Reinforced ARP main studs for MAZDA MZR LF

The Mazda MZR and LF engines

ARP lists this kit for Mazda's 16-valve 2.3-litre from 2003, in MZR and MZR DISI versions (L3-VE and the direct-injection turbo L3-VDT), and for the 2.0-litre LF. They are inline four-cylinder engines with double overhead camshafts. According to the NGK catalogue, they power in particular the Mazda 3, 5, 6, CX-7, MX-5 (NC), Tribute, MPV, Premacy and Biante.

Originally, the main caps are held by plastic-yield bolts, Mazda reference L3G2-10-352 for the L3-VDT and LF; being angle-tightened, they are single use. The kit's 10 mm studs replace them.

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.

The kit replaces the original main cap fasteners with 10 mm ARP 8740 steel studs. The block threads are cleaned and checked first, then the studs are screwed in by hand only, before the caps are fitted. Threads, washers and nuts receive ARP Ultra-Torque, preferred to engine oil. The nuts are first tightened by hand, then in three equal steps following the manufacturer's sequence up to 81 Nm (60 ft-lb): 27, 54 and 81 Nm. 8740 is a chrome-moly steel heat treated by ARP to 200,000 psi tensile strength, with threads rolled after heat treatment. To put this figure in context, standard engine fasteners mostly belong to property classes 10.9 and 12.9, i.e. 150,800 to 176,900 psi minimum tensile strength. On this four-cylinder engine, the 8740 studs in 10 mm 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. A Loctite thread locker may be used for a permanent installation according to the ARP instructions; tightening must then be completed before the product hardens.

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 align honed if needed, always with the final fasteners and lubricant.

The torque figures above come from the ARP 218-5402 instruction sheet, revision applicable at publication. The official instruction sheet is authoritative in the event of any difference. Read the ARP 218-5402 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	Reinforced ARP studs
Assembly	The original bolt tightens in the plastic range and cannot be reused	The stud is tightened in three measured steps, outside the plastic range
Load on the block	The bolt works in the thread at each tightening	The stud stays in place, only the nut is turned
Fitting the caps	Each bolt is guided under the cap	The studs first centre the caps

When studs become necessary

• Mazda 2.3 MZR DISI turbo whose output is far above stock
• Building a 2.0 LF or a 2.3 for the track or competition
• Bottom end removed to change crankshaft, rods or pistons
• Single-use original main bolts that must not go back in
• Main caps loaded by high revs and high load

Vehicle compatibility

OEM reference: L3G2-10-352

Why is it the most cost-effective solution?

A 2.3 turbo or a 2.0 pushed far from stock values puts the bottom end under heavy strain. Strengthening the main cap fastening costs far less than redoing a damaged crankshaft or block.