Main Studs

Main studs and main bolts clamp the main bearing caps to the block, holding the crankshaft in line against combustion and inertia loads. The ARP kits in this section cover main stud and main bolt kits, cap side bolts for cross-bolted blocks and crankcase through-bolts, each listed by engine in the title.

Each firing stroke drives the crank towards the main caps, and at high speed the inertia of the moving parts adds loads of its own. If the clamp load falls short, a cap can shift fractionally on every stroke, which is known as cap walk. The parting faces then fret, the bearing bore loses its shape and the bearings take the damage.

A stud is screwed into the block by hand, and all the tightening happens at the nut. The block threads are not turned under load or worn each time the caps come off, and the clamp load is more even from cap to cap. Because the clamp load differs from the original, check the main bores with the studs tightened, and line-hone them where needed.

The listings set out these choices:

• Studs or bolts: stud kits bring the advantages above, while bolt kits upgrade the fastener and keep the original assembly method.
• Cap layout: kits suit 2-bolt or 4-bolt mains, splayed caps, strap caps or a single cap. Side bolts and through-bolts are listed where the block design calls for them.
• Sump and girdle: some studs are longer to mount a windage tray or a main girdle, and some kits are made for a particular sump layout.
• Material: mostly 8740 chrome moly or ARP2000, with a nickel-based superalloy for specialist use.
• Nuts and drive: 12-point or hex nuts in inch or metric socket sizes. Many studs have a hex socket in the end, such as 3/16 in, 5/32 in or 5 mm, for screwing them in.

Chase the block threads, then blow out the blind holes, as trapped oil or debris can stop a stud from seating fully. Screw each stud in by hand with a hex key, without torque. Lubricate the nut threads and faces with the specified lubricant, then tighten in the maker’s sequence and in steps.

Measure bearing clearances with the caps tightened before final assembly, then check crank end float and that the crank turns freely. Where a windage tray or girdle is fitted, confirm that it clears the rods and crank through a full rotation.