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 main studs

The main bearings take every load the connecting rods pass on. On each combustion cycle the main cap works in tension and tries to shift on its parting face: this is what peens the mating surfaces and eventually damages the bearing bore.

This kit replaces the original bolts with 14 ARP 8740 steel studs and their nuts. The stud is screwed hand tight into the block, the cap is lowered onto it, then the nut is torqued. The resulting preload is higher and more consistent, it limits cap walk and preserves the block threads.

Technical specifications

Tightening torque

Tightening builds up in three equal stages to 81 Nm, following the tightening order the manufacturer specifies for the main caps. Those figures assume ARP assembly grease on the threads, the nuts and the washers.

The full procedure is set out in the ARP instruction sheet attached to this product, under the Download tab.

Technical guide

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.

The 8740 steel material

8740 is a chrome-moly steel, heat treated by ARP to reach a tensile strength of 200,000 psi. Its threads are rolled after heat treatment, which markedly improves fatigue strength.

The fitting method

The ARP procedure runs in this order: clean and if necessary chase the block threads, inspect the hardware, screw the studs into the block hand tight only and with no torque, using the 4 mm hex socket in their end, fit the main caps and check there is no binding or misalignment, lubricate threads, nuts and washers, then tighten in three equal stages.

Thread locker may be used for a permanent mounting of the studs, provided the fasteners are torqued before it sets.

ARP adds a point few retailers mention: moving to stronger fasteners changes the clamping force, so the main bearing bores must be checked after fitting the studs, for size and out-of-round, and align honed if necessary. That honing should be done with the same fasteners and lubricant used for final assembly.

Advantages over the original bolts

Criterion	Original main bolts	ARP studs
Tightening	Torque then angle, in one go	Three torque passes, gradual build-up
After removal	Bolt has stretched, fit for the bin	Stud is intact, refitted as is
Bearing on the cap	Bolt head straight onto the cast iron	Hardened parallel washer supplied
Load passed to the cap	The head rubs and tries to drag it along	The nut turns on the washer, the cap stays put
Threads in the block	Worked again at every tightening	Screwed in once, by hand
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 it is the most cost-effective solution

Reaching the main caps means pulling the engine and turning it over: the cost is labour, not fasteners. Better to fit studs that load the block thread only once.

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: