Reinforced ARP cylinder head studs Nissan RB20 RB25

The engines concerned

The RB20 and RB25 are Nissan 24-valve inline six-cylinder engines, naturally aspirated (RB20DE, RB25DE) or turbocharged (RB20DET, RB25DET): the RB20 displaces 2.0 litres (original bore 78 mm), the RB25 2.5 litres (original bore 86 mm). They power the Nissan Skyline R32, R33 and R34, the Laurel C33, C34 and C35, the Cefiro A31 and the Stagea WC34, among others. ARP offers two stud kits: the 202-4301 in 8740, for the RB20DE, RB20DET, RB25DE and RB25DET, and the 202-4309 in ARP2000, for the RB25DE and RB25DET.

As original equipment, the RB25DET cylinder head is held by bolts with part number 11056-58S01, tightened in several passes with an intermediate loosening, then finished with an angle.

Reinforced ARP cylinder head studs

The job of the head fasteners is to keep the cylinder head locked to the block despite the pressure inside the combustion chambers. They keep the gasket compressed; as soon as their clamping force drops, leaks appear between head and block.

Both ARP kits are studs with 12-point nuts: the 202-4301 in 7/16 in, in 8740, and the 202-4309 in M11 x 1.5, in ARP2000. Their block-side threads differ: the 202-4309 instruction sheet asks you to compare the kit number with the one on the box before fitting.

For both kits, the instruction sheets ask you to screw the studs into the block by hand before fitting the cylinder head, then to tighten the nuts in three equal passes, up to 80 ft-lb for the 202-4301 and up to 90 ft-lb for the 202-4309.

Material of the fasteners: 8740, a chrome-molybdenum steel that ARP specifies at 190,000 psi tensile strength. Factory-fitted fasteners usually fall into classes 10.9 and 12.9, with 150,800 to 176,900 psi minimum tensile strength.

Developed by ARP, ARP2000 alloy steel is heat treated to 220,000 psi tensile strength, and its threads are rolled once the treatment is done. Compared with the 10.9 and 12.9 classes of standard fasteners, between 150,800 and 176,900 psi minimum tensile strength, the gap reaches 24 to 46 %.

In 7/16 in, the 8740 studs hold up to 900 hp on this six-cylinder engine (calculated estimate). On this six-cylinder engine, the limit of the ARP2000 studs in M11 x 1.5 lies at 1,000 hp (calculated estimate).

Technical specifications

Tightening torque

For both kits: no torque is applied to the studs as they go into the block: they are screwed in by hand. The instruction sheet leaves the option of thread locker to mount them permanently, provided tightening is done before the product hardens.

For the 202-4301, the instruction sheet says to clean up the block threads with an ARP thread chaser if needed. If the studs open into a water jacket, the instruction sheets say to coat their block-side threads with ARP thread sealer.

For the 202-4309, the instruction sheet cites the ARP thread chaser 912-0005 (M11 x 1.5) to clean up the block threads if needed.

Final tightening is done in three successive stages, following the manufacturer's tightening sequence; these torque figures only hold with ARP assembly lubricant on the threads and under the nuts.

These torque figures are those of ARP instruction sheets 202-4301 and 202-4309; only the official sheets in force apply, as for the complete procedure and the tightening sequence. Read the ARP 202-4301 instruction sheet ; Read the ARP 202-4309 instruction sheet

Technical guide

To obtain the intended preload, ARP assembly grease must be present on the threads and under the nuts: the torque figures in the table are established with it.

The ARP brand

Founded in the early 1970s by Gary Holzapfel, who came from the aerospace industry, ARP (Automotive Racing Products) set itself the mission of eliminating engine failures caused by inadequate fasteners in competition. Its thousands of parts are designed and produced in California, in several material grades depending on the application.

Advantages over the original bolts

Criterion	Original head bolts	8740 studs	ARP2000 studs
Tightening	Several passes with an intermediate loosening, then an angle (RB25DET)	Torque only, in three passes	Torque only, in three passes
Block threads	Stressed at every tightening	Spared (studs screwed in by hand)	Spared (studs screwed in by hand)

When studs become necessary

Standard fasteners remain suitable as long as the engine keeps its original combustion pressure. They reach their limits in the following cases:

• Power raised on an RB20DET or an RB25DET (higher boost pressure, hard-tuned build)
• Compression ratio raised for more power
• Reinforced MLS head gasket, which requires a high and even preload
• Cylinder head that has already lifted or gasket that has already leaked
• Racing or track use, with repeated heat-ups and frequent teardowns

Vehicle compatibility

OEM reference: 11056-58S01 (RB20, RB25)

Why is it the most cost-effective solution?

The original RB20 and RB25 head bolts are sized for stock power: as soon as combustion pressure rises with forced induction, the gaskets and the fasteners are the first to suffer. ARP studs in 8740 or ARP2000 bring a higher clamp load and keep the gasket pressed down; redoing a six-cylinder cylinder head, with gasket and labour, costs far more than the kit.