Reinforced ARP cylinder head studs BMW S14B20 S14B23 S14B25

The BMW engine concerned

The S14 is BMW's inline four-cylinder petrol engine with sixteen valves, offered as a 2.0-litre (S14B20, 1,990 cm³), 2.3-litre (S14B23, 2,302 cm³, 93.4 mm bore, 84 mm stroke) and 2.5-litre (S14B25, 2,467 cm³). Naturally aspirated and built from 1986 to 1991, it powers the 320is, the M3 E30 and its Evo versions.

As standard, the cylinder head is held by M12 bolts (part number 11 12 1 307 135), which must be replaced, washed and oiled. They are tightened to 50 Nm, then 80 Nm, and after a 15-minute wait to 100 Nm.

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.

The ARP 201-4605 kit contains 10 studs in 8740 steel, 10 nuts (12-point) and 10 washers, in M12 x 1.75 thread. According to the instruction sheet, the studs are screwed into the block by hand first, then the head is fitted and the nuts are torqued: the preload is higher and the block threads are spared. 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. On this four-cylinder engine, the limit of the 8740 studs in M12 x 1.75 lies at 600 hp (calculated estimate).

Technical specifications

Tightening torque

The studs are fitted into the block by hand, without a torque wrench. For a permanent installation, the instruction sheet accepts thread locker, with tightening to be completed before it sets.

When a stud protrudes into a water jacket, the instruction sheet calls for its block threads to be coated with ARP Thread Sealer.

Tightening to torque is done in three steps, in the order set by the engine manufacturer, with ARP lubricant applied to the threads and under the nuts, a condition for the table values to apply.

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

Values taken from ARP instruction sheet 201-4605, in the revision current on the date of publication. In the event of any discrepancy, the official instruction sheet prevails; it also details the full procedure and the tightening sequence. Read the ARP 201-4605 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
Tightening	Torque in three stages (50 Nm, 80 Nm, 15 min wait, 100 Nm)	Torque only, in three equal stages
Block threads	Stressed at every tightening	Preserved (studs fitted hand tight)

When studs become necessary

On an engine left standard, the original fasteners do their job. Switching to reinforced fasteners becomes worthwhile as soon as:

• Boost pressure exceeds the original value, or a turbo or supercharger is added
• 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

OEM reference: 11 12 1 307 135

Why is it the most cost-effective solution?

When combustion pressure rises, the original head bolts are put under heavy strain. Stronger, the ARP 8740 fasteners withstand a much higher pressure, forced induction included: they limit the risk of gasket leaks or breakage, and so avoid a rebuild far more expensive 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: