Reinforced ARP bolts for OEM rods SUBARU EJ25 EJ255 EJ257

The Subaru engines concerned

This kit is intended for the turbocharged 2.5-litre Subaru EJ25 flat-four (99.5 mm bore, 79 mm stroke, 2,457 cc), in its EJ255 versions fitted from 2004 to 2014 to the Forester XT, Impreza WRX, Legacy GT and Outback XT, and its EJ257 versions fitted from 2007 to 2021 to the Impreza WRX STi.

The original connecting rod bolt of these engines carries the reference 12109AA062; the ARP kit replaces it on each of the rods.

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.

Each ARP2000 bolt in this kit is best tightened by the stretch read with a gauge, ARP giving a torque of 56.9 Nm for the case where that instrument is not at hand. These bolts are made from ARP2000, an ARP-specific alloy steel heat treated to 220,000 psi tensile strength, with threads rolled once the treatment is complete. Classes 10.9 and 12.9 of standard fasteners start at 150,800 and 176,900 psi: ARP2000 exceeds them by 24 to 46%.

Technical specifications

According to the ARP instruction sheet for this kit, each bolt is fitted without a washer.

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 260-6302 instruction sheet, revision in force on the day of publication. Should any difference appear, the official instruction sheet takes precedence. Read the ARP 260-6302 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

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): 12109AA062

Why is it the most cost-effective solution?

On an EJ255 or EJ257 run at a higher boost pressure, the rod bolts see tensile loads that grow with engine speed, and a single bolt letting go ruins the rod, the piston and the boxer's block. A set of ARP2000 bolts protects that point for a cost far below that of rebuilding the bottom end.

Matching connecting rod bearings

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