Reinforced ARP main studs for BMC B-Series 3-main

The BMC B-Series engines with three mains

ARP publishes this kit as "B Series (3 cap main)": it targets BMC B-Series engines with three main caps, with no engine code, model or year given. Part number 206-5402 is specific to this application; the five-main version falls under a different ARP part number.

From the factory these caps are held by the manufacturer's own hardware; in the kit, 1/2 inch studs take over.

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.

Kit 206-5402 contains ARP 8740 steel studs, 1/2 inch. After cleaning the block threads, they are screwed in by hand only, then the caps, washers and nuts go on, lubricated with ARP Ultra-Torque and not engine oil. The nuts are tightened in three equal passes, following the engine manufacturer's sequence, up to 149 Nm (110 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 three-main BMC B-Series engines; no strength figure is published for this part number.

Technical specifications

Tightening torque

The studs are screwed into the block by hand, with no torque. The ARP instruction sheet allows a thread locker for a permanent installation, provided the nuts are fully tightened 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.

Stronger fasteners change the clamping force: the size and roundness of the main bearing bores are checked after installation, and the bores are honed in line if necessary, using the final fasteners and lubricant.

Torque figures taken from the ARP 206-5402 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 206-5402 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
Putting the cap on	Depends on the original setup	Cap slid over the studs
Block threads	Depends on the original setup	Stud screwed in by hand, load taken by the nut
Material	Depends on the original setup	Heat-treated 8740 steel

When studs become necessary

• BMC B-Series three-main engine revved or boosted well beyond stock
• Build for track, historic rally or hill climb
• Full overhaul with the crankshaft or rods to replace
• Block threads marked by successive removals
• Main caps to be secured before line boring

Vehicle compatibility

Why is it the most cost-effective solution?

A B-Series block pushed on revs or boost loads its three caps harder. ARP studs remain a small budget against a crankshaft, a block or a line bore to redo.