Reinforced ARP bolts for OEM rods BMW M20 M50

The BMW engines concerned

The 2.5-litre M20B25 (325i E30, 525i E34, Z1) and the 2.0 and 2.5-litre M50 engines, with or without VANOS, of the 3 Series (E36) and 5 Series (E34) are the BMW inline six-cylinder petrol engines covered by this kit.

This ARP kit replaces the original connecting rod bolt, reference 11 24 1 713 342 (M9 x 53 mm), which BMW requires to be replaced at every disassembly, washed and oiled, then tightened to 5 Nm, 20 Nm and 70° of angle; depending on the rod type, the M50 also receives a 44 mm bolt (reference 11 24 1 739 729) that this kit does not replace.

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.

The ARP2000 bolts in this kit are tightened by stretch measured with a gauge, ARP's priority method, the 48.8 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 engine speed allowed by the ARP2000 bolts goes up to 10,250 rpm (calculated estimate).

Technical specifications

No washer is provided under the head of these bolts. On the rod cap, the ARP instruction sheet asks for a chamfer wide enough around the hole for the head radius to sit clear.

Tightening torque and stretch

ARP recommends tightening to a measured stretch, which gives a more accurate preload than torque alone, given as a fallback. Torque and stretch figures assume ARP lubricant.

Figures taken from the ARP 201-6301 instruction sheet, revision in force on the day of publication. Should any difference appear, the official instruction sheet takes precedence. Read the ARP 201-6301 instruction sheet

The ARP instruction sheet does not require a thread locker for this kit; only ARP Ultra-Torque grease is applied, on the threads as well as under the bolt 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

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

An engineer with an aerospace background, Gary Holzapfel launched ARP (Automotive Racing Products) in the early 1970s with one goal: to eliminate engine failures caused by weak fasteners in racing. ARP designs and manufactures thousands of part numbers in California, cylinder head, main and rod bolts, in several materials depending on the use, and has become known for the consistency of its manufacturing.

Vehicle compatibility

OEM reference (rod bolt): 11 24 1 713 342 (M9 x 53 mm); 11 24 1 739 729 (M50, 44 mm, not covered by this kit)

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?

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: