Reinforced ARP main studs and bolts for BMW MINI PSA N12 to N18 and EP6

The 1.6-litre Prince engines

This kit covers the 1.6-litre engines of the Prince family: N12B16, N13B16, N14B16, N16B16 and N18B16 at BMW and MINI, EP6, EP6C, EP6DT, EP6DTS and EP6CDTX at Peugeot and Citroën. They are inline four-cylinder petrol engines of 1,598 cc (77 mm bore, 85.8 mm stroke); the N14B16 and N18B16 are turbocharged.

Originally, the main bearings are held by M9 bolts 115 mm long (BMW reference 11 11 7 525 890), fitted at 30 Nm then through 150°. This angle stretches the bolts permanently: they are single-use. The kit replaces this hardware with ten M9 studs for the main caps and eighteen M6 bolts for the bedplate.

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 become indented, then the bearing bore goes out of round.

This kit brings together 28 ARP2000 fasteners numbered by the instruction sheet: 10 M9 studs (positions 1 to 10) for the main caps and 18 M6 bolts (positions 11 to 28) for the bedplate. Once the block is cleaned and its threads chased with a tap if necessary, the studs are screwed in by hand only. Washers, threads and nuts are coated with ARP Ultra-Torque, which the instructions prefer to engine oil. Material: ARP2000, an alloy steel developed by ARP, rated at 220,000 psi tensile strength. Against classes 10.9 and 12.9 of standard fasteners, between 150,800 and 176,900 psi minimum, it offers 24 to 46% more strength. On this four-cylinder engine, the ARP2000 studs in M9 / M6 are rated for 900 hp at most (calculated estimate).

Technical specifications

Tightening torque

Fitting the studs: screwed into the block by hand, without tightening torque. For a permanent installation, the ARP instruction sheet allows a Loctite thread locker, provided the tightening is completed before the product cures.

The M9 studs are tightened in three equal steps up to 68 Nm (50 ft-lb): 23, 45, then 68 Nm. The M6 bedplate bolts follow in three equal steps up to 16 Nm (12 ft-lb): 5, 11, then 16 Nm.

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.

Moving to reinforced fasteners changes the clamping force on the caps. After installation, the size and roundness of the bores are checked, then the bores are line-honed if needed, always with the final fasteners and lubricant.

Torque figures taken from the ARP 201-5401 instruction sheet, in the revision in force on the day of publication. In the event of any difference, the manufacturer's instruction sheet prevails. Read the ARP 201-5401 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

ARP, short for Automotive Racing Products, was born in the early 1970s on the initiative of Gary Holzapfel, an engineer from the aerospace sector, in response to the engine failures caused in racing by inadequate fasteners. Today the brand produces thousands of part numbers in California, offered in several materials depending on the application.

Advantages over the original bolts

Criterion	Original main bolts	Reinforced ARP studs
Tightening	Bolts tightened to 30 Nm then 150°, with the block thread loaded on every turn	Only the nut turns on the stud, the block thread stays intact
Refitting	Each removal and refit works the block thread	The stud stays in place: the block thread is preserved
Fitting the caps	The cap is held by hand while the bolts go in	The studs guide the cap and keep it in position

When studs become necessary

• Turbo N14, N18 or EP6 raised well above stock output
• Regular use on the track, in rally or in hill climbs
• Bottom end opened to change the crankshaft, rods or pistons
• Original main bearing bolts to be replaced, since they are angle-tightened
• Main caps and bedplate loaded by a heavily stressed engine

Vehicle compatibility

OEM reference: 11 11 7 525 890

Why is it the most cost-effective solution?

Tightened by torque then angle, the original main bolts are stretched permanently. Reinforced and therefore stronger, the ARP ARP2000 studs withstand much higher connecting rod loads and keep the caps in place: the risk of a fastener-related bottom end failure falls, and with it the risk of a repair costing far more than the kit.