Reinforced ARP cylinder head studs BMW MINI Toyota B48B20 B46B20 B48A20

The engines concerned

The B48B20 is BMW's 2.0-litre turbocharged inline four-cylinder petrol engine (1,998 cm³, 82 mm bore, 94.6 mm stroke), in production since 2014. It powers many BMW models, from the 1 Series to the 7 Series, the X3 to X6 and the Z4, as well as the Toyota GR Supra 2.0; under the B48A20 code, it also powers the MINI Cooper S and John Cooper Works. The B46B20, listed with it by ARP, is part of the application list.

As standard, the B48B20 cylinder head is held by ten M11 bolts, 188 mm long.

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.

Four ARP kits are offered for these engines, each with 10 studs and 12-point nuts, M11 diameter. Kits 201-4704 (BMW and MINI) and 203-4707 (Toyota GR Supra) are in ARP2000; kits 401-4704 (BMW and MINI) and 403-4707 (Toyota GR Supra) are in CA625+.

According to the instruction sheets, the cylinder head and its gasket are fitted first; the studs are screwed into the block afterwards.

Developed by ARP, ARP2000 alloy steel is heat treated to 220,000 psi tensile strength, and its threads are rolled once the treatment is done. Compared with the 10.9 and 12.9 classes of standard fasteners, between 150,800 and 176,900 psi minimum tensile strength, the gap reaches 24 to 46 %.

Material of the CA625+ version: Custom Age 625+, a super-alloy that ARP specifies at 260,000 to 280,000 psi typical tensile strength. ARP highlights its fatigue life, its toughness, its complete resistance to atmospheric corrosion and oxidation, and its immunity to hydrogen embrittlement and stress corrosion. Factory-fitted fasteners usually fall into classes 10.9 and 12.9, with 150,800 to 176,900 psi minimum tensile strength.

In M11, the ARP2000 studs hold up to 700 hp on this four-cylinder engine (calculated estimate). On this four-cylinder engine, the limit of the CA625+ studs in M11 lies at 800 hp (calculated estimate).

Technical specifications

Tightening torque

For all four kits: each stud is screwed into the block by hand, without torque. This kit's instruction sheet does not mention thread locker; ARP states that some engine builders use a medium-strength (blue) one on the block end, but never on the nuts, never with ARP grease, and only if tightening is finished before it hardens.

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.

Fit the washers perfectly dry and clean both faces; degrease the head seats too, otherwise the tightening obtained would not be even.

These torque figures are taken from ARP instruction sheets 201-4704, 401-4704, 203-4707 and 403-4707, in the revision current on the day of publication. In the event of any difference, the official instruction sheet applies; it also describes the full procedure and the tightening sequence. Read the ARP 201-4704 instruction sheet ; Read the ARP 401-4704 instruction sheet ; Read the ARP 203-4707 instruction sheet ; Read the ARP 403-4707 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	ARP2000 studs	CA625+ studs
Main fasteners	10 M11 bolts	10 M11 studs	10 M11 studs
Block threads	Stressed at every tightening	Preserved (studs fitted hand tight)	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:

• Higher boost pressure, through an engine remap or a turbo change
• 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 fasteners reach their limit as soon as combustion pressure rises, for example with a larger turbo or an engine remap. Reinforced, and therefore stronger, ARP2000 or CA625+ fasteners withstand a much higher pressure: the risk of gasket leaks or breakage drops, and with it the risk of an expensive repair.