Reinforced ARP main studs for Nissan RB25DET RB26DETT

The Nissan engines concerned

This kit is made for the Nissan inline six-cylinders of 2.5 and 2.6 litres, 24 valves, turbo (RB25DET) or twin-turbo (RB26DETT), fitted in particular to the Skyline GT-R R32, R33 and R34, the Skyline GTS-25t and GT-t, the Stagea, Laurel, Cedric and Gloria.

On the stock RB25DET and RB26DETT the main caps are secured with bolts; the ARP studs take their place.

Reinforced ARP studs for main bearings

Each main bearing takes a share of the loads the connecting rods pass on at every combustion. Pulled downwards, the main cap tends to slide on its parting line: over time the mating faces brinell and the bearing bore loses its shape.

This kit replaces the original main cap fasteners with ARP2000 M10 x 1.5 studs and their nuts. After the block threads are cleaned, the studs are screwed in by hand only, then the caps are fitted. Threads, washers and nuts receive the ARP Ultra-Torque lubricant, then the nuts are tightened in three equal passes, in the manufacturer's sequence. Only the nut turns: the torque read stays truer to the preload and the block thread no longer takes the twisting load of the bolt. ARP2000 is an alloy steel developed by ARP and heat treated to 220,000 psi tensile strength, with threads rolled after heat treatment. Standard engine fasteners commonly belong to property classes 10.9 and 12.9, i.e. 150,800 to 176,900 psi minimum tensile strength: ARP2000 therefore sits 24 to 46% above that. On this inline six-cylinder engine, the ARP2000 studs in M10 x 1.5 are rated for 1,400 hp at most (calculated estimate).

Technical specifications

Tightening torque

No torque is applied to the stud as it goes into the block: it is screwed in by hand. For a permanent installation, the instruction sheet leaves the option of a thread locker of the blue Loctite type on the block end, with the nuts fully tightened before it cures, never on the nuts and never mixed with ARP grease.

ARP instruction sheet 202-5403 covers the Nissan RB25DET and RB26DETT inline sixes and sets the final tightening at 81 Nm.

The nuts are tightened following the order set by the manufacturer for the caps; every step is given in the ARP instruction sheet.

With stronger fasteners, the clamping force is no longer what it was originally: the size and roundness of the main bearing bores must be checked once installation is complete, and align honed if necessary with the final fasteners and lubricant.

Torque figures taken from the ARP 202-5403 instruction sheet, in the revision in force on the day of publication. In the event of any difference, the manufacturer's instruction sheet prevails. Read the ARP 202-5403 instruction sheet

Technical guide

Tightening is done with ARP assembly grease: the torque figures in the table were established with it, not with engine oil.

The ARP brand

Coming from the aerospace industry, Gary Holzapfel founded ARP (Automotive Racing Products) in the early 1970s to put an end to engine failures caused by weak fasteners in motorsport. The company designs and manufactures thousands of part numbers in California, in several material grades depending on the application.

Advantages over the original bolts

Criterion	Original main bolts	Reinforced ARP studs
Tightening method	Bolt turned into the block	Nuts tightened progressively, in three passes
Block thread	Loaded by the turning bolt	Stud placed by hand, no torque load
Refitting the caps	Cap to be held while screwing	Cap positioned by the studs

When studs become necessary

• Power increase from a turbo, supercharger or higher boost
• Stroker crankshaft or higher rev limit
• Complete bottom end strip-down
• Bearing bores or block threads already fatigued
• Intensive motorsport use

Vehicle compatibility

Why is it the most cost-effective solution?

On an RB tuned well beyond its original output, the original fasteners become the weak link of the bottom end: ARP2000 studs hold the main caps with a much higher clamping load and remove that risk of failure. The price difference is minimal next to the cost of rebuilding the bottom end.

Consider the crankshaft main bearings

To complete the bottom end overhaul, consider the tri-metal crankshaft main bearings suited to these engines: