Reinforced ARP cylinder head studs Ford Cosworth YB N5B

The engine concerned

The Ford Cosworth YB, also designated N5B, is Ford's 2.0-litre (1,993 cc) turbocharged 16-valve inline four-cylinder, built from 1985 to 1996. It powers the Sierra RS Cosworth and its Sapphire RS Cosworth 4x4 version, as well as the Escort RS Cosworth.

As standard, the cylinder head is held by cylinder head bolts, which the ARP stud kit replaces.

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 this engine ARP offers a single stud kit, reference 251-4701, in 8740 with 12-point nuts. According to its instruction sheet, the studs are first started into the block by hand and the cylinder head is then fitted; the nuts and washers, coated with the ARP Ultra-Torque lubricant included in the kit, are then tightened in three equal passes, in the order laid down by the manufacturer. Material of the fasteners: 8740, a chrome-molybdenum steel that ARP specifies at 190,000 psi tensile strength. Factory-fitted fasteners usually fall into classes 10.9 and 12.9, with 150,800 to 176,900 psi minimum tensile strength. In M12, the 8740 studs hold up to 600 hp on this four-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 you to clean all the block threads and, if necessary, to chase them with the ARP thread cleaning tool.

ARP thread sealer is reserved for studs that open into the water jacket.

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.

Source of these values: ARP instruction sheet 251-4701, revision current at the time of publication. The official instruction sheet applies if any discrepancy appears; it covers the entire procedure and the tightening sequence. Read the ARP 251-4701 instruction sheet

Technical guide

All torque values on this page are given with ARP assembly grease on the threads and under the nuts; another lubricant would make them wrong.

The ARP brand

Founded in the early 1970s by Gary Holzapfel, who came from the aerospace industry, ARP (Automotive Racing Products) set itself the mission of eliminating engine failures caused by inadequate fasteners in competition. Its thousands of parts are designed and produced in California, in several material grades depending on the application.

Advantages over the original bolts

Criterion	Original head bolts	Reinforced ARP studs
Block thread	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:

• Power raised on a turbocharged four-cylinder (higher boost pressure, remapped engine management)
• 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 turbocharged four-cylinder, the original head fasteners are sized for stock power: when boost pressure rises, the head gasket and the fasteners are the first to be stressed. ARP 8740 studs give a higher clamp load and keep the gasket pressed down; redoing a cylinder head, gasket and labour included, costs far more than the kit.

Consider a reinforced head gasket

Reinforced fasteners are only fully worthwhile with a head gasket able to resist the same pressure. We offer the following part for this engine: