Reinforced ARP cylinder head studs FORD DURATEC MAZDA KL V6

The engines concerned

This sheet covers two 2.5-litre 24-valve V6 engines. Ford's Duratec 25 (2,544 cc as originally built, 82.4 mm bore, 79.5 mm stroke) powers the Mondeo I and II, the Mondeo ST200 and the Cougar. Mazda's KL (2,497 cc, 84.5 mm bore, 74.2 mm stroke) powers the Mazda 626, MX-6 and Xedos 9, as well as the Ford Probe II. ARP offers two stud kits: the 253-4701 in 8740 for Ford's Duratec V6 and the 218-4704 in ARP2000 for Mazda's KL.

Each of these V6 engines has two cylinder heads: the ARP studs replace the original head bolts on both banks of cylinders. The two kits are not interchangeable, because their block-side threads differ (10 mm diameter for the 253-4701, M11 x 1.5 for the 218-4704); before fitting, the number printed on the box must therefore match the kit ordered.

Reinforced ARP cylinder head studs

Every combustion cycle pushes the cylinder head upwards. The head fasteners resist it by keeping a constant clamping pressure on the gasket; if that pressure drops, the seal is no longer guaranteed.

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 12-point nuts in three equal passes, up to 60 ft-lb for the 253-4701 and up to 85 ft-lb for the 218-4704. In M10, the 8740 studs hold up to 700 hp on this V6 engine (calculated estimate). On this V6 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 253-4701, the instruction sheet cites an ARP thread chaser to clean up the block threads if needed.

For the 218-4704, the instruction sheet cites the ARP thread chaser 912-0005 (M11 x 1.5) to clean up the block threads if needed. Four short studs, 5.950 inches or about 151 mm long, go in positions 5 and 6, at the front of each cylinder head, to avoid any interference with the camshaft sprockets.

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.

These values come from ARP instruction sheets 253-4701 and 218-4704; the official version of those sheets, checked before assembly, takes precedence for the whole procedure. Read the ARP 253-4701 instruction sheet; Read the ARP 218-4704 instruction sheet

Technical guide

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

The available grades

ARP undercut studs

An undercut stud has a reduced-diameter shank. This profile, ARP explains, spreads stretch more evenly across the studs: the cylinder head is clamped with a steadier force, gasket compression is better compensated and the gasket is less likely to fail.

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	Spared (studs screwed in by hand)
Tightening	As per the manufacturer's manual	Three equal passes up to the instruction sheet torque

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:

• Duratec V6 or KL built for fast road or track use (forced induction, modified engine map, raised engine speed)
• 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?

The original head bolts of a naturally aspirated V6 are sized for stock power: as soon as a build raises combustion pressure, the gaskets and the heads are the first to suffer. ARP studs in 8740 or ARP2000 keep each head pressed down with a steadier clamping load; reopening a V6 and its two heads costs far more than these kits.