Reinforced ARP cylinder head studs Nissan VR38DETT

The Nissan engine concerned

The VR38DETT is Nissan's 3.8-litre twin-turbo V6, with 24 valves and twin overhead camshafts (3,799 cc, 95.5 mm bore, 88.4 mm stroke). It powers the Nissan GT-R (R35), in its standard and NISMO versions, since 2007. ARP offers a single kit for it, the 202-4305, in CA625+ studs.

As original equipment, the VR38DETT cylinder head is held by TTY-type bolts, tightened to the yield point.

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.

The ARP 202-4305 kit combines CA625+ studs (Inconel 625) in M12 x 1.25 with their 12-point nuts and washers.

According to the instruction sheet, the studs are screwed into the block by hand before the cylinder head is fitted; the nuts are then tightened in three equal passes, up to 115 ft-lb.

Material of the CA625+ version: Custom Age 625+, a super-alloy that ARP specifies at 260,000 to 280,000 psi typical tensile strength. ARP highlights its fatigue life, its toughness, its complete resistance to atmospheric corrosion and oxidation, and its immunity to hydrogen embrittlement and stress corrosion. Factory-fitted fasteners usually fall into classes 10.9 and 12.9, with 150,800 to 176,900 psi minimum tensile strength.

On this V6 engine, the limit of the CA625+ studs in M12 x 1.25 lies at 1,200 hp (calculated estimate).

Technical specifications

Tightening torque

The studs are screwed into the block by hand, without torque. Their instruction sheet allows thread locker if they are to be fixed permanently, on the sole condition that tightening is finished before it sets.

The instruction sheet cites the ARP thread chaser 912-0006 (M12 x 1.25) to clean up the block threads if needed. If the studs open into a water jacket, it says to coat their block-side threads with ARP thread sealer.

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.

The instruction sheet also calls for ARP lubricant on the washers.

Values taken from ARP instruction sheet 202-4305, in the revision current on the date of publication. In the event of any discrepancy, the official instruction sheet prevails; it also details the full procedure and the tightening sequence. Read the ARP 202-4305 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	CA625+ studs
Type of fastener	TTY-type cylinder head bolt	Studs and nuts, three torque passes
Block threads	Stressed at every tightening	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:

• VR38DETT with raised boost or 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-JF01A (VR38DETT)

Why is it the most cost-effective solution?

The original VR38DETT head bolts are sized for stock power: once stronger boost or hard tuning raises the combustion pressure, the gasket and the fasteners are the first to suffer. CA625+ studs bring a much higher clamp load and keep the gasket pressed down; redoing the cylinder heads of a GT-R, with gaskets and labour, costs far more than the kit.