Reinforced ARP cylinder head studs Toyota F

The engine concerned

The Toyota F is a 3.9-litre inline six-cylinder (3,878 cc in the NGK catalogue), fitted to the Land Cruiser FJ40, FJ43, FJ45 and FJ55. ARP matches this engine to its 203-4001 stud kit, made of 8740 and supplied with hex nuts.

As built, the F cylinder head is secured by the standard fasteners; ARP kit 203-4001 replaces them with studs and nuts.

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.

ARP kit 203-4001, in 8740, uses a 1/2 in thread. According to the sheet, the studs are first run into the block by hand; the head is then fitted, and threads, nuts and washers take the supplied ARP Ultra-Torque before torque tightening in three equal passes. 8740 is a chrome-molybdenum steel, rated by ARP at 200,000 psi tensile strength. As a reference point, an engine's standard fasteners mostly belong to property classes 10.9 and 12.9, with a minimum tensile strength between 150,800 and 176,900 psi. In 1/2 in, the 8740 studs hold up to 1,100 hp on this six-cylinder engine (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 asks for the block threads to be cleaned before fitting, if needed with the ARP thread chaser.

When a stud reaches into the water jacket, its block-side threads are coated with ARP Thread Sealer; Loctite is only an option for a permanent fit, with the nuts torqued before it sets.

Follow the tightening sequence recommended by the manufacturer and bring to torque in three stages; the stated values assume ARP lubricant on the threads and under the nuts.

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

The torque figures above come from ARP instruction sheet 203-4001, revision applicable on the day the page went online. The official instruction sheet prevails in case of difference and describes the complete procedure, tightening sequence included. Read the ARP 203-4001 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

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

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

• F 3.9 litres pushed: output beyond stock
• 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

Why is it the most cost-effective solution?

On a tuned F, it is the original fasteners that cap the load on the gasket. 8740 studs of 1/2 in carry it much higher, for a budget out of all proportion to a cylinder head to be skimmed and a gasket to be replaced.