Reinforced ARP cylinder head studs BMW M50B20 M50B20TU M50B25 M50B25TU S50B30US S52B32US

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 (30 Nm, 90°, then 90° again). This method stretches them for good, and BMW requires their replacement every time the head is removed.

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 201-4302 kit contains 14 ARP2000 studs with their 12-point nuts and washers, in M10 x 1.5, one stud for each original bolt. According to its instruction sheet, the head goes on before the studs: the gasket and head are fitted first, then the studs are screwed into the block by hand, the washers are placed on the head and the nuts are tightened. The ARP2000 range relies on an alloy steel developed by ARP: after heat treatment it reaches 220,000 psi tensile strength, and the threads are rolled afterwards. As standard bolts generally belong to classes 10.9 and 12.9, i.e. 150,800 to 176,900 psi minimum, ARP2000 exceeds them by 24 to 46 %. In M10 x 1.5, the ARP2000 studs hold up to 900 hp on this six-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.

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.

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

Source of these values: ARP instruction sheet 201-4302, 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 201-4302 instruction sheet

Technical guide

All torque values on this page are given with ARP assembly grease on the threads and under the nuts; another lubricant would make them wrong.

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 then angle (30 Nm, 90°, then 90° again)	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:

• M50 inline-six boosted or tuned naturally aspirated: combustion pressure beyond what BMW designed for
• 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 1 721 938

Why is it the most cost-effective solution?

The original head bolts, tightened to torque then angle, remain permanently stretched. Stronger, the ARP2000 fasteners withstand a much higher combustion 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

To get the most out of reinforced fasteners, pair them with a head gasket that withstands the same pressure. Here are the gaskets we offer for these engines: