Reinforced ARP cylinder head studs BMW M10B20

The BMW engine concerned

The M10 is BMW's inline four-cylinder petrol engine with eight valves; its M10B20 version is a 2.0-litre unit (1,990 cm³, 89 mm bore, 80 mm stroke). It powers the BMW 2000 and 2002, including the 2002 Turbo, as well as the 320 E21 and the 520 E12.

As standard, the cylinder head is held by M12 bolts of 156 mm (part number 11 12 0 621 144), which must be replaced, washed and oiled. They are tightened to 60 Nm; after a 15-minute wait comes a 33° angle, then the engine is warmed up for 25 minutes before a final 25° angle.

Reinforced ARP cylinder head 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 ARP 201-4601 kit contains 10 studs in 8740 steel, 10 nuts (12-point) and 10 washers, in 12 mm diameter, one for each original bolt. According to the instruction sheet, the studs are first screwed into the block by hand; the head is then fitted, followed by the nuts, which are tightened to torque. The block threads are thus spared.

ARP makes these fasteners in 8740, a chrome-molybdenum steel whose tensile strength reaches 190,000 psi. To put this figure in context, original engine fasteners generally belong to classes 10.9 and 12.9, whose minimum ranges from 150,800 to 176,900 psi.

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.

If a stud opens into a water jacket of the block, the instruction sheet asks for its threads to be coated with ARP Thread Sealer.

Final tightening is done in three successive stages, following the manufacturer's tightening sequence; these torque figures only hold with ARP assembly lubricant on the threads and under the nuts.

According to the instruction sheet, ARP lubricant is also applied to the washers, as on the threads.

Source of these values: ARP instruction sheet 201-4601, 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 201-4601 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

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	8740 studs
Tightening	Torque then angle (60 Nm, 15 min wait, 33°, 25 min warm, 25°)	Torque only, in three equal stages
Block threads	Stressed at every tightening	Preserved (studs fitted hand tight)

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:

• Forced induction added, or boost pressure increased
• 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

OEM reference: 11 12 0 621 144

Why is it the most cost-effective solution?

Tightening to torque then angle stretches the original head bolts for good. Reinforced, the ARP 8740 fasteners cope with a much higher combustion pressure, even with forced induction: the risk of gasket leaks or breakage drops, and with it the risk of a repair costing far more than the kit.