Reinforced ARP main studs for MAZDA B6 BP

The Mazda B6 and BP engines

This kit is intended for the 16-valve double overhead camshaft four-cylinder engines of Mazda: the B6 (1.6 litres) and the BP (1.8 litres), fitted to the MX-5 (Miata) and, according to the NGK catalogue, to the 323 (BG and BA series). ARP lists them together as "1.6 (B6) and 1.8L (BP) DOHC Miata".

Originally, Mazda fasteners hold the crankshaft main caps; the kit's 10 mm studs replace them.

Reinforced ARP studs for main bearings

Each main bearing takes a share of the loads the connecting rods pass on at every combustion. Pulled downwards, the main cap tends to slide on its parting line: over time the mating faces brinell and the bearing bore loses its shape.

This kit replaces the original main cap fasteners of the B6 and BP with 10 mm ARP 8740 steel studs. The block threads are first cleaned and checked; the studs are then screwed in by hand only, before the caps go on. Studs, washers and nuts are coated with ARP Ultra-Torque, which the instructions prefer to engine oil. The 12-point nuts are tightened in three equal steps, in the manufacturer's order, up to 81 Nm (60 ft-lb): 27, 54, then 81 Nm. Material: 8740, a chrome-moly steel that ARP specifies at 200,000 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. The 8740 studs in 10 mm hold up to 1,000 hp on this four-cylinder engine (calculated estimate).

Technical specifications

Tightening torque

The studs are screwed into the block by hand, with no torque. For a permanent assembly, ARP's instructions permit a Loctite thread locker, provided full tightening is done before the product cures.

Follow the cap tightening sequence recommended by the manufacturer when bringing the nuts up to torque; the kit's instruction sheet details each step.

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.

Values taken from the ARP 218-5401 instruction sheet in force at the date of publication; the official instruction sheet prevails should any difference appear. Read the ARP 218-5401 instruction sheet

Technical guide

Tightening is done with ARP assembly grease: the torque figures in the table were established with it, not with engine oil.

The ARP brand

Founded in the early 1970s by Gary Holzapfel, who came from the aerospace industry, ARP (Automotive Racing Products) set out to prevent fastener-related engine failures in motorsport. Its thousands of part numbers are designed and manufactured in California, in materials chosen for each use.

Advantages over the original bolts

Criterion	Original main bolts	Reinforced ARP studs
Tightening	Each screwing loads the block thread	Only the nut works on the stud: the block is left untouched
Successive removals	The block thread suffers at every removal	The stud stays screwed in, the block keeps its thread intact
Fitting the cap	The bolt has to be guided by hand under the cap	The studs position the cap before tightening

When studs become necessary

• B6 or BP engine built for the track, rally or drag racing
• Power or revs well above stock
• Block opened to rework the whole crankshaft
• Main caps that move or damaged threads
• Rebuild with a forged crankshaft and new forged parts

Vehicle compatibility

Why is it the most cost-effective solution?

An MX-5 pushed well beyond stock asks more of its main caps than Mazda planned for. Studs that clamp them better protect the crankshaft and the block, for a far lower cost than rebuilding a bottom end.