Rod Bolts

Rod bolts clamp the cap to the big end of each connecting rod, holding the bearing bore together against the inertia of the piston and rod. ARP’s rod bolt kits are listed by engine, by aftermarket rod or by bolt size, in materials from 8740 chrome moly up to Custom Age 625+ and MP35N.

The peak load comes at the top of the exhaust stroke. The piston and the top of the rod must be stopped and sent back down with no combustion pressure to cushion them, so their inertia pulls on the cap. That pull rises with the square of engine speed and returns thousands of times a minute at high revs, which makes fatigue the real threat.

The defence is preload. A bolt stretched to its design figure clamps the cap harder than inertia can pull on it. The joint then never opens, and the bolt sees only a small part of each load cycle. Too little stretch lets the cap move; too much yields the bolt.

Check these points:

• Application: kits name the engine in the title. Replacement bolts for aftermarket rods name the rod maker, and often a rod part number or a bolt size such as 1/4 or 7/16 in.
• Material: 8740 chrome moly is the base grade, and ARP2000 the usual step up. L19 is stronger again, but it must be kept oiled, as it is prone to stress corrosion and hydrogen embrittlement.
• Premium alloys: Custom Age 625+, a nickel-based superalloy, and MP35N, a nickel-cobalt alloy, pair very high strength with resistance to corrosion.
• Wrenching: hex, such as 1/2 or 9/16 in, or compact 12-point, such as 7/16 in or 12 mm, which can help clearance on a long-stroke crank.
• Design and quantity: some kits use ARP’s Wave-Loc design, named in the title. Kits come as pairs for one rod or as complete engine sets.

New bolts change the clamping on the big end, so check the bore for size and roundness with the bolts tightened, and have the rods resized where needed. On the bench, hold the rod in a rod vice or soft jaws while tightening. Lubricate the threads and the underside of the head with the lubricant the kit specifies.

Where both ends of the bolt can be reached, tighten to the stretch figure with a stretch gauge, since stretch tracks the clamp load whatever the friction. Note each bolt’s free length before fitting; one that has grown permanently has yielded and must be replaced.