Reinforced ARP main studs for BMW M50B20 M50B25 M52B25 M52B28

The BMW engines concerned

The BMW M50 (2.0 and 2.5 litres) and M52 (2.5 and 2.8 litres) are the petrol inline-six engines with two-bolt main caps fitted to the 3 Series (E36), 5 Series (E34 and E39), 7 Series (E38) and the Z3. The North American E36 M3 engines (S50B30US, then S52B32US) share the same architecture.

As standard, the crankshaft is held by 14 main cap bolts (reference 11 11 1 735 525, M10 x 75 hex bolts), tightened by torque then angle. This method stretches them for good: they cannot be reused and must be replaced every time the bottom end is dismantled.

Reinforced ARP studs for main bearings

The main bearings take the whole of the load transmitted by the connecting rods. On every combustion event the bearing cap works in tension and tries to move on its parting line: this is what brinells the mating faces and eventually ruins the bearing bore.

This kit replaces the original bolts with 14 ARP studs in 8740 steel and their nuts. The stud is hand-threaded into the block, the cap is placed onto it, then the nut is torqued: preload becomes higher and more consistent, the cap no longer moves and the block threads are protected. 8740 is a chrome-moly steel heat-treated by ARP to 200,000 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: 8740 therefore sits 13 to 33 % above that. These studs take up to 1,400 hp on this six-cylinder engine (calculated estimate).

Technical specifications

Tightening torque

The studs screw into the block by hand, with no torque. ARP allows a thread locker of the blue Loctite type on the end that goes into the block for a permanent mounting, provided the nuts are torqued before it cures, never on the nuts and never together with the ARP grease.

The nuts are tightened in the sequence specified by the manufacturer for the main bearing caps. The instruction sheet supplied details the procedure.

A stronger fastener changes the clamping force: check the size and the roundness of the main bearing bores after fitting, and align hone if necessary, using the fasteners and the lubricant of the final assembly.

Torque figures taken from the ARP 201-5000 instruction sheet, in the revision applicable on the day of publication. Should any discrepancy appear, the manufacturer's instruction sheet is the reference. View the ARP 201-5000 instruction sheet

Technical guide

The torque figures given here are only valid with ARP assembly grease, never with engine oil.

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. Every kit comes with its assembly grease and instructions covering the steps and tightening torque.

Advantages over the original bolts

Criterion	Original main bolts	ARP studs
Tightening	Torque then angle, in one go	Three torque passes, gradual build-up
Threads in the block	Worked again at every tightening	Screwed in by hand, no torque
Reassembling the bottom end	Cap held in place, then bolted	Cap guided by the two studs

When studs become necessary

• Forced induction added, or boost pressure raised
• Raising the rev limit or the crankshaft stroke
• Bottom end rebuild, where the original fasteners have to be replaced anyway
• A block showing signs of cap walk or tired threads
• Competition use, with repeated load cycles

Vehicle compatibility

OEM reference: 11 11 1 735 525

Why is it the most cost-effective solution?

Tightened to a torque value and then through an angle, the original main bolts stretch permanently. Reinforced and stronger, ARP studs in 8740 steel take far higher connecting rod loads and keep the caps in place: the likelihood of a fastener-related bottom end failure drops, which avoids a far more expensive repair.

Consider the crankshaft main bearings

A bottom end rebuild is done with new bearings. We offer the tri-metal crankshaft main bearings matching these engines: