Reinforced ARP bolts and nuts for OEM rods Mazda B6 BP

The Mazda 1.6 B6 and 1.8 BP engines concerned

This kit is for the two double-overhead-cam 16-valve four-cylinder engines from Mazda: the 1.6-litre B6 (1,597 cm³, 88 to 125 hp depending on the version) and the 1.8-litre BP (1,840 cm³, 103 to 209 hp, naturally aspirated or turbo), fitted to the MX-5 NA and NB and to the 323 BG and BA series.

This ARP kit takes the place of the original rod bolts of the B6 and BP engines, whose reference is not published. ARP presents it as a set of bolts to be pressed into the rods for the Mazda 1.6-litre (B6) and 1.8-litre (BP) DOHC.

Reinforced ARP bolts and nuts for connecting rods

Few parts of the bottom end work as hard as the rod bolt: at every revolution, it is what stops the cap from pulling away under the inertia loads of the rod and piston assembly. The original bolt is designed for stock power.

Made in 8740, this set of bolts is pressed into the rods before the nuts are tightened. ARP favours measuring the stretch; without a gauge, its instruction sheet gives 51.5 Nm. 8740 is a chrome-moly steel that ARP heat treats to 180,000 psi tensile strength, with threads rolled after heat treatment. As a reference point, standard engine fasteners mostly belong to classes 10.9 and 12.9, i.e. 150,800 to 176,900 psi minimum strength. In 8740, these bolts are rated for 9,500 rpm at most (calculated estimate).

Technical specifications

Tightening torque and stretch

For this part, ARP favours tightening to a measured stretch, the key to an exact preload; torque is given only when no gauge is available. The figures in the table assume ARP lubricant.

Source of the torque and stretch figures: ARP 118-6401 instruction sheet, in the revision applicable at publication. The manufacturer's instruction sheet prevails in the event of any difference. Read the ARP 118-6401 instruction sheet

No thread locker is called for by the ARP instruction sheet for this kit: the bolts are fitted with ARP Ultra-Torque grease, on their threads and on the bearing face of the nuts.

The ARP instructions for this kit require the big ends to be checked and resized once the fasteners are fitted: this job belongs to an equipped machine shop.

Measuring bolt stretch

Stretch measurement supplements torque tightening and gives the most accurate preload. It is done with the bolt in place: the gauge, fitted with a dial indicator, bears on both ends of the bolt and shows the stretch achieved, to be brought to the value in the table above. This avoids the variations caused by friction, which matters on a highly stressed engine.

Technical guide

All the torque and stretch values on this page assume ARP assembly grease on the bolt threads and on the bearing face of the nuts; engine oil does not replace it.

The ARP brand

An engineer with an aerospace background, Gary Holzapfel launched ARP (Automotive Racing Products) in the early 1970s with one goal: to eliminate engine failures caused by weak fasteners in racing. ARP designs and manufactures thousands of part numbers in California, cylinder head, main and rod bolts, in several materials depending on the use, and has become known for the consistency of its manufacturing.

Vehicle compatibility

Why is it the most cost-effective solution?

A tuned MX-5 or a turbo 323 GT-R loads each rod bolt on every cycle, and a single bolt that lets go wrecks the rod, the piston and the block. Moving to an 8740 kit costs far less than rebuilding the bottom end.

Matching connecting rod bearings

Removing the connecting rods is the right time to replace their bearings: here are the ones that fit this engine.

Why not step up to forged connecting rods?

A forged rod, hot formed under high pressure from a steel such as 4340, gains density and homogeneity compared with a stock rod. Where the original rod, often made of sintered metal, is aimed first at cost, a 4340 forged rod shows about +19% yield strength, +8% tensile strength and +19 to +37% fatigue strength. It is the cornerstone of a solid high-power build. For your engine: