the BMW M3 (E36) and Z3 M sold in North America, which the ARP instruction sheet associates with the same bolt.

The original connecting rod bolt of the M52B28 carries the reference 11 24 7 589 671 (M9 x 47 mm) and that of the S52B32 the reference 11 24 1 739 729 (44 mm): BMW prescribes replacing these bolts each time they are removed, washing and oiling them, then tightening them to 5 Nm, 20 Nm, then 70° of angle.

Reinforced ARP bolts for connecting rods

At every engine revolution, the rod and piston assembly pulls on the rod cap, and only its bolts hold it against these inertia loads. The original ones are intended for stock power and for a single installation.

This kit replaces the original bolts of the six rods with ARP2000 bolts that the ARP instruction sheet has you tighten by measured stretch, using a gauge, the 54.2 Nm torque being used only when no gauge is available. Material: ARP2000, an alloy steel developed by ARP, heat treated to 220,000 psi tensile strength, with threads rolled after heat treatment. Against classes 10.9 and 12.9 of original fasteners, between 150,800 and 176,900 psi minimum, the margin is 24 to 46%. The ARP2000 bolts hold up to 10,250 rpm (calculated estimate).

Technical specifications

This bolt is fitted without a washer: the ARP instruction sheet requires sufficient chamfer around the bolt hole of the rod cap to clear the radius under the head.

Tightening torque and stretch

ARP gives priority to stretch control for an accurate preload; the torque in the table is used only when no gauge is available. Torque and stretch figures in the table apply with ARP lubricant.

The values above come from the ARP 201-6201 instruction sheet applicable at the date of publication; it is authoritative in the event of any difference. Read the ARP 201-6201 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.

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 7 589 671 (M52B28, M9 x 47 mm); 11 24 1 739 729 (S52B32, 44 mm)

Why is it the most cost-effective solution?

As original equipment, the rod bolt is tightened by torque then angle up to its yield point, which stretches it permanently. In ARP2000, the ARP bolts, reinforced and stronger, accept clearly higher engine speeds and inertia loads: less risk of a fastener-related failure, and therefore an engine repair far more expensive than the kit avoided.

Matching connecting rod bearings

New bolts, new bearings: find below the connecting rod bearings suited to 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: