Reinforced ARP cylinder head studs Nissan RB26DETT

The engine concerned

The RB26DETT is Nissan's 2.6-litre turbocharged inline six-cylinder petrol engine (2,568 cc, 86 mm bore, 73.7 mm stroke), built from 1989 to 2002. It powers the Nissan Skyline GT-R R32, R33 and R34, as well as the Stagea 260RS. ARP offers two stud kits for it: the 202-4207 in ARP2000 and the 202-4208 in CA625+.

As original equipment, the cylinder head is held by 14 M12 x 1.5 bolts with part number 11056-05U01, tightened in several passes with an intermediate loosening, then finished with an angle.

Reinforced ARP cylinder head studs

Cylinder head fasteners work constantly: they hold the head tight against the block, crush the gasket to the right value and hold back the push of the gases at every combustion. If they lose even a little tension, the gasket starts to leak.

Each of the two ARP kits is supplied as 14 studs with their 12-point nuts, in M12 x 1.5.

The two instruction sheets do not call for the same fitting order. For the 202-4207, the studs are first screwed into the block by hand, then the cylinder head is fitted; for the 202-4208, the gasket and the cylinder head are fitted first, then the studs are screwed into the block by hand, without torque.

ARP2000 is an ARP proprietary alloy, heat treated to reach 220,000 psi tensile strength, with threads rolled after heat treatment. Standard engine fasteners mostly belong to property classes 10.9 and 12.9 (150,800 to 176,900 psi minimum): ARP2000 therefore offers 24 to 46 % more margin.

CA625+ (Custom Age 625+) is a super-alloy that ARP rates at a typical tensile strength of 260,000 to 280,000 psi, with long fatigue life and high toughness. It is fully resistant to atmospheric corrosion and oxidation, and ARP states it is immune to both hydrogen embrittlement and stress corrosion. As a reference point, standard engine fasteners mostly belong to property classes 10.9 and 12.9, i.e. a minimum tensile strength of 150,800 to 176,900 psi.

On this six-cylinder engine, the limit of the ARP2000 studs in M12 x 1.5 lies at 1,100 hp (calculated estimate). These CA625+ studs in M12 x 1.5 are calculated to handle up to 1,400 hp on this six-cylinder engine (calculated estimate).

Technical specifications

Tightening torque

Kit 202-4207: the studs are fitted into the block by hand, without a torque wrench. For a permanent installation, the instruction sheet accepts thread locker, with tightening to be completed before it sets.

For the 202-4207, the instruction sheet asks you to clean all the block threads, with the ARP thread chaser 912-0007 (M12 x 1.5) if needed. If the studs open into a water jacket, their block-side threads are coated with ARP thread sealer. The stud threads, the nuts and the washers receive ARP Ultra-Torque lubricant before fitting.

Kit 202-4208: each stud is screwed into the block by hand, without torque. This kit's instruction sheet does not mention thread locker; ARP states that some engine builders use a medium-strength (blue) one on the block end, but never on the nuts, never with ARP grease, and only if tightening is finished before it hardens.

For the 202-4208, the instruction sheet asks you to clean the block threads, with the ARP thread chaser 912-0007 (M12 x 1.5) if needed, then the washer seats of the cylinder head with solvent. The hexagonal socket at the end of each stud is only for fitting and removing the studs. The washers go on clean and dry, with no oil or grease; only the stud threads and the underside of the nuts receive ARP Ultra-Torque lubricant.

Tightening to torque is done in three steps, in the order set by the engine manufacturer, with ARP lubricant applied to the threads and under the nuts, a condition for the table values to apply.

These torque figures are those of ARP instruction sheets 202-4207 and 202-4208; only the official sheets in force apply, as for the complete procedure and the tightening sequence. Read the ARP 202-4207 instruction sheet ; Read the ARP 202-4208 instruction sheet

Technical guide

The stated tightening torques only make sense with ARP assembly grease, applied to the threads and under the nuts.

The ARP brand

Gary Holzapfel, a former aerospace engineer, created ARP (Automotive Racing Products) in the early 1970s to tackle engine failures caused by insufficiently strong fasteners in racing. Today the brand develops and manufactures thousands of parts in California, available in several material grades as needed.

Advantages over the original bolts

Criterion	Original head bolts	ARP2000 studs	CA625+ studs
Tightening	Several passes with an intermediate loosening, then an angle	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

On an engine left standard, the original fasteners do their job. Switching to reinforced fasteners becomes worthwhile as soon as:

• Power raised on an RB26DETT (higher boost pressure, larger turbos, hard-tuned build)
• The compression ratio is increased
• A reinforced MLS gasket is fitted, with its need for a high and even preload
• The head has already lifted or the gasket has already failed
• The engine is used in competition, with intense heat cycles and repeated teardowns

Vehicle compatibility

OEM reference: 11056-05U01 (RB26DETT)

Why is it the most cost-effective solution?

The original RB26DETT 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 ARP2000 or CA625+ 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.