Reinforced ARP main studs for AUSTIN HEALEY 6-cylinder

The Austin Healey six-cylinder

ARP lists this kit for the Austin Healey six-cylinder ("Austin Healey 6-cylinder" in the catalogue). According to the NGK catalogue, this 2,639 to 2,912 cc six-cylinder powers the Austin-Healey 100 (1957 to 1959) and the 3000 MK.I, MK.II and MK.III (1959 to 1968). Part number 206-5405 is specific to this application.

On this six-cylinder, the original fasteners hold the main caps; the studs in the kit replace them.

Reinforced ARP studs for main bearings

The bottom end rests on its main bearings: combustion after combustion, the loads coming from the connecting rods end up on them. Under this stress the main cap tries to move on its parting line, which eventually brinells the mating faces and distorts the bearing bore.

For kit 206-5405, the ARP 8740 steel studs are fitted as follows: block threads cleaned with a chaser if needed, hand tightening only, caps put in place, then washers and nuts coated with ARP Ultra-Torque. Engine oil is not recommended. The nuts are tightened in three equal passes, following the manufacturer's sequence, up to 149 Nm (110 ft-lb): 50, 99 then 149 Nm. 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. The kit replaces the original main cap fasteners of the six-cylinder and is tightened as the ARP instruction sheet describes; no strength figure is published for this part number.

Technical specifications

Tightening torque

The studs are fitted into the block by hand, with no torque tightening. For a permanent installation, the ARP instruction sheet allows a thread locker to be used, with tightening completed before it sets.

The nuts are tightened following the order set by the manufacturer for the caps; every step is given in the ARP instruction sheet.

After moving to stronger fasteners, the size and roundness of the main bearing bores are checked; if needed, the main bearing line is honed with the final fasteners and lubricant fitted.

Torque figures taken from the ARP 206-5405 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 206-5405 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

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 fasteners	Reinforced ARP studs
Taking the load	Depends on the original fastener of the six-cylinder	The nut takes the load, the block thread stays at rest
Block upkeep	Depends on the original fastener of the six-cylinder	Block threads preserved by hand fitting
Cap engagement	Depends on the original fastener of the six-cylinder	Engaged on studs that align it

When studs become necessary

• Austin-Healey 100 or 3000 entered in historic rallying, on track or on the road
• Six-cylinder pushed beyond its original output
• Bottom end rebuild during an overhaul or a crankshaft change
• Main cap bores to check after fitting the studs
• Forged rods and pistons to install in the six-cylinder's original block

Vehicle compatibility

Why is it the most cost-effective solution?

A six-cylinder pushed hard loads its main caps more. ARP studs are a modest outlay compared with a crankshaft or a block to replace.