Reinforced ARP main studs for BMC A-Series

The BMC A-Series engines

ARP lists this kit for the BMC A-Series ("BMC A-series"), with no engine code, model or year given. Part number 206-5401 is specific to this application.

As built, these main caps rest on the manufacturer's original hardware, which the kit's 7/16 inch studs replace.

Reinforced ARP studs for main bearings

In a bottom end, the main bearings absorb the thrust of the connecting rods at every combustion, and it is the main caps, clamped to the block, that hold the crankshaft in place. When their clamping gives way, the cap works on its parting line: the mating faces become marked, then the bearing bore distorts.

Kit 206-5401 contains ARP 8740 steel studs, 7/16 inch, which go into the block by hand only, after the threads have been cleaned. The caps, washers and nuts follow, coated with ARP Ultra-Torque rather than engine oil, then tightening in three equal passes, in the order set by the engine manufacturer, up to 108 Nm (80 ft-lb). 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 kit strengthens the main cap fixing of built BMC A-Series engines; 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. The ARP instruction sheet accepts a thread locker when the installation is to be permanent, provided the nuts are fully tightened before it hardens.

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 206-5401 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 206-5401 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

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
Cap fixing	Varies with the engine	Stud fixed in the block, nut tightened on the cap side
Steel	Varies with the engine	8740 chrome-moly steel, heat-treated for strength
Seating the cap	Varies with the engine	Cap located by the studs already screwed in

When studs become necessary

• Building a BMC A-Series engine for track, rally or hill climb
• A-Series engine revved well beyond stock
• Short block stripped for a complete rebuild
• Main caps that no longer seat as they should
• Forged crankshaft, rods or pistons going into the block

Vehicle compatibility

Why is it the most cost-effective solution?

The bearings of a built A-Series engine work harder. A few ARP studs cost little next to replacing a crankshaft or a block.