Reinforced ARP cylinder head studs Toyota 2AZ-FE

The engine concerned

The 2AZ-FE is Toyota's 2.4-litre petrol four-cylinder (2,362 cc, bore 88.5 mm, stroke 96 mm), naturally aspirated, with sixteen valves and double overhead camshafts, built from 2000 to 2012; it powers the Toyota Camry and RAV4, among others. ARP devotes two ARP2000 stud kits to it, with 12-point nuts, according to the model year: the 203-4303 for 2006 and earlier, the 203-4306 for 2007 and later.

On this engine the cylinder head is originally held down by head bolts, which the ARP studs of these two kits replace.

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.

Two ARP kits are offered for this engine, both in ARP2000, with an M11 x 1.5 thread and 12-point nuts. Their instruction sheets differ on the fitting order: the 203-4303 studs are screwed in by hand before the head is fitted, those of the 203-4306 after the gasket and head are in place. 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 M11 x 1.5, the ARP2000 studs hold up to 800 hp on this four-cylinder engine (calculated estimate). This figure applies to the 203-4303 kit; as an estimate, the 203-4306 kit, of the same ARP2000 grade and the same M11 x 1.5 thread, suits tuned four-cylinder engines like the 203-4303 kit; its admissible power is not quantified.

Technical specifications

Tightening torque

Studs 203-4303: 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.

Before fitting, the sheet has the block threads cleaned, with the ARP M11 x 1.5 chaser if needed.

If a stud opens into the water jacket, the part screwed into the block is coated with ARP thread sealer.

Threads, nuts and washers are lubricated with the Ultra-Torque supplied in the kit, never with engine oil; tightening is done in three equal passes up to 100 ft-lb.

Studs 203-4306: the studs are fitted by hand into the block threads, without a torque wrench. The instruction sheet makes no provision for thread locker. According to ARP, some installers apply a blue one to the block-end thread only; never on the nuts or together with ARP grease, and tightening must be completed before it sets.

Before fitting, the sheet has the block threads cleaned, with the ARP M11 x 1.5 chaser if needed, along with the spot faces on the head where the washers seat.

Once the studs are screwed in by hand, both sides of each washer are wiped dry: they go on without oil or lubricant; the Ultra-Torque is applied only to the stud threads and the underside of the nuts.

Tightening is done in three passes, at 35, 70 then 100 ft-lb.

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 follow ARP instruction sheets 203-4303 and 203-4306; for the full procedure, including the manufacturer's tightening sequence, the official sheets in force remain the reference. Read the ARP 203-4303 instruction sheet; Read the ARP 203-4306 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	Reinforced ARP studs
Block threads	Stressed at every tightening	Preserved: studs fitted 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:

• 2AZ-FE tuned, with or without added boost: output beyond stock
• 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

Why is it the most cost-effective solution?

On a tuned 2AZ-FE, the load the gasket can take is capped by the original bolts. ARP2000 studs of 11 mm, with their 12-point nuts, carry it much higher, for a cost out of all proportion to a cylinder head to be skimmed and a gasket to be replaced.

Consider a reinforced head gasket

Reinforced fasteners and the head gasket work together: the gasket must hold the same pressure as the fasteners. The matching gasket for this engine: