Reinforced ARP cylinder head bolts and studs Ford YVDA 39CD C23HD0D N38H N48H

The engines concerned

The YVDA, 39CD, C23HD0D, N38H and N48H are the variants of Ford's 2.3-litre turbocharged EcoBoost four-cylinder (2,261 cc, bore 87.5 mm, stroke 94 mm, sixteen valves), fitted to the Focus RS, the Mustang and the Zenos E10. ARP offers three kits for it: the 151-4301 stud kit in 8740, the 251-4303 stud kit in CA625+ and the 251-3701 bolt kit in ARP2000.

Originally, the cylinder head of the YVDA is held by single-use head bolts, tightened to 7 Nm, 15 Nm and then 55 Nm, before two 90° angles, which each of the three ARP kits replaces.

Reinforced ARP cylinder head bolts and studs

The job of the head fasteners is to keep the cylinder head locked to the block despite the pressure inside the combustion chambers. They keep the gasket compressed; as soon as their clamping force drops, leaks appear between head and block.

The 151-4301 stud kit has ten M10 x 1.25 studs with 12-point nuts; the 251-4303 stud kit, in CA625+, also takes 12-point nuts, while the 251-3701 bolt kit replaces the original bolts with 12-point head bolts, no nuts. In M10 x 1.25, the 8740 studs hold up to 600 hp on this four-cylinder engine (calculated estimate). As an estimate, equivalent CA625+ cylinder head studs hold up to 600 hp on this four-cylinder engine (calculated estimate).

Technical specifications

Tightening torque

No torque when screwing the studs into the block: hand tight is enough. The instruction sheet says nothing about thread locker; ARP mentions that some engine builders use a blue product on the block end, excluding the nuts and any mixing with its assembly grease, with tightening to be completed before the product sets.

Studs 151-4301: each washer goes on with the stamped side against the cylinder head, and the hex socket at the end of the stud is only for fitting and removal, never for tightening. The sequence includes a loosening: after three passes at 34, 68 then 102 Nm on nuts 1 to 10, all the nuts are loosened from 10 to 1, then the three passes are repeated in the same order.

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 or bolt heads.

The washers go on dry, clean on both faces, onto degreased cylinder head seats; an oil film at this point would distort the tightening.

The torque and passes in the table come from ARP sheet 151-4301; its full procedure, tightening sequence included, is to be followed to the letter. Read the ARP 151-4301 instruction sheet

Technical guide

To obtain the intended preload, ARP assembly grease must be present on the threads and under the nuts or bolt heads: the torque figures in the table are established with it.

The available grades

ARP undercut studs

The undercut version is recognisable by its reduced shank between the threads. According to ARP, this profile equalises stretch from one stud to the next and makes the clamping more consistent, while compensating for head gasket compression during installation: engine sealing benefits.

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	Reinforced ARP bolts
Fastener	Original head bolt	Stud and 12-point nut	12-point head bolt
Engine block threads	Stressed at every tightening	Preserved: the stud is screwed in by hand	Stressed at every tightening

When to upgrade the cylinder head fasteners

The original fasteners are sized for the engine as it leaves the factory. They need to be reinforced when:

• YVDA, 39CD, C23HD0D, N38H or N48H, tuned: boost pressure raised on a 2.3-litre EcoBoost
• The compression ratio rises as the build progresses
• The original gasket is replaced by a reinforced MLS gasket, which requires a high and uniform preload
• The engine has already suffered a lifted head or a gasket leak
• Competition use imposes severe heat cycles and regular head removals

Vehicle compatibility

Why is it the most cost-effective solution?

On a 2.3-litre EcoBoost run at a higher boost pressure, the single-use original head bolts limit what the gasket can hold. An ARP kit, in studs or bolts, moves that limit for an outlay far below that of reopening a Focus RS or a Mustang after a failed head gasket.

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: