Reinforced ARP cylinder head studs BMW M50B20 M50B25 M50B25TU

The BMW engines concerned

The BMW M50 (2.0 and 2.5 litres) is the cast-iron block petrol inline-six fitted to the 3 Series (E36) and 5 Series (E34). The North American E36 M3 engines (S50B30US, then S52B32US) share the same architecture.

As standard, the cylinder head is held down by 14 head bolts (reference 11 12 1 721 938, M10 x 1.5 thread), tightened by torque then angle. This method stretches them for good: they cannot be reused, and BMW requires replacement every time the head is removed.

Reinforced ARP cylinder head studs

Cylinder head fasteners have one simple job: to press the head onto the block hard enough to compress the gasket and contain combustion pressure, which is constantly trying to separate the two parts. Any loss of clamp load shows up as a blown head gasket.

This kit replaces the original bolts with 14 ARP2000 studs and their 12-point nuts. Once the gasket and cylinder head are in place, the studs are hand-threaded into the block, then the nuts are torqued, which gives a higher preload. ARP2000 is an alloy steel developed by ARP, heat-treated to 220,000 psi tensile strength, with threads rolled after heat treatment. Standard engine fasteners commonly fall into property classes 10.9 and 12.9, that is 150,800 to 176,900 psi minimum tensile strength: ARP2000 therefore sits 24 to 46 % above that. These reinforced M10 x 1.5 studs are designed to withstand up to 900 hp on this six-cylinder engine (calculated estimate).

Technical specifications

Tightening torque

The studs screw into the block by hand only, with no torque: the hex broached into the end of the stud is there to ease fitting, not to apply torque. The kit instruction sheet does not call for thread locker. ARP states that some engine builders use one, blue grade, on the end that screws into the block; it forbids any thread locker on the nuts, forbids combining it with the ARP grease, and requires tightening to be completed before it cures.

The nuts are tightened in three successive stages, following the manufacturer's recommended sequence. These figures assume the use of ARP assembly lubricant on the threads and under the nuts.

The washers must go on perfectly dry, wiped on both faces, and the seats on the cylinder head must be degreased: any trace of oil there makes the clamping load uneven.

These figures come from ARP instruction sheet 201-4302, in the revision current on the day of publication. The official instruction sheet prevails in the event of any discrepancy, and sets out the full procedure as well as the tightening sequence. Read the ARP 201-4302 instruction sheet

Technical guide

The torque figures given here assume ARP assembly grease on the threads and under the nuts, and only hold with it.

The ARP brand

Coming from the aerospace industry, Gary Holzapfel founded ARP (Automotive Racing Products) in the early 1970s to tackle a scourge of motorsport: engine failure caused by insufficiently strong fasteners. Today ARP designs and manufactures in the USA (California) thousands of part numbers, offered in several material grades according to the application.

Advantages over the original bolts

Criterion	Original head bolts	ARP2000 studs
Tightening method	Torque then angle	Torque only, in 3 stages
Block threads	Stressed at every tightening	Preserved (stud fitted hand tight)

When studs become necessary

The original fasteners are adequate as long as cylinder pressure stays close to the factory figure. Reinforced studs become necessary in several cases:

• Adding forced induction, or raising boost pressure on an already turbocharged engine
• Raising the compression ratio, particularly on a naturally aspirated build
• Fitting a reinforced MLS head gasket, which needs a high and even preload to seal
• An engine that has already lifted its head or blown a gasket
• Competition use, with repeated heat cycles and frequent head removals

Vehicle compatibility

OEM reference: 11 12 1 721 938

Why is it the most cost-effective solution?

The original head bolts are tightened to a torque value and then through an angle, a method that stretches them permanently. Reinforced ARP2000 studs withstand far higher combustion pressure, boost included: they reduce the risk of a head gasket leak or a fastener-related failure, and so spare you a rebuild that costs far more than the kit.

Consider a reinforced head gasket

Reinforced studs show their full worth alongside a head gasket able to take the same pressure. We offer the two references matching these engines: