Reinforced ARP main studs for BMW M10B20 S14B20 S14B23 S14B25

The BMW engines concerned

The M10B20 (2.0 litres, 1,990 cc) and the S14B20 (2.0 litres, 1,990 cc), S14B23 (2.3 litres, 2,302 cc) and S14B25 (2.5 litres, 2,467 cc) are petrol inline-four engines from BMW. Built from 1965 to 1982, the M10B20 powers the 2002, 2000 Coupe, 320 and 520 of its day; the S14 family, from 1986 to 1993, is the one found in the E30 M3 and the 320is.

As standard, the crankshaft is held by 10 main cap bolts (reference 11 11 1 735 525, M10 x 75 bolts), tightened to 60 Nm on the M10B20 and by torque then angle (20 Nm + 50°) on the S14. On the S14, this method stretches them permanently: they are replaced every time the bottom end is dismantled.

Reinforced ARP studs for main bearings

On a crankshaft, the main caps are what hold the shaft against the loads passed on by the connecting rods. If their clamping force drops, the cap shifts slightly on its parting line at every cycle: the mating faces brinell, then the bearing bore goes out of round.

This kit replaces the original bolts with 10 ARP studs in 8740 steel and their nuts. Each stud is screwed hand-tight into the block and the caps are fitted over them; threads and nuts are then lubricated and the nuts are torqued. Preload becomes higher and more consistent, and the block threads are protected.

Material: 8740, a chrome-moly steel that ARP specifies at 190,000 psi tensile strength, with threads rolled after heat treatment. Factory-fitted fasteners usually belong to classes 10.9 and 12.9, with 150,800 to 176,900 psi minimum tensile strength. On this four-cylinder engine, the 8740 studs in M10 x 1.5 are rated for 1,000 hp at most (calculated estimate).

Technical specifications

Tightening torque

No torque is applied to the stud as it goes into the block: it is screwed in by hand. For a permanent installation, the instruction sheet leaves the option of a thread locker of the blue Loctite type on the block end, with the nuts fully tightened before it cures, never on the nuts and never mixed with ARP grease.

The nuts are tightened in the sequence the manufacturer specifies for the main caps; the instruction sheet supplied with the kit describes the full procedure.

Stronger fasteners change the clamping force: the size and roundness of the main bearing bores are checked after installation, and the bores are align honed if necessary, using the final fasteners and lubricant.

The torque figures above come from the ARP 201-5001 instruction sheet, revision applicable at publication. The official instruction sheet is authoritative in the event of any difference. Read the ARP 201-5001 instruction sheet

Technical guide

All the torque values on this page assume ARP assembly grease; with engine oil they are no longer correct.

The ARP brand

Coming from the aerospace industry, Gary Holzapfel founded ARP (Automotive Racing Products) in the early 1970s to put an end to engine failures caused by weak fasteners in motorsport. The company designs and manufactures thousands of part numbers in California, in several material grades depending on the application.

Advantages over the original bolts

Criterion	Original main bolts	ARP studs
Tightening	60 Nm on the M10B20, torque then angle on the S14	Three torque passes, steady build-up
Threads in the block	Loaded by the turning bolt at every tightening	Stud screwed in by hand, no torque
Refitting the caps	Cap held by hand while the bolts go in	Cap guided by the two studs

When studs become necessary

• Power increase from a turbo, supercharger or higher boost
• Stroker crankshaft or higher rev limit
• Complete bottom end strip-down
• Bearing bores or block threads already fatigued
• Intensive motorsport use

Vehicle compatibility

OEM reference: 11 11 1 735 525

Why is it the most cost-effective solution?

ARP studs in 8740 steel hold the main caps more firmly than the original bolts, which lowers the risk of bottom-end failure on a tuned engine. The outlay stays small compared with the cost of repairing a crankshaft and a block.